Topic
11
Chemical Equilibrium
Part A Unit-based exercise
Unit 39 An introduction to
chemical equilibrium
5
The direction in which a net reaction will proceed
to achieve equilibrium can be predicted by
constant (Kc).
Fill in the blanks
only are known as
2
a) When Qc < Kc, a net
Chemical reaction that take place in one direction
irreversible
forward
reaction
Unit 39
1
Part A
comparing reaction quotient (Qc) and equilibrium
must occur until equilibrium is reached.
reactions.
b) When Qc > Kc, a net
The reaction between ethanoic acid and ethanol
backward
reaction
must occur until equilibrium is reached.
does not go to completion no matter how long
the reaction mixture is heated under reflux. This
3
is known as a
reversible
A
equilibrium is reached when
dynamic
the forward and backward reactions occur at the
same rate.
4
True or false
reaction.
Decide whether each of the following statements is
true or false.
6
A dynamic equilibrium is reached when
the forward and backward reactions occur
at the same rate.
T
7
For a system at equilibrium, the
concentrations of the reactants and the
products must be the same.
F
8
Equilibrium can only be established in an
open system.
F
9
The value of Kc for a reaction can be
used to judge the rate at which
equilibrium is attained.
F
10 If Qc is greater than Kc, the system is not
at equilibrium.
T
In an aqueous solution of potassium chromate,
the following equilibrium system is established:
2CrO42–(aq) + 2H+(aq)
Cr2O72–(aq) + H2O(l)
a) When a little dilute sulphuric acid is added to
the system, the colour of the solution changes
from
to
yellow
orange
indicates that the concentration of
. This
Cr2O72–(aq)
ions has increased while the concentration
of
2–
ions has decreased.
CrO4 (aq)
b) When a little dilute sodium hydroxide solution
is added to the resulting solution, the colour
of the solution changes from
to
yellow
orange
. This indicates that the
concentration of
CrO 4 2– (aq)
ions
has increased while the concentration
of
Cr2O72–(aq)
ions has decreased.
1
Multiple choice questions
11 Which of the following statements is true for a
reaction system at equilibrium?
Part A
A All reactions cease.
B The reactions have gone to completion.
C The rates of the forward and backward
reactions are equal.
D The amount of products equals the amount
of reactants.
C
12 Consider the following reaction:
Unit 39
2SO3(g)
2SO2(g) + O2(g)
Initially, SO3(g) is placed in an empty flask. How do
the rates of the forward and backward reactions
change as the system proceeds to equilibrium?
A
B
C
D
Forward
reaction rate
Backward
reaction rate
Increases
Increases
Decreases
Decreases
increases
decreases
decreases
increases
D
2HI(g)
In experiment A, 1.00 mol dm–3 H2(g) and 1.00
mol dm–3 I2(g) were initially added to a flask and
equilibrium was established. In experiment B, 2.00
mol dm–3 HI(g) were initially added to a second
flask and equilibrium was established. Which of
the following statements is always true about the
equilibrium concentrations?
A [H2(g)] equals [HI(g)] in experiment A.
B [HI(g)] equals 2[H2(g)] in experiment A.
C [HI(g)] in experiment A equals [HI(g)] in
experiment B.
1
D [HI(g)] in experiment A equals
[I2(g)] in
2
experiment B.
C
A
B
C
D
A closed system
A constant temperature
Equal forward and backward reaction rates
Equal concentrations of reactants and
products
D
N2(g) + 3H2(g)
Which of the following graphs represents [H2(g)]
after equilibrium has been established?
2NH3(g)
16 What are the changes in the rate of the forward
reaction and the concentration of N2(g) as the
system approaches equilibrium?
5JNF
<)H
>
<)H
>
D
5JNF
N2(g) + 3H2(g)
A 1.00 dm3 container is initially filled with NH3(g).
<)H
>
5JNF
C
Directions: Questions 16 and 17 refer to the following
information.
Consider the following reaction:
B
<)H
>
A
A
B
C
D
5JNF
B
2
H2(g) + I2(g)
15 Which of the factors below is NOT a condition
necessary for equilibrium?
13 Consider the following equilibrium system:
2NH3(g)
14 Two experiments were performed involving the
following equilibrium. The temperature was the
same in both experiments.
Rate of
forward reaction
Concentration of N2(g)
Decreases
Decreases
Increases
Increases
increases
decreases
increases
decreases
A
17 At equilibrium, there is 0.0400 mole of N2(g)
present. What is the concentration of H2(g)?
A
B
C
D
–3
0.0400 mol dm
–3
0.0600 mol dm
–3
0.0800 mol dm
–3
0.120 mol dm
21 CO2(g) + 3H2(g)
What is the equilibrium constant, Kc, for this
reaction?
[CH3OH(g)][H2O(g)]
[CO2(g)][H2(g)]3
B Kc =
[CO2(g)][H2(g)]3
[CH3OH(g)][H2O(g)]
C Kc =
[CH3OH(g)] + [H2O(g)]
[CO2(g)] + 3[H2(g)]
D Kc =
[CO2(g)] + 3[H2(g)]
[CH3OH(g)] + [H2O(g)]
18
Part A
A Kc =
D
$PODFOUSBUJPO
CH3OH(g) + H2O(g)
A
Unit 39
22 Consider the following equilibrium system:
CaCO3(s) + 2HF(g)
CaF2(s) + H2O(g) + CO2(g)
5JNF
Which of the following chemical reactions is
consistent with the above graph?
Which of the following expressions represents the
equilibrium concentration of CO2(g)?
A
B
C
D
A [CO2(g)] =
Kc[H2O(g)]
2
[HF(g)]
B [CO2(g)] =
Kc[HF(g)]
[H2O(g)]
C [CO2(g)] =
Kc[CaCO3(s)][HF(g)]2
[H2O(g)]
D [CO2(g)] =
Kc[CaCO3(s)][HF(g)]2
[CaF2(s)][H2O(g)]
X(g)
X(g)
2X(g)
2X(g)
Y(g) + 2Z(g)
Y(g) + 3Z(g)
Y(g) + Z(g)
2Y(g) + Z(g)
2
B
19 Consider the hypothetical reaction:
2A(g) + B(g)
3C(g)
What is the equilibrium constant, Kc?
A Kc =
[C(g)]3
[A(g)]2[B(g)]
B Kc =
[A(g)] [B(g)]
3
[C(g)]
C Kc =
[C(g)]
[A(g)]2 + [B(g)]
D Kc =
[A(g)] + [B(g)]
3
[C(g)]
23 For which of the following systems does Kc =
[O2(g)]?
A
B
C
D
2
3
2
A
20 What is the equilibrium constant, Kc, for the
reaction below?
2S(s) + 3O2(g)
2SO3(g)
A Kc =
2[SO3(g)]
2[S(s)] + 3[O2(g)]
B Kc =
2[SO3(g)]
3[O2(g)]
C Kc =
[SO3(g)]
[S(s)]2[O2(g)]3
D Kc =
2
[SO3(g)]
[O2(g)]3
B
O2(l)
O2(g)
3O2(g)
2O3(g)
2O2(g) + N2(g)
N2O4(g)
2Hg(s) + O2(g)
2HgO(s)
A
24 1 mole of N2O4(g) was placed in an empty 1 dm3
container and allowed to reach equilibrium
according to the following equation:
N2O4(g)
2NO2(g)
At equilibrium, x mole of N2O4(g) had dissociated.
What is the value of the equilibrium constant, Kc,
at the temperature of the experiment?
A
2x
(1 – x)
C
4x
(1 – x)2
2
B
2x
2
(1 – x)
D
4x
(1 – x)
2
2
D
D
3
SO3(g) + NO(g)
SO2(g) + NO2(g)
was analyzed and found to contain 0.0800 mole
of SO2(g), 0.200 mole of NO2(g), 0.250 mole of
NO(g) and 0.480 mole of SO3(g) in a 10.0 dm3
container. What is the equilibrium constant, Kc,
for this reaction?
Part A
A
B
C
D
7.52
1.14
0.302
0.133
D
Unit 39
26 The reaction below reaches equilibrium in a closed
reaction vessel of volume 2.50 dm3.
2NO(g) + O2(g)
2NO2(g)
At equilibrium, there are 2.83 moles of NO(g),
3.00 moles of O2(g), and 18.0 moles of NO2(g).
What is the equilibrium constant, Kc, for the
reaction?
0.218 dm3 mol–1
3
–1
1.83 dm mol
3
–1
13.4 dm mol
3
–1
33.7 dm mol
2.00 moles of each of H2(g) and I2(g) are allowed to
react in a 1.00 dm3 container at a certain temperature.
3.50 moles of HI(g) are present at equilibrium.
28 What is the value of the equilibrium constant,
Kc?
A
B
C
D
5.10 x 10–3
3.74
56.0
196
D
29 Which of the following graphs shows how the
rates of the forward and backward reactions
change when hydrogen and iodine are mixed?
LFZ
GPSXBSESFBDUJPO
CBDLXBSESFBDUJPO
A
B
3BUF
A
B
C
D
Directions: Questions 28 and 29 refer to the following
information.
3BUF
25 An equilibrium mixture at constant temperature
and pressure,
D
5JNF
NO2(g) + CO(g)
In an experiment, 4.00 moles of NO(g) and 0.900
mole of CO2(g) are placed in a 2.00 dm3 reaction
vessel.
At equilibrium, 0.100 mole of CO2(g) is present.
What is the equilibrium constant, Kc, for the
reaction?
A
B
C
D
4
0.500
1.60
2.00
5.00
D
3BUF
NO(g) + CO2(g)
C
3BUF
27 NO(g) and CO2(g) react according to the following
equation:
5JNF
5JNF
A
30 X2(g) and Y2(g) react according to the following
equation:
X2(g) + Y2(g)
C
5JNF
2XY(g)
A mixture containing 4.00 moles each of X2(g)
and Y2(g) is heated in a closed container. The
system is allowed to reach equilibrium. The graph
shows how the number of moles of each gas
varies with time.
/VNCFSPGNPMFT
33 What is the value of Kc for the reaction?
9:H
A
B
C
D
34 PCl5(g) decomposes to form PCl3(g) and Cl2(g)
according to the equation:
5JNF
D
31 Consider the following reaction:
3+
–
Fe (aq) + SCN (aq)
50.0 cm3 of 0.100 mol dm–3 Fe3+(aq) are added
to 30.0 cm3 of 0.200 mol dm–3 SCN–(aq). At
equilibrium, the concentration of [Fe(SCN)]2+(aq)
is found to be 0.0500 mol dm–3.
What is the equilibrium constant, Kc, for the
reaction?
A
B
C
D
[PCl5(g)]
[PCl3(g)]
[Cl2(g)]
0.10
0.15
0.20
0.30
0.40
0.20
0.30
0.60
0.10
0.30
0.15
0.20
C
35 For which system does the equilibrium constant,
Kc, have units of dm3 mol–1?
D
Directions: Questions 32 and 33 refer to the following
information.
Equal volumes of two 1.00 mol dm–3 solutions of
W and X are mixed. The reaction rapidly reaches
equilibrium.
W(aq) + X(aq)
Four different flasks, A, B, C and D, at the same
temperature, contain a mixture of PCl5(g), PCl3(g)
and Cl2(g). The concentration, in mol dm–3, of
these components in each of the flasks is shown
below. In three of the four flasks, the mixture of
gases is at equilibrium.
In which one is the mixture of gases NOT at
equilibrium?
2+
[Fe(SCN)] (aq)
6.25 x 10–3 dm3 mol–1
0.400 dm3 mol–1
2.50 dm3 mol–1
160 dm3 mol–1
PCl3(g) + Cl2(g)
Unit 39
0.0278
0.167
6.00
36.0
PCl5(g)
Part A
What is the equilibrium constant, Kc, for the
reaction?
A
B
C
D
B
9H
BOE:H
A
B
C
D
9.00
2.25
0.360
0.184
Y(aq) + Z(aq)
The concentration of Z(aq) is found to be 0.300
mol dm–3.
A
B
C
D
H2(g) + I2(g)
2HI(g)
2NO2(g)
N2O4(g)
2SO3(g)
2SO2(g) + O2(g)
CH3COOC2H5(l) + H2O(l)
CH3COOH(l) + C2H5OH(l)
B
36 For which system does the equilibrium constant,
Kc, have NO units?
A
B
C
D
C(s) + H2O(g)
CO(g) + H2(g)
SO3(g) + NO(g)
SO2(g) + NO2(g)
2+
2+
Cu (aq) + 4NH3(aq)
[Cu(NH3)4] (aq)
N2O4(g)
2NO2(g)
B
32 What is the equilibrium concentration of
W(aq)?
A
B
C
D
0.100
0.200
0.500
0.700
mol
mol
mol
mol
dm–3
dm–3
dm–3
dm–3
B
5
37 The equilibrium constant, Kc, for the reaction
N2O4(g)
2NO2(g)
–3
–3
is 6.10 x 10 mol dm at 25 °C. What is the
Kc for the following reaction?
3
A 327 dm 2 mol
3
B 164 dm 2 mol
Part A
C 12.8 dm
3
2
1
2
1
–
2
1
–
2
3
2
–
2NO(g) + O2(g)
mol
–3
D 3.05 x 10
dm
mol
–
1
2
C
Unit 39
38 Consider the following hypothetical equilibrium
systems:
A(g)
B(g)
B(g)
C(g)
Kc = 2.00
Kc = 0.0100
What is the value of K c for the following
reaction?
2C(g)
A
B
C
D
2A(g)
2 500
200
50.0
4.00 x 10–4
A
COCl2(g) Kc = 22
4HCl(g) + O2(g)
Kc = 5.0 x 10–4
C 2HBr(g)
H2(g) + Br2(g)
Kc = 7.0 x 10–20
D N2(g) + O2(g)
2NO(g)
Kc = 1.0 x 10–31
A CO(g) + Cl2(g)
B 2H2O(g) + 2Cl2(g)
A
40 An equal number of moles of methane and steam
are placed in a closed container at 800 °C. The
following reaction occurs.
CO(g) + 3H2(g)
Kc = 1.78 x 10–3 at 800 °C
Which of the following is correct at equilibrium
at this temperature?
6
B
2NO2(g)
At equilibrium, the concentration of NO(g) is 0.600
mol dm–3 and that of O2(g) is 0.300 mol dm–3.
What is the equilibrium concentration of
NO2(g)?
A
B
C
D
19.0 mol dm–3
3.44 mol dm–3
2.65 mol dm–3
0.526 mol dm–3
C
42 1.50 moles of CS2(g) and 3.00 moles of Cl2(g)
are mixed and the following equilibrium is
established:
CS2(g) + 3Cl2(g)
39 Starting with equal concentrations of reactants,
which of the following will be closest to
completion at equilibrium?
CH4(g) + H2O(g)
[CO(g)] > [CH4(g)]
[CH4(g)] > [H2(g)]
[CH4(g)] = [CO(g)]
[CO(g)] = 3[H2(g)]
41 At a particular temperature, the equilibrium
constant, Kc, for the reaction below is 65.0
dm3 mol–1.
1
N2O4(g)
2
NO2(g)
A
B
C
D
S2Cl2(g) + CCl4(g)
At equilibrium, 0.300 mole of CCl4(g) is found.
How much Cl2(g) is present?
A
B
C
D
0.900 mole
1.80 moles
2.10 moles
2.70 moles
C
43 At a certain temperature, the equilibrium constant,
Kc, for the reaction
2NH3(g) + Cl2(g)
N2H4(g) + 2HCl(g)
is 4.00.
An equilibrium mixture in a 1.00 dm3 container
has 2.60 moles of NH3(g), 4.00 moles of Cl2(g)
and 5.90 moles of N2H4(g).
What is the equilibrium concentration of
HCl(g)?
A
B
C
D
4.28
5.34
10.7
12.0
mol
mol
mol
mol
dm–3
dm–3
dm–3
dm–3
A
44 When 1.00 mole of phosphorus pentachloride
was heated to 523 K in a closed vessel, 50.0%
dissociated as shown.
PCl5(g)
PCl3(g) + Cl2(g)
How many moles of gas were present in the
equilibrium mixture?
0.50
1.00
1.50
2.00
C
2SO2(g) + O2(g)
2SO3(g)
In an experiment, 0.10 mole of O2(g) and 0.10
mole of SO3(g) are added to an empty 1.0 dm3
flask and then the flask is sealed. The reaction
goes backward to establish equilibrium. Which
of the following must be true at equilibrium?
A
B
C
D
[SO2(g)] = [O2(g)] = [SO3(g)]
[O2(g)] < [SO3(g)]
[O2(g)] = [SO2(g)]
[SO3(g)] < [O2(g)]
Part A
A
B
C
D
48 Consider the following reaction:
D
45 Consider the following reaction:
49 Propanone can be made from propan-2-ol.
2NO(g) + Cl2(g)
Kc = 0.0100
Initially, some NOCl(g) was placed in a 1.00 dm3
container. At equilibrium, there were 0.860 mole
of NOCl(g), 0.0300 mole of NO(g) and 0.0150
mole of Cl2(g). How many moles of NOCl(g) were
initially added to the container?
C3H8O(g)
A
B
C
D
Which of the following change(s) will occur as
the system proceeds towards equilibrium?
0.785
0.815
0.890
0.905
mole
mole
mole
mole
C
46 Consider the following equilibrium system:
2O3(g)
3O2(g)
C3H6O(g) + H2(g)
In an experiment, 6.00 moles of C3H8O(g), 0.150
mole of C3H6O(g) and 0.100 mole of H2(g) are
placed in a 1.00 dm3 container and allowed to
establish equilibrium.
A
B
C
D
[C3H6O(g)]
[C3H6O(g)]
[C3H8O(g)]
[C3H8O(g)]
increases while [H2(g)] decreases.
and [H2(g)] both increase.
and [H2(g)] both increase.
and [C3H6O(g)] both decrease. B
Kc = 1
Which of the following correctly compares the
equilibrium concentrations of the two species?
50 Consider the following reaction:
3NO2(g)
N2O5(g) + NO(g)
Kc = 1.0 x 10–11 dm3 mol–1
A [O3(g)] = [O2(g)]
3
B [O3(g)] = [O2(g)] 2
2
C [O3(g)] = [O2(g)] 3
3
D [O3(g)] 2 = [O2(g)]
B
47 Consider the following equilibrium system:
N2O4(g)
2NO2(g)
Initially, some NO2(g), N2O5(g) and NO(g) are placed
in a container and allowed to reach equilibrium.
When equilibrium is established, it is found that
the pressure has increased. Which of the following
combinations is correct as the system proceeds
towards equilibrium?
Reaction
quotient Qc
Kc = 4.50
Initially, 0.500 mole of N2O4(g) and 0.500 mole
of NO2(g) are placed in a 1.00 dm3 container.
Which of the following describes the changes in
concentrations as the system proceeds towards
equilibrium?
A
B
C
D
Unit 39
2NOCl(g)
[N2O4(g)]
[NO2(g)]
Decreases
Decreases
Increases
Increases
decreases
increases
decreases
increases
B
A
B
C
D
Qc
Qc
Qc
Qc
>
<
>
<
Kc
Kc
Kc
Kc
Direction
of net reaction
net
net
net
net
forward reaction
forward reaction
backward reaction
backward reaction
C
7
Directions: Questions 51 and 52 refer to the following
information.
The equilibrium constant for the following reaction is
Kc = 2.50 x 10–4 at 25 °C.
N2(g) + C2H2(g)
2HCN(g)
In an experiment, a mixture of N2(g), C2H2(g) and
HCN(g), all of initial concentrations 1.00 mol dm–3,
are allowed to reach equilibrium.
Part A
51 Which of the following statements is correct as
the system proceeds towards equilibrium?
Unit 39
A A net
Kc.
B A net
< Kc.
C A net
Kc.
D A net
> Kc.
forward reaction occurs because Qc <
backward reaction occurs because Qc
forward reaction occurs because Qc >
backward reaction occurs because Qc
D
52 What is the concentration of HCN(g) at
equilibrium?
A
B
C
D
0.0158 mol dm–3
–3
0.0235 mol dm
–3
1.28 mol dm
–3
1.53 mol dm
(1) Constant temperature
(2) Equal concentrations of reactants and
products
(3) Equal rates of forward and backward
reactions
A system at equilibrium must have
8
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
2ICl(g)
B
I2(g) + Cl2(g)
Initially, some ICl(g) is placed in an empty flask.
Which of the following statements describe(s)
the change(s) occurring as the system proceeds
towards equilibrium?
(1) The rate of the backward reaction
increases.
(2) Concentration of ICl(g) increases.
(3) Concentration of Cl2(g) increases.
A
B
C
D
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
C
55 Consider the following reaction:
2SO2(g) + O2(g)
2SO3(g)
Kc = 4.00
In an experiment, 0.600 mole of SO2(g), 0.300
mole of O2(g) and 0.600 mole of SO3(g) are placed
in a 1.00 dm3 container. The system is allowed
to reach equilibrium. Which of the following are
correct as the system approaches equilibrium?
(1) Concentration of SO2(g) increases.
(2) Concentration of O2(g) decreases.
(3) Concentration of SO3(g) increases.
B
53 Consider the following:
A
B
C
D
54 Consider the following reaction:
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
C
Directions :
A
B
C
D
Each question (Questions 56 – 60) consists of two separate statements. Decide whether each
of the two statements is true or false; if both are true, then decide whether or not the second
statement is a correct explanation of the first statement. Then select one option from A to D
according to the following table :
Both statements are true and the 2nd statement is a correct explanation of the 1st statement.
Both statements are true but the 2nd statement is NOT a correct explanation of the 1st statement.
The 1st statement is false but the 2nd statement is true.
Both statements are false.
Part A
1st statement
2nd statement
At equilibrium, the rate of the forward
reaction and the rate of the backward
reaction are equal.
57 When a small amount of dilute sulphuric acid
is added to an equilibrium system containing
CrO42–(aq) ions and Cr2O72–(aq) ions, the colour
of the system changes from yellow to orange.
When a small amount of dilute sulphuric acid
A
is added to the equilibrium system, the
concentration of CrO42–(aq) ions decreases while
that of Cr2O72–(aq) ions increases.
58 When H2(g) and I2(g) are allowed to react in a
closed container, the rate of the reaction between
H2(g) and I2(g) becomes zero when equilibrium
is attained.
At equilibrium, both the forward and
backward reactions stop.
D
59 A reaction with a large Kc can attain equilibrium
rapidly.
The value of Kc can be used to predict the
rate at which equilibrium is reached.
D
60 If Qc is greater than Kc, the system will undergo
a net forward reaction until equilibrium is
reached.
If Qc is greater than Kc, the system is not
at equilibrium.
C
Unit 39
56 All equilibrium systems have equal concentrations
of reactants and products.
C
9
Unit 40 Factors affecting
chemical equilibrium
systems
6
When the temperature of an equilibrium
system with an endothermic forward reaction is
increased,
a) a net
Fill in the blanks
1
b) the position of equilibrium will shift to
the
When the concentration of a reactant in an
equilibrium system is increased,
a) a net
reaction will occur;
forward
;
c) the value of equilibrium constant, K c ,
will
reaction will occur;
forward
right
increase
.
b) the position of equilibrium will shift to the
Part A
2
True or False
.
right
Unit 40
equilibrium system is increased,
Decide whether each of the following statements is
true or false.
a) a net
7
If the position of equilibrium lies close
to the product side, the value of Kc is
small.
F
8
When the concentration of a reactant in
an equilibrium system is increased, a net
forward reaction will occur.
T
9
At a constant temperature, increasing
F
the concentration of a reactant in an
equilibrium system will cause Kc to increase.
When the concentration of a product in an
reaction will occur;
backward
b) the position of equilibrium will shift to the
.
left
3
An increase in pressure (i.e. a / an
decrease
in volume) will bring about a net reaction
that
the number of moles of gas,
decreases
i.e. the position of equilibrium will shift to the
side of the equation with a
fewer
number
A decrease in pressure (i.e. a / an
increase
of moles of gas.
4
in volume) will bring about a net reaction
that
the number of moles of gas,
increases
10 Removing some CaCO3(s) from an
equilibrium system of CaCO3(s), CaO(s)
and CO2(g) will cause the position of
equilibrium to shift to the CaCO3(s) side.
F
11 An increase in volume of an equilibrium
system involving gases will bring about
a net reaction that increases the number
of moles of gas.
T
12 For an equilibrium system with an
exothermic forward reaction, decreasing
the temperature will shift the position of
equilibrium to the right.
T
13 Decreasing the temperature will cause
Kc for an endothermic reaction to
increase.
F
i.e. the position of equilibrium will shift to the
side of the equation with a
greater
number
of moles of gas.
5
When the temperature of an equilibrium
system with an exothermic forward reaction is
increased,
a) a net
reaction will occur;
backward
b) the position of equilibrium will shift to
the
left
;
c) the value of equilibrium constant, K c ,
will
10
decrease
.
14 In the conversion of sulphur dioxide to
sulphur trioxide in the Contact process,
a pressure of 200 atmospheres is usually
used.
F
15 The optimum conditions for the Haber
process are 10 atm and 500 °C.
F
16 A catalyst will not affect the percentage
of the product in an equilibrium mixture.
T
20 Consider the following equilibrium system:
N2(g) + 3H2(g)
Some NH3(g) is added to the system. Which of
the following combinations is correct?
A
B
C
D
Decreases
Decreases
Increases
Increases
17 Consider the following equilibrium system:
2NO(g) + 2H2O(g)
A
18 Consider the following equilibrium system:
CO(g) + H2O(g)
CO2(g) + H2(g)
Adding CO(g)
Adding H2O(g)
Removing CO2(g)
Removing H2O(g)
D
19 Consider the following equilibrium system:
2KClO3(s)
2KCl(s) + 3O2(g)
Which of the following actions will cause the
position of equilibrium to shift to the left?
A
B
C
D
Adding O2(g)
Adding KCl(s)
Removing KClO3(s)
Removing KCl(s)
Adding which of the following substances will
cause the equilibrium concentration of Ca2+(aq)
ions to increase?
A
B
C
D
H2O(l)
HCl(aq)
KOH(s)
Ca(OH)2(s)
B
22 Consider the following system at equilibrium:
H2O(g) + CO(g)
Which of the following actions would cause the
concentration of H2(g) to decrease?
A
B
C
D
Ca2+(aq) + 2OH–(aq)
Unit 40
Which of the following actions can cause the
position of equilibrium to shift to the right?
Adding N2H4(g)
Adding H2O(g)
Removing N2H4(g)
Removing O2(g)
increases
remains constant
increases
remains constant D
Part A
Ca(OH)2(s)
A
B
C
D
Value of Kc
Net change of [H2(g)]
21 Consider the following equilibrium system:
Multiple choice questions
N2H4(g) + 2O2(g)
2NH3(g)
CO2(g) + H2(g)
The position of equilibrium shifts to the right as
the result of the addition of some extra H2O(g).
How will this shift affect the concentrations of
the other gases?
A
B
C
D
[CO(g)]
[CO2(g)]
[H2(g)]
Increases
Increases
Decreases
Decreases
decreases
increases
decreases
increases
decreases
decreases
increases
increases
D
23 Consider the following equilibrium system:
2CrO42–(aq) + 2H+(aq)
Cr2O72–(aq) + H2O(l)
A solution of Ba(NO3)2 is added, and a BaCrO4
precipitate forms. Which of the following
combinations is correct?
A
Shift of
position of equilibrium
A
B
C
D
To the left
To the left
To the right
To the right
Value of Kc
remains constant
increases
remains constant
decreases
A
11
24 Consider the following equilibrium system:
H2(g) + I2(g)
27 The following equilibrium exists in aqueous
bromine.
2HI(g)
Which of the following graphs represents what
happens when some HI(g) is removed and a new
state of equilibrium is established?
B
<)*H
>
<)*H
>
A
5JNF
Part A
<)*H
>
D
Unit 40
5JNF
5JNF
C2H4(g) + H2(g)
Some H2(g) is injected into the system. Which of
the combinations is correct?
Shift of position
of equilibrium
Change of [H2(g)] relative
to previous equilibrium
To the left
To the left
To the right
To the right
increases
decreases
increases
decreases
A
12
CO(g)
CO2(g)
H2(g)
H2O(g)
A
B
C
D
Rate of
backward reaction
Increases
Increases
Decreases
Decreases
increases
decreases
increases
decreases
D
29 Consider the following equilibrium system:
2NO2(g)
An equilibrium mixture of NO(g), O2(g) and NO2(g)
is transferred from a 1 dm3 container to a 2 dm3
container.
Which of the following combinations describes
what happens as the system proceeds towards
a new state of equilibrium?
CO(g) + H2O(g)
Which of the following, when added to the system
above, would result in a decrease in [H2O(g)]
relative to the previous equilibrium?
A
B
C
D
2HI(g)
Rate of
forward reaction
2NO(g) + O2(g)
26 Consider the following equilibrium system:
CO2(g) + H2(g)
C
How will the rates of the forward and backward
reactions change when the volume of the reaction
vessel is increased?
25 Consider the following equilibrium system:
A
B
C
D
HCl.
KBr.
AgNO3.
NaOBr.
H2(g) + I2(g)
B
C2H6(g)
–
28 Consider the following equilibrium system:
<)*H
>
C
+
Br (aq) + 2H (aq) + OBr (aq)
colourless
colourless
The yellow-brown colour of aqueous bromine
would fade on adding a few drops of a
concentrated solution of
A
B
C
D
5JNF
–
Br2(aq) + H2O(l)
yellow-brown
A
A
B
C
D
Shift of position
of equilibrium
Number of
moles of NO2(g)
To the right
To the right
To the left
To the left
increases
decreases
increases
decreases
D
Directions: Questions 30 and 31 refer to the following
information.
At room temperature, N2O4(g) and NO2(g) exist in
equilibrium as follows:
N2O4(g)
pale yellow
2NO2(g)
dark brown
A gas syringe is filled with a pale brown mixture of
N2O4(g) and NO2(g) at equilibrium.
HBTTZSJOHF
SVCCFSDBQ
QMVOHFS
first darkens, and then lightens.
first lightens, and then darkens.
31 Which of the following graphs represents how
the concentration of NO2(g) in the mixture varies
until a new state of equilibrium is established?
B
U
5JNF
</0H
>
</0H
>
D
5JNF
Shift of position
of equilibrium
Number of
moles of PCl3(g)
To the right
To the right
To the left
To the left
increases
decreases
increases
decreases
A
FeO(s) + H2(g)
Fe(s) + H2O(g)
Which of the following combinations describes the
effect of decreasing the volume of the system?
A
B
C
D
Shift of position
of equilibrium
Concentration of H2(g)
To the right
To the right
No change
No change
increases
decreases
increases
decreases
C
34 Consider the following equilibrium system:
H2(g) + I2(g)
colourless purple
2HI(g)
colourless
ΔH < 0
Which of the following combinations describes the
effect of increasing the pressure of the system?
</0H
>
</0H
>
5JNF
C
U
A
B
C
D
first darkens, and then becomes
C
U
Which of the following combinations describes
the effect of decreasing the pressure by increasing
the volume?
Unit 40
first lightens, and then becomes
A
PCl3(g) + Cl2(g)
33 Consider the following equilibrium system:
30 What would be observed when the plunger is
quickly pushed in at time t?
A The mixture
colourless.
B The mixture
colourless.
C The mixture
D The mixture
PCl5(g)
Part A
/0H
/0H
32 Consider the following equilibrium system:
U
A
B
C
D
Colour intensity
of the mixture
Value of Kc
Decreases
Decreases
Increases
Increases
decreases
unchanged
unchanged
decreases
C
5JNF
C
13
35 In which of the following systems will the position
of equilibrium shift to the right when the pressure
is increased at constant temperature?
CaCO3(s)
CaO(s) + CO2(g)
2NO2(g)
2NO(g) + O2(g)
H2(g) + I2(g)
2HI(g)
PCl3(g) + Cl2(g)
PCl5(g)
CH4(g) + H2O(g)
D
36 In which of the following would the position of
the equilibrium NOT be affected by a volume
change at constant temperature?
Part A
A
B
C
D
2NF2(g)
N2F4(g)
C2H6(g)
C2H4(g) + H2(g)
2NOCl(g)
2NO(g) + Cl2(g)
CO(g) + H2O(g)
H2(g) + CO2(g)
Unit 40
a catalyst is used.
the temperature changes.
the concentrations of products change.
the volume changes.
B
38 Consider the following equilibrium system:
N2(g) + 3H2(g)
2NH3(g)
ΔH < 0
Which of the following sets of conditions will
favour the formation of the product?
A
B
C
D
Low pressure and low temperature
Low pressure and high temperature
High pressure and high temperature
High pressure and low temperature
2SO3(g)
14
Equilibrium yield of H2(g)
Reaction rate
Decreases
Decreases
Increases
Increases
decreases
increases
decreases
increases
A
B
C
D
B
41 Which of the following combinations describes
the effect of increasing the temperature of the
system?
Equilibrium yield of H2(g)
Reaction rate
Decreases
Decreases
Increases
Increases
decreases
increases
decreases
increases
A
B
C
D
D
42 Consider the following graph which relates to
this equilibrium system:
Fe3+(aq) + SCN–(aq)
[Fe(SCN)]2+(aq) ΔH < 0
<'F4$/
>
D
4$/m
'F
ΔH = –197 kJ
Which of the following combinations describes
the effect of increasing the temperature of the
system?
A
B
C
D
ΔH > 0
40 Which of the following combinations describes the
effect of increasing the pressure of the system?
39 Consider the following equilibrium system:
2SO2(g) + O2(g)
CO(g) + 3H2(g)
D
37 The value of equilibrium constant for a gaseous
reaction will change when
A
B
C
D
The hydrogen used in the Haber process is made by
the following reaction:
$PODFOUSBUJPO
A
B
C
D
Directions: Questions 40 and 41 refer to the following
information.
Shift of
position of equilibrium
Value of Kc
To the left
To the left
To the right
To the right
increases
decreases
increases
decreases
U
Which of the following actions would cause the
concentration changes at time t?
A
B
C
D
B
5JNF
Addition of Fe3+(aq) ions
Removal of [Fe(SCN)]2+(aq) ions
Increase in temperature
Increase in pressure
C
43 Consider the following equilibrium system:
–
46 Consider the following reaction:
2–
Co (aq) + 4Cl (aq)
pink
CoCl4 (aq)
blue
2NO2(g)
When the temperature is increased, the solution
turns dark blue. It can be deduced that the
forward reaction is
value of K c has
value of K c has
value of K c has
value of K c has
C
CO(g) + 3H2(g) ΔH > 0
What is the effect of a small increase in
temperature on the rate of the forward reaction
(Rf), rate of the backward reaction (Rb), and the
equilibrium constant (Kc)?
A
B
C
D
Rf
Rb
Kc
Lower
Higher
Higher
Higher
higher
lower
higher
higher
lower
unchanged
higher
lower
C
4HCl(g) + O2(g) ΔH > 0
The temperature of the system is lowered and
the amount of Cl2(g) changes by 1 mole. Which
of the following combinations is correct?
Amount
of Cl2(g)
Amount
of HCl(g)
Amount
of O2(g)
A Increase by
1 mole
B Increase by
1 mole
C Decrease by
1 mole
D Decrease by
1 mole
decrease by
0.5 mole
decrease by
2 moles
increase by
0.5 mole
increase by
2 moles
decrease by
2 moles
decrease by
0.5 mole
increase by
2 moles
increase by
0.5 mole
+
+
–
–
increases
decreases
increases
decreases
A
B
C
D
A
47 Methanol can be produced by the reaction
between carbon monoxide and hydrogen.
CO(g) + 2H2(g)
CH3OH(g)
Two experiments were conducted.
Experiment 1: Some CO(g) and H2(g) were placed
in a sealed vessel and the reaction allowed to
proceed at constant temperature.
Experiment 2: Experiment 1 was repeated, but
at a different temperature.
45 Consider the following equilibrium system:
2H2O(g) + 2Cl2(g)
Sign of ΔH
Product yield as
temperature increases
Unit 40
44 Consider the following system at fixed pressure:
CH4(g) + H2O(g)
Which of the following combinations concerning
the forward reaction is correct?
Part A
A exothermic and the
increased.
B exothermic and the
decreased.
C endothermic and the
increased.
D endothermic and the
decreased.
2NO(g) + O2(g)
The value of the equilibrium constant for the
reaction is 1.45 x 10–6 at 227 °C and 0.938 at
727 °C.
The graph below shows the amount of methanol
produced over the course of Experiments 1 and
2.
"NPVOUPG$)0)H
2+
&YQFSJNFOU
&YQFSJNFOU
5JNF
These results show that Experiment 2 was
conducted at a
B
A lower temperature than Experiment 1 and the
forward reaction is endothermic.
B lower temperature than Experiment 1 and the
forward reaction is exothermic.
C higher temperature than Experiment 1 and
the forward reaction is endothermic.
D higher temperature than Experiment 1 and
the forward reaction is exothermic.
D
15
48 Consider the following graph which relates to
this equilibrium system:
2NH3(g)
ΔH < 0
A CO2(g) + H2(g)
CO(g) + H2O(g)
ΔH
B C2H6(g)
C2H4(g) + H2(g)
ΔH
C C2H4(g) + H2O(g)
C2H5OH(g) ΔH
D 2SO2(g) + O2(g)
2SO3(g)
ΔH
</)H
>
<)H
>
5JNF
A
Consider the following equilibrium system:
CH3OH(g)
ΔH = –91 kJ
Increase temperature, increase volume
Increase temperature, decrease volume
Decrease temperature, decrease volume
Decrease temperature, increase volume
C
3BUF
2NH3(g)
BUN
BUN
BUN
BUN
5JNF
U
5JNF
3BUF
U
16
5JNF
D
3BUF
C
U
5JNF
B
B
5FNQFSBUVSFž$
C
ΔH < 0
BUN
BUN
BUN
BUN
BUN
BUN
BUN
BUN
5FNQFSBUVSFž$
D
5FNQFSBUVSFž$
U
C
Which of the following graphs shows the effect
of temperature and pressure on the percentage
of NH3(g) in the equilibrium mixture?
A
B
3BUF
Low pressure and low temperature
Low pressure and high temperature
High pressure and low temperature
High pressure and high temperature
N2(g) + 3H2(g)
50 Which of the following graphs represents the rate
of the forward reaction when the temperature of
the system is increased at time t?
A
A
B
C
D
53 Consider the following equilibrium system:
49 Which of the following actions will each cause the
position of equilibrium to shift to the right?
A
B
C
D
ΔH < 0
Which of the following sets of conditions will
favour the formation of phosgene?
Directions: Questions 49 and 50 refer to the following
information.
CO(g) + 2H2(g)
COCl2(g)
PG/)H
Unit 40
N2(g)
H2(g)
temperature
volume
CO(g) + Cl2(g)
PG/)H
Part A
Which of the following actions will cause the
concentration changes at time t?
Addition of
Removal of
Decrease in
Decrease in
0
0
0
0
52 Phosgene (COCl2(g)) is manufactured by passing
carbon monoxide and chlorine through a bed of
carbon which acts as a catalyst.
PG/)H
U
A
B
C
D
>
>
<
<
B
</H
>
PG/)H
$PODFOUSBUJPO
N2(g) + 3H2(g)
51 In which of the following systems will the position
of equilibrium shift to the left upon an increase
in pressure, but to the right upon an increase in
temperature?
BUN
BUN
BUN
BUN
5FNQFSBUVSFž$
D
54 Consider the following equilibrium system:
PCl3(g) + Cl2(g)
PCl5(g)
57 Consider the following equilibrium system:
ΔH < 0
PCl3(g) + Cl2(g)
1$MH
$MH
1$MH
5JNF
1$MH
$MH
D
A
B
C
D
1$MH
$MH
1$MH
U
Ag+(aq) + Cl–(aq)
Consider the following reaction:
ΔH < 0
(1) Adding NaCl(aq)
(2) Adding AgNO3(aq)
(3) Removing Cl–(aq)
Adding a catalyst
Decreasing the temperature
Decreasing the volume of the system
Adding SO3(g) to the system
A
B
C
D
D
N2H4(g) + 2O2(g)
B
(1)
(2)
(3)
(4)
A
B
C
D
3BUF
3BUF
U
5JNF
U
5JNF
U
5JNF
2NO(g) + 2H2O(g)
N2H4(g)
O2(g)
NO(g)
H2O(g)
(1) and (3) only
(2) and (4) only
(1), (2) and (3) only
(2), (3) and (4) only
D
3BUF
D
3BUF
C
A
Some O2(g) is added to the equilibrium system
and a new state of equilibrium is established.
Which of the following substances have a net
increase in concentration, relative to the previous
equilibrium concentrations?
56 Which of the following graphs shows the rate of
the backward reaction when a catalyst is added
to the equilibrium system at time t?
A
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
59 Consider the following equilibrium system:
55 How can the number of moles of O 2(g) at
equilibrium in the system be increased?
A
B
C
D
AgCl(s)
Which of the following actions will increase the
amount of solid silver chloride of the equilibrium
system?
5JNF
Directions: Questions 55 and 56 refer to the following
information.
2SO3(g)
B
58 Consider the following equilibrium system:
5JNF
C
2SO2(g) + O2(g)
Adding more Cl2(g)
Adding a catalyst
Increasing the pressure
Decreasing the temperature
Unit 40
5JNF
Which of the following will NOT cause the position
of equilibrium to shift to the right?
1$MH
U
1$MH
U
1$MH
$MH
ΔH < 0
Part A
$PODFOUSBUJPO
U
C
B
$PODFOUSBUJPO
$PODFOUSBUJPO
A
$PODFOUSBUJPO
Which of the following graphs shows the
relationship between concentration and time as
a result of adding a catalyst at time t?
PCl5(g)
U
5JNF
B
17
60 The dissociation of a weak acid in aqueous solution
is represented by the following equation:
HA(aq)
+
63 Consider the following equilibrium system:
NH4Cl(s)
When the above system is at equilibrium, which
of the following statements are correct?
(1) Both HA(aq) and H (aq) are present in the
system.
(2) More HA(aq) dissociates upon the addition
of NaOH(aq).
(3) Decreasing the pressure shifts the position
of equilibrium to the right.
(1) Adding a small amount of NH4Cl(s)
(2) Increasing the temperature
(3) Increasing the pressure
A
B
C
D
A
Unit 40
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
N2O4(g)
A
C2H5OH(g)
ΔH < 0
Which of the following action(s) will increase the
mass of C2H5OH(g) in the equilibrium mixture?
(1) Adding a catalyst
(2) Decreasing the volume of the reaction
vessel
(3) Increasing the temperature
18
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
2NO2(g)
B
B
ΔH > 0
/0H
/0H
U
5JNF
Which of the following would cause the
concentration changes at time t?
(1) A change in temperature
(2) A change in pressure
(3) A change in the concentration of NO2(g)
62 The following reaction reaches equilibrium in a
closed reaction vessel.
A
B
C
D
only
only
and (3) only
and (3) only
64 Consider the following graph which relates to
this equilibrium system:
(1) CaCO3(s)
CaO(s) + CO2(g)
(2) CH4(g) + H2O(g)
CO(g) + 3H2(g)
(3) HCl(g) + H2O(l)
H3O+(aq) + Cl–(aq)
C2H4(g) + H2O(g)
(1)
(2)
(1)
(2)
$PODFOUSBUJPO
Part A
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
61 For which of the following equilibrium systems
will a decrease in volume at constant temperature
cause a decrease in the amounts of products?
A
B
C
D
ΔH > 0
Which of the following actions would favour the
formation of NH3(g)?
+
A
B
C
D
NH3(g) + HCl(g)
–
H (aq) + A (aq)
A
B
C
D
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
A
65 Hydrogen iodide is formed when hydrogen
and iodine react according to the following
equation.
H2(g) + I2(g)
2HI(g)
66 Which of the following statements concerning a
catalyst is / are correct?
(1) It decreases the enthalpy change of the
reaction.
(2) It increases the rate of formation of the
products.
(3) It increases the concentration of the
products.
ΔH = –10 kJ
Which of the following statements about the
equilibrium system is / are correct?
(1) The use of a catalyst would have no effect
on the yield of HI(g).
(2) Increasing the total pressure increases the
yield of HI(g).
(3) Increasing the temperature increases the
yield of HI(g).
only
only
and (3) only
and (3) only
Directions :
A
B
C
D
only
only
and (3) only
and (3) only
B
A
Unit 40
(1)
(2)
(1)
(2)
(1)
(2)
(1)
(2)
Part A
A
B
C
D
A
B
C
D
Each question (Questions 67 – 74) consists of two separate statements. Decide whether each
of the two statements is true or false; if both are true, then decide whether or not the second
statement is a correct explanation of the first statement. Then select one option from A to D
according to the following table :
Both statements are true and the 2nd statement is a correct explanation of the 1st statement.
Both statements are true but the 2nd statement is NOT a correct explanation of the 1st statement.
The 1st statement is false but the 2nd statement is true.
Both statements are false.
1st statement
2nd statement
67 At a constant temperature, increasing the
concentration of a reactant in an equilibrium
system will cause Kc to increase.
Increasing the concentration of a reactant
will cause the position of equilibrium to shift
to the product side.
C
68 Adding some H2(g) to an equilibrium system of
H2(g), I2(g) and HI(g) will cause a net decrease
in the concentration of I2(g).
When some H2(g) is added to the equilibrium
system, a net reaction will occur to consume
some of the H2(g).
A
69 When the pressure of an equilibrium system
involving gases is increased, the position of
equilibrium must shift to the right.
When the pressure of an equilibrium system
involving gases is increased, the rate of the
forward reaction will increase.
C
19
Part A
Unit 40
20
70 At a constant temperature, changing the
pressure of an equilibrium system of N2(g), H2(g)
and NH3(g) has no effect on the position of
equilibrium.
The value of Kc for a reversible reaction
depends only on temperature.
C
71 Decreasing the pressure of an equilibrium system
of N2O4(g) and NO2(g) will cause more NO2(g)
to form.
Decreasing the pressure will bring about a
net reaction that increases the number of
moles of gas.
A
72 Increasing the volume of the reaction vessel of
an equilibrium system of H2(g), I2(g) and HI(g)
will cause the rate of the reaction between
H2(g) and I2(g) to increase.
Increasing the volume of the reaction vessel
of the equilibrium system of H2(g), I2(g) and
HI(g) will cause the position of equilibrium to
shift to the right.
D
73 When the temperature of an equilibrium system
with an exothermic forward reaction is increased
by 10 °C, the equilibrium constant Kc doubles.
For an equilibrium system with an exothermic
forward reaction, increasing the temperature
will shift the position of equilibrium to the
right.
D
74 Using iron as a catalyst in the Haber process can
increase the yield of ammonia.
A catalyst can increase the rate of the reaction
between nitrogen and hydrogen.
C
Part B
Topic-based exercise
Multiple choice questions
1
Phosphorus reacts with chlorine according to the
following equation:
Directions: Questions 4 and 5 refer to the following
information.
P4(s) + 6Cl2(g)
Consider the following reaction:
4PCl3(g)
What is the equilibrium constant, Kc, for the
reaction?
CO2(g) + H2(g)
A
4[PCl3(g)]
[P4(s)]6[Cl2(g)]
1.00 mole of CO2(g) and 2.00 moles of H2(g) are
placed in a 2.00 dm3 container. At equilibrium, the
concentration of CO(g) is 0.280 mol dm–3.
B
4[PCl3(g)]
6[Cl2(g)]
4
C
[PCl3(g)]
[P4(s)][Cl2(g)]6
D
[PCl3(g)]4
[Cl2(g)]6
4
2NO2(g)
A
B
C
D
D
Nitrogen dioxide decomposes on heating according
to the following equation:
5
2NO(g) + O2(g)
A
B
C
D
0.357 mol dm–3
–3
0.655 mol dm
–3
1.53 mol dm
–3
2.80 mol dm
A
0.220
0.360
0.440
0.720
mol
mol
mol
mol
dm–3
dm–3
dm–3
dm–3
0.00810
0.495
2.02
123
Which of the following statements is INCORRECT
for a system at equilibrium?
A The system has a constant mass.
B The system acts so as to oppose
disturbances.
C The forward and backward reactions proceed
at the same rate.
D The reactant and product concentrations vary
with time.
D
B
Directions: Questions 6 and 7 refer to the following
information.
In a reversible reaction, propanoic acid and ethanol
react producing an ester and water.
propanoic acid + ethanol
3
A
What is the equilibrium constant, Kc, for the
reaction?
A
B
C
D
When 6.80 moles of NO2(g) were put into a
1.00 dm3 container and heated, the equilibrium
mixture contained 1.20 moles of O2(g).
What is the equilibrium constant, Kc, for the
reaction?
What is the equilibrium concentration of
CO2(g)?
Part B
2
CO(g) + H2O(g)
ester + water
In an experiment, 1.00 mole of propanoic acid, 1.00
mole of ethanol and 2.00 moles of water were mixed.
At equilibrium, 2.40 moles of water were found to
be present.
6
The equilibrium constant, Kc, for the reaction in
this experiment has the value
A
B
C
D
2.67.
1.56.
0.643.
0.375.
A
21
7
Which of the following graphs best represents
how the rates of the forward and backward
reactions change over time?
LFZ
GPSXBSESFBDUJPO
CBDLXBSESFBDUJPO
B
5JNF
3BUF
Part B
A
B
C
D
5JNF
C
Equal volumes of 1.00 mol dm–3 solutions of
W and X are mixed. The reaction below rapidly
reaches equilibrium.
W(aq) + X(aq)
Y(aq) + Z(aq)
At equilibrium, the concentration of Y(g) is found
to be 0.400 mol dm–3. What is the value of Kc
for the above reaction?
A
B
C
D
9
0.250
0.440
4.00
16.0
D
2NO2(g)
Equal number of moles of N2(g) and O2(g) are
placed, under certain conditions, in a closed
container. Which of the following describes the
changes which occur as the system proceeds
towards equilibrium?
22
A
3O2(g)
Kc = 36.0 mol dm–3
2.32
4.54
3.87
8.70
x
x
x
x
10–3
10–3
10–2
10–2
mol
mol
mol
mol
dm–3
dm–3
dm–3
dm–3
A
11 An equal number of moles of steam and chlorine
are placed in a closed container at 375 K. The
following reaction occurs.
2H2O(g) + 2Cl2(g)
4HCl(g) + O2(g)
Kc = 5 x 10–4 mol dm–3 at 375 K
Which of the following relates [Cl 2(g)] and
[HCl(aq)] at equilibrium?
A
B
C
D
[Cl2(g)] < 2[HCl(g)]
2[Cl2(g)] > [HCl(g)]
[Cl2(g)] = 2[HCl(g)]
2[Cl2(g)] = [HCl(g)]
B
12 Propanoic acid and ethanol react producing an
ester and water.
Consider the following reaction:
N2(g) + 2O2(g)
increases
increases
decreases
decreases
What is the concentration of O3(g) when the
equilibrium concentration of O2(g) is 5.80 x 10–2
mol dm–3?
5JNF
5JNF
8
Increases
Decreases
Increases
Decreases
A
B
C
D
2O3(g)
D
3BUF
C
Concentration
of NO2(g)
10 Consider the following equilibrium system:
3BUF
3BUF
A
Rate of
backward reaction
propanoic acid + ethanol
ester + water
At a certain temperature, the equilibrium constant,
Kc, for the reaction is 2.45.
In an experiment, 2.00 moles of propanoic acid,
2.00 moles of ethanol and 2.40 moles of water
are mixed. What is the equilibrium concentration
of the ester?
A
B
C
D
0.900 mol dm–3
1.10 mol dm–3
1.47 mol dm–3
3.30 mol dm–3
A
13 2SO2(g) + O2(g)
2SO3(g)
Kc = 4.50 dm3 mol–1
In an experiment, a mixture of 4.00 moles of
sulphur dioxide and 1.50 moles of oxygen is
allowed to reach equilibrium.
The amount of sulphur trioxide present at
equilibrium would be
A
B
C
D
5.50 moles.
4.00 moles.
3.00 moles.
less than 3 moles.
D
14 Given the following equilibrium constants:
H+(aq) + HS–(aq)
Kc = 9.50 x 10–8 mol dm–3
HS–(aq)
H+(aq) + S2–(aq)
Kc = 1.00 x 10–19 mol dm–3
What is the equilibrium constant for the following
reaction?
S2–(aq) + 2H+(aq)
A
B
C
D
9.50
9.75
9.50
1.05
x
x
x
x
H2S(aq)
10
10–14
1011
1026
forward reaction occurs because Qc <
forward reaction occurs because Qc >
backward reaction occurs because Qc
backward reaction occurs because Qc
A
17 Consider the following equilibrium system:
Fe2O3(s) + 3CO(g)
2Fe(l) + 3CO2(g)
Which of the following actions will cause the
position of equilibrium to shift to the right?
Adding Fe2O3(s)
Adding Fe(l)
Removing CO(g)
Removing CO2(g)
D
Directions: Questions 18 and 19 refer to the following
information.
2NH3(g)
Kc = 64.0 dm6 mol–2
A 1.00 dm3 container is filled with 0.280 mole
of N2(g), 0.600 mole of H2(g) and 0.540 mole
of NH 3(g). The system is allowed to reach
equilibrium.
Which of the following combinations is correct
as the system proceeds towards equilibrium?
A
B
C
D
A A net
K c.
B A net
K c.
C A net
< Kc.
D A net
> Kc.
D
15 Consider the following reaction:
N2(g) + 3H2(g)
A flask is filled with NOCl(g), NO(g) and Cl2(g).
Initially there is a total of 5.00 moles of gases
present. When equilibrium is reached, there is a
total of 6.00 moles of gases present. Which of
the following explains this observation?
A
B
C
D
–27
2NO(g) + Cl2(g)
2NOCl(g)
Part B
H2S(aq)
16 Consider the following equilibrium:
Direction
of net reaction
Pressure of system
Forward
Forward
Backward
Backward
increases
decreases
increases
decreases
CaCO3(s)
CaO(s) + CO2(g)
Kc = 0.100 mol dm–3
18 Which of the following changes will cause the
position of equilibrium to shift to the left?
A
B
C
D
Adding more CaO(s)
Removing CaCO3(s)
Decreasing the volume
Increasing the surface area of CaO(s)
C
19 Initially, 30.0 g of CaCO3(s) were placed in a 2.00
dm3 container. What mass of CO2(g) would be
present at equilibrium?
(Relative atomic masses: C = 12.0, O = 16.0)
B
A
B
C
D
0.100 g
2.20 g
8.80 g
15.0 g
C
23
A little dilute sulphuric acid is added to the above
equilibrium system.
20 What would be observed?
A The
and
B The
and
C The
D The
mixture first becomes orange in colour,
then colourless.
mixture first becomes yellow in colour,
then colourless.
mixture becomes orange in colour.
mixture becomes yellow in colour.
C
U
U
5JNF
5JNF
U
5JNF
C
U
5JNF
U
5JNF
D
5JNF
24 Tooth enamel, Ca 5(PO 4) 3OH, establishes the
following equilibrium:
Ca5(PO4)3OH(s)
Which of the following, when added to the
above equilibrium system, will cause the position
of equilibrium to shift to the right?
U
5JNF
Directions: Questions 22 and 23 refer to the following
information.
The following equilibrium exists in aqueous bromine.
Br–(aq) + 2H+(aq) + OBr–(aq)
A little dilute sodium hydroxide solution is added to
the above equilibrium system.
A
B
C
D
H+(aq)
OH–(aq)
Ca2+(aq)
Ca5(PO4)3OH(s)
A
25 Each of the following equilibrium systems is
disturbed by increasing the pressure as a result
of decreasing the volume.
In which of the systems will the number of moles
of products increase?
A
B
C
D
24
B
5Ca2+(aq) + 3PO43–(aq) + OH–(aq)
B
Br2(aq) + H2O(l)
A
C
U
D
<$S0mBR
>
C
5JNF
first darkens, and then lightens.
first lightens, and then darkens.
gets lighter.
get darker.
C
23 Which of the following graphs represents how the
concentration of Br–(aq) ions in the mixture varies
until a new state of equilibrium is established?
U
<$S0mBR
>
B
<$S0mBR
>
A
<$S0mBR
>
Part B
21 Which of the following graphs represents how
the concentration of CrO 42–(aq) ions in the
mixture varies until a new state of equilibrium is
established?
mixture
mixture
mixture
mixture
<#SmBR
>
Cr2O72–(aq) + H2O(l)
The
The
The
The
<#SmBR
>
2CrO42–(aq) + 2H+(aq)
A
B
C
D
<#SmBR
>
Consider the following equilibrium system:
22 What would be observed?
<#SmBR
>
Directions: Questions 20 and 21 refer to the following
information.
2CO2(g)
2CO(g) + O2(g)
N2F4(g)
2NF2(g)
Si(s) + 2Cl2(g)
SiCl4(g)
N2(g) + C2H2(g)
2HCN(g)
C
Directions: Questions 26 and 27 refer to the following
information.
28 The system below reaches equilibrium in a closed
reaction vessel.
The diagram below shows a gas syringe containing
a pale brown mixture of N2O4(g) and NO2(g) at
equilibrium at room temperature:
4HCl(aq) + MnO2(s)
Cl2(g) + 2H2O(l) + Mn2+(aq) + 2Cl–(aq)
ΔH < 0
N2O4(g)
pale yellow
Which of the following actions will increase the
mass of Cl2(g) in the equilibrium mixture?
2NO2(g)
dark brown
A
B
C
D
HBTTZSJOHF
SVCCFSDBQ
/0H
/0H
QMVOHFS
Some N2O4(g) is added to the gas syringe at time t
while the volume and the temperature of the mixture
are both kept constant.
CH3COOH(aq) + H2O(l)
CH3COO–(aq) + H3O+(aq)
ΔH < 0
first darkens, and then lightens.
first lightens, and then becomes
<)0BR
>
A The mixture
B The mixture
colourless.
C The mixture
D The mixture
29 Consider the following graph which relates to
this equilibrium system:
Part B
26 What would be observed?
Adding some MnO2(s)
Increasing the temperature
Decreasing the volume of the reaction vessel
Adding something that precipitates the
Mn2+(aq) ions
D
gradually gets darker.
gradually becomes colourless.
C
27 Which of the following graphs represents how
the concentration of NO2(g) in the mixture varies
until a new state of equilibrium is established?
B
</0H
>
U
5JNF
5JNF
U
5JNF
Addition of HCl(aq)
Decreasing the temperature
Addition of CH3COO–(aq) ions
Increasing the volume of the container
C
</0H
>
D
</0H
>
C
U
5JNF
Which of the following actions caused the change
in the concentration of H3O+(aq) at time t?
A
B
C
D
</0H
>
A
U
U
5JNF
C
25
Directions: Questions 30 and 31 refer to the following
information.
The hydrogen used in the Haber process is made by
the following reaction:
CH4(g) + H2O(g)
CO(g) + 3H2(g)
ΔHO = +206 kJ
30 Which of the following sets of conditions will
favour the formation of hydrogen?
A
B
C
D
Low pressure and low temperature
Low pressure and high temperature
High pressure and low temperature
High pressure and high temperature
B
Part B
31 Equal amounts of CH4(g) and H2O(g) are placed in
a reaction vessel and allowed to react. After 10
minutes, equilibrium has been reached. At that
time, some H2(g) is added to the mixture and a
new state of equilibrium is established.
32 Consider the following equilibrium system in a
closed reaction vessel:
CH3CHO(g)
A The amounts of all substances increase, relative
to their previous equilibrium amounts.
B The value of Kc increases.
C The amounts of CH3CHO(g) and CO(g) both
increase, relative to their previous equilibrium
amounts.
D The amounts of CH3CHO(g) and CH4(g) both
increase, relative to their previous equilibrium
amounts.
D
33 The graph below shows the effect of temperature
and pressure on the equilibrium yield of the
product in a gaseous equilibrium system.
BUN
<)H
>
5JNF
NJO
<)H
>
$PODFOUSBUJPO
NPMENm
C
<$)H
>
5JNF
NJO
BUN
BUN
<$)H
>
5FNQFSBUVSF
D
5JNF
NJO
<$)H
>
<)H
>
5JNF
NJO
B
26
:JFMEPGQSPEVDU
$PODFOUSBUJPO
m
NPMEN <$)H
>
<)H
>
$PODFOUSBUJPO
m
NPMEN $PODFOUSBUJPO
NPMENm
B
ΔH > 0
What would happen if some CH4(g) is added to
the system?
Which of the following graphs represents the
changes in the concentrations of CH4(g) and H2(g)
in the reaction mixture?
A
CH4(g) + CO(g)
Which of the following reactions would have the
relationship between the yield, temperature and
pressure shown in the graph?
A
B
C
D
H2(g) + I2(g)
2HI(g)
PCl3(g) + Cl2(g)
PCl5(g)
N2O4(g)
2NO2(g)
CH3CH(OH)CH3(g)
CH3COCH3(g) + H2(g)
ΔH > 0
ΔH < 0
ΔH > 0
ΔH < 0
C
Directions: Questions 34 and 35 refer to the following
information.
Directions: Questions 37 and 38 refer to the following
information.
Methanol is manufactured by the reaction of carbon
monoxide with hydrogen in the presence of a ZnO /
Cr2O3 catalyst:
The equilibrium constant, K c , for the thermal
decomposition of calcium carbonate is 2.70 x 10–3
mol dm–3 at 1 000 K.
CO(g) + 2H2(g)
ZnO / Cr2O3
CaCO3(s)
CH3OH(g)
34 Which of the following combinations describes the
effect of increasing the pressure of the system?
A
B
C
D
Equilibrium yield
Reaction rate
Decreases
Decreases
Increases
Increases
increases
decreases
increases
decreases
Decreases
No change
No change
Decreases
decreases
decreases
no change
no change
C
(Relative atomic masses: C = 12.0, O = 16.0, Ca
= 40.1)
A
B
C
D
1.80%
4.05%
9.00%
13.9%
C
38 Which of the following actions will cause the
percentage of decomposition of CaCO3(s) of the
equilibrium system to increase?
(1) Adding some CaCO3(s)
(2) Increasing the temperature
(3) Removing some CaO(s)
B
36 Transport of oxygen in the body involves
the complex molecules haemoglobin and
oxyhaemoglobin.
haemoglobin + oxygen
15.0 g of CaCO3(s) are introduced into a 5.00 dm3
evacuated vessel, and the system is allowed to attain
equilibrium at 1 000 K.
Part B
A
B
C
D
Reaction rate
ΔH > 0
37 What is the percentage of decomposition of
CaCO3(s) in the equilibrium system?
35 Which of the following combinations describes
the effect of removing the catalyst from the
system?
Equilibrium yield
CaO(s) + CO2(g)
oxyhaemoglobin
If carbon monoxide (CO) is present in the air,
poisoning can occur because
A the equilibrium constant for the reaction is
reduced.
B CO reacts with oxygen to form CO2, driving
the equilibrium to the left.
C the position of equilibrium shifts to the left
because haemoglobin bonds strongly with
CO.
D CO catalyzes the decomposition of
oxyhaemoglobin into haemoglobin and
oxygen.
C
A
B
C
D
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
B
39 Consider the following reaction:
2NOCl(g)
2NO(g) + Cl2(g)
Initially, some NOCl(g) is placed in an empty flask.
Which of the following statements describe the
changes which occur as the system proceeds
towards equilibrium?
(1) The rate of the backward reaction
increases.
(2) The concentration of NOCl(g) increases.
(3) The concentration of NO(g) increases.
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
B
27
40 Consider the following equilibrium system:
NO2Cl(g) + NO(g)
43 Consider the following equilibrium system:
NOCl(g) + NO2(g)
PCl3(g) + Cl2(g)
Adding which of the following substances will
cause the equilibrium concentration of NO2(g) to
increase?
Which of the following statements is / are correct
when the volume of the system is decreased?
(1) The position of equilibrium shifts to the
right.
(2) The rate of the backward reaction is greater
than that of the forward reaction.
(3) The value of Kc decreases.
(1) NOCl(g)
(2) NO(g)
(3) NO2Cl(g)
A
B
C
D
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
D
PCl5(g)
A
B
C
D
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
A
41 Consider the following equilibrium system:
2SO2(g) + O2(g)
Part B
2SO3(g)
Kc = 1.20 x 104 dm3 mol–1
44 For which of the following equilibrium systems
will the amounts of reactants increase with an
increase in the container volume?
Some SO2(g) is added to the equilibrium system.
Which of the following statements are correct?
(1) The position of equilibrium shifts to the
right.
(2) The rate of the forward reaction
increases.
(3) The value of Kc increases.
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
2NO2(g)
dark brown
A
B
C
D
A
28
only
only
and (3) only
and (3) only
D
3F2(g) + Cl2(g)
ΔHO = +159 kJ
Which of the following statements is / are
correct?
ΔH = +58 kJ
(1) an increase in the volume of the system.
(2) a release of heat by the system to the
surroundings.
(3) a decrease in the average relative molecular
mass of the gas molecules in the system.
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
2ClF3(g)
(1) The decomposition is a redox reaction.
(2) When an equilibrium mixture is heated,
its colour fades.
(3) When the volume of the container of
an equilibrium mixture is increased, more
Cl2(g) will form.
Shifting the position of equilibrium to the left is
accompanied by
A
B
C
D
(1)
(2)
(1)
(2)
2CO(g)
2NH3(g)
2NO2(g)
45 Chlorine trifluoride, a colourless gas, can be
decomposed into its element.
42 Consider the following equilibrium system:
N2O4(g)
pale yellow
(1) C(g) + CO2(g)
(2) N2(g) + 3H2(g)
(3) 2NO(g) + O2(g)
B
A
B
C
D
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
C
46 Consider the following equilibrium system:
2NO(g) + Br2(g)
2NOBr(g)
49 Consider the following equilibrium system:
ΔH < 0
CO(g) + H2O(g)
Which of the following actions will cause the
position of equilibrium to shift to the right?
(1) Adding some NO(g)
(2) Increasing the volume
(3) Decreasing the temperature
A
B
C
D
B
47 Consider the following equilibrium system in a
closed reaction vessel:
CH3OH(g)
Which of the following will cause a shift in the
position of equilibrium?
(1) Adding a catalyst
(2) Changing the temperature
(3) Changing the volume
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
CO(g) + 2H2(g)
ΔH < 0
A
B
C
D
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
only
only
and (3) only
and (3) only
B
(1) Enzymes are proteins.
(2) Enzymes increase the rate of biochemical
reactions.
(3) Enzymes increase the equilibrium constant
of biochemical reactions.
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
Part B
(1) Adding some CO(g)
(2) Decreasing the temperature
(3) Transferring the reaction mixture to a vessel
of larger volume
(1)
(2)
(1)
(2)
50 Which of the following statements about enzymes
are correct?
Which of the following action(s) can cause an
increase in the value of Kc?
A
B
C
D
CO2(g) + H2(g)
ΔH = –41 kJ mol–1
A
B
48 Consider the following equilibrium system:
Cu2+(aq) + 4Br–(aq)
blue
colourless
CuBr42–(aq)
green
Cooling the system changes its colour from green
to blue.
Which of the following statements is / are
correct?
(1) The forward reaction is exothermic.
(2) The value of Kc decreases when the system
is cooled.
(3) A net backward reaction occurs when the
system is cooled.
A
B
C
D
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
D
29
Directions :
A
B
C
D
Each question (Questions 51 – 60) consists of two separate statements. Decide whether each
of the two statements is true or false; if both are true, then decide whether or not the second
statement is a correct explanation of the first statement. Then select one option from A to D
according to the following table :
Both statements are true and the 2nd statement is a correct explanation of the 1st statement.
Both statements are true but the 2nd statement is NOT a correct explanation of the 1st statement.
The 1st statement is false but the 2nd statement is true.
Both statements are false.
1st statement
Part B
30
2nd statement
51 All chemical equilibrium systems have equal
concentrations of reactants and products.
At equilibrium, both the forward and backward
reactions stop.
D
52 Adding some O2(g) to an equilibrium system of
SO2(g), O2(g) and SO3(g) will cause a net increase
in the concentration of SO3(g).
When some O2(g) is added to the equilibrium
system, a net reaction will occur to consume
some of the O2(g).
A
53 Adding some CaO(s) to an equilibrium system
of CaCO3(s), CaO(s) and CO2(g) will cause the
position of equilibrium to shift to the left.
Adding some CaO(s) to the equilibrium system
will cause the value of Kc to increase.
D
54 Keeping the volume constant, adding some
NO2(g) to an equilibrium system of N2O4(g) and
NO2(g) will cause the mixture to get darker
gradually.
The new equilibrium concentration of NO2(g)
will increase relative to its previous equilibrium
concentration.
C
55 Decreasing the volume of the container of an
equilibrium system of H2(g), I2(g) and HI(g) will
cause the purple colour of the system to become
deeper.
Decreasing the volume of the container of
the equilibrium system will cause the
concentration of I2(g) to increase.
A
56 When the volume of the gas syringe containing
an equilibrium system of N2O4(g) and NO2(g) is
increased, the mixture first darkens and then
lightens.
Increasing the volume of the gas syringe
containing the equilibrium system will cause
more NO2(g) to form.
C
57 For an equilibrium system with an endothermic
forward reaction, increasing the temperature
will cause the position of equilibrium to shift
to the left.
For an equilibrium system with an endothermic
forward reaction, increasing the temperature
will cause the value Kc to increase.
C
58 A temperature of 1 000 °C is usually used in
the Haber process.
Increasing the temperature will increase the
yield of ammonia in the Haber process.
D
59 The pressure used for converting sulphur dioxide
to sulphur trioxide in the Contact process is
usually 200 atmospheres.
Increasing the pressure will increase the yield
of sulphur trioxide.
C
60 Catalysts are used in many industrial processes.
Catalysts will not affect the percentage of the
product in the equilibrium mixture.
B
Short questions
61 For each of the following equilibrium system,
(i) write an expression for the equilibrium constant, Kc;
(ii) State the units of Kc.
a) Fe2O3(s) + 3CO(g)
2Fe(s) + 3CO2(g)
[CO2(g)]3
Kc =
[CO(g)]3
(2 marks)
(1)
(1)
b) MgCO3(s)
MgO(s) + CO2(g)
(2 marks)
Kc = [CO2(g)]
(1)
–3
The units of Kc are mol dm .
(1)
c) 3O2(g)
Kc =
[O3(g)]2
[O2(g)]3
2O3(g)
(2 marks)
(1)
3
–1
The units of Kc are dm mol .
d) CO(g) + 2H2(g)
Kc =
Part B
Kc has no units.
(1)
CH3OH(g)
(2 marks)
[CH3OH(g)]
[CO(g)][H2(g)]2
(1)
6
–2
The units of Kc are dm mol .
(1)
62 Given the following reactions and their corresponding equilibrium constants at 1 292 °C:
2H2O(g)
2H2(g) + O2(g)
Kc1 = 1.60 x 10–11 mol dm–3
2CO2(g)
2CO(g) + O2(g)
Kc2 = 1.30 x 10–10 mol dm–3
Calculate the equilibrium constant, Kc3, for the following reaction
CO2(g) + H2(g)
H2O(g) + CO(g)
at the same temperature.
Kc1
Kc2
(3 marks)
[H2(g)]2[O2(g)]
=
[H2O(g)]2
[CO(g)]2[O2(g)]
=
[CO2(g)]2
31
[H2O(g)][CO(g)]
[CO2(g)][H2(g)]
1
x Kc2
=
Kc1
1
=
–11
–3
1.60 x 10 mol dm
Kc3 =
(1)
x
1.30 x 10–10 mol dm–3
(1)
= 2.85
(1)
63 When bromine is dissolved in water, the following equilibrium system is established:
Br2(l) + H2O(l)
yellow-brown
H+(aq) + Br–(aq) + HOBr(aq)
Part B
colourless
In an experiment, dilute sodium hydroxide solution and then dilute hydrochloric acid are added to the system.
The observations made are listed below.
Step
Procedure
Observation
I
Adding dilute sodium hydroxide solution
A colourless solution is formed
II
Adding dilute hydrochloric acid to the resulting solution
A yellow-brown solution is formed
Explain the observations using Le Chatelier’s principle.
(6 marks)
When dilute sodium hydroxide solution is added, the hydroxide ions react with the hydrogen ions to form water. Thus, the
concentration of hydrogen ions decreases.
(1)
The system responds by reducing this change.
(1)
A net forward reaction occurs to produce more hydrogen ions.
(1)
A colourless solution is observed as the concentration of Br2(aq) decreases.
(1)
When hydrogen ions are added, the system responds by reducing this change.
32
A net backward reaction occurs to consume some of the hydrogen ions.
(1)
A yellow-brown solution is observed as the concentration of Br2(aq) increases.
(1)
64 Each of the following equilibrium systems is disturbed by increasing the pressure as a result of decreasing
the volume of the reaction vessel. Decide whether the number of moles of reaction product(s) will increase,
decrease, or remain the same. Explain your answer in each case.
a) 2CO2(g)
2CO(g) + O2(g)
(3 marks)
An increase in pressure will bring about a net reaction that decreases the number of moles of gas. This helps to reduce
the pressure.
(1)
A net backward reaction occurs.
(1)
Thus, the number of moles of reaction products will decrease.
(1)
CO2(g) + H2(g)
(3 marks)
The number of moles of gas is the same on both sides of the equation,
(1)
so changing the pressure has no effect on the position of equilibrium.
(1)
Thus, the number of moles of reaction products will remain the same.
(1)
c) Si(s) + 2Cl2(g)
SiCl4(g)
Part B
b) CO(g) + H2O(g)
(3 marks)
An increase in pressure will bring about a net reaction that decrease the number of moles of gas. This helps to reduce
the pressure.
(1)
A net forward reaction occurs.
(1)
Thus, the number of moles of reaction product will increase.
(1)
33
65 Consider the reaction between copper(II) ions and chloride ions:
Cu2+(aq) + 4Cl–(aq)
blue
CuCl42–(aq)
yellow
The Cu2+(aq) ion is blue while the CuCl42–(aq) ion is yellow, so a mixture of these two appears green.
The table below lists the colour of two samples of equilibrium mixture of the same composition kept at
different temperatures.
Temperature
Colour of equilibrium mixture
10 °C
blue
90 °C
green
Deduce and explain whether the forward reaction is exothermic or endothermic.
When the system is cooled, it appears blue because the concentration of Cu2+(aq) ions increases.
(3 marks)
(1)
Part B
It can be deduced that when the temperature is decreased, the system will undergo a net backward reaction so as to raise
the temperature.
(1)
Thus, the backward reaction should be an exothermic reaction. / The forward reaction should be an endothermic reaction. (1)
66 Consider the following equilibrium system in a closed container:
CaCO3(s)
CaO(s) + CO2(g)
ΔH > 0
Complete the table to describe the effect of various actions on the position of equilibrium of the system.
(5 marks)
Action
Position of equilibrium
(a) The volume of the reaction vessel is increased.
shifts to the right
(1)
no effect
(1)
shifts to the left
(1)
(d) A few drops of NaOH(aq) are added.
shifts to the right
(1)
(e) The temperature is increased.
shifts to the right
(1)
(b) Some CaCO3(s) is removed.
(c) Some CO2(g) is added.
34
67 In the manufacture of nitric acid, ammonia is oxidized to nitrogen monoxide by the following reaction:
4NH3(g) + 5O2(g)
ΔHO = –906 kJ
4NO(g) + 6H2O(g)
In an experiment, a mixture of NH3(g), O2(g), NO(g) and H2O(g) is allowed to reach equilibrium in a cylinder
fitted with a movable piston. Give the effect of each of the following changes on items listed in the table
below:
a) Adding a catalyst
(2 marks)
b) Increasing the temperature
(2 marks)
c) Increasing the pressure by decreasing the volume
(2 marks)
Effect of change on
(b) Increasing the
temperature
(a) Adding a catalyst
(c) Increasing the pressure
increases
(0.5)
increases
(0.5)
increases
(0.5)
the rate of the
backward reaction
increases
(0.5)
increases
(0.5)
increases
(0.5)
the position of
equilibrium
no effect
(0.5)
the equilibrium
constant, Kc
no effect
(0.5)
shifts to the left (0.5)
decreases
(0.5)
Part B
the rate of the
forward reaction
shifts to the left (0.5)
no effect
(0.5)
68 A particular industrial process involves the following steps.
B(g)
A(g)
B(g)
Reaction 2
Reaction 1
A(g) + B(g)
ΔH = +100 kJ
2C(g) + B(g)
2C(g)
2D(g)
ΔH = –150 kJ
temperature 300 °C
pressure 10 atm
separation chamber
product D(g)
B(g) and C(g) recycling
35
a) It is possible to alter the temperature and pressure at which Reaction 2 occurs.
In the table below, indicate what effect the following changes would have on the rate, equilibrium yield
and value of the equilibrium constant, Kc, for Reaction 2.
(3 marks)
Would the rate of
Reaction 2 become
higher, lower or remain
unchanged?
Would the equilibrium
yield of Reaction 2
become higher, lower or
remain unchanged?
Would the value of Kc
of Reaction 2 become
higher, lower or remain
unchanged?
The temperature of
Reaction 2 is increased to
600 °C.
higher
(0.5)
lower
(0.5)
lower
(0.5)
The pressure of Reaction
2 is decreased to 5 atm
at constant temperature.
lower
(0.5)
lower
(0.5)
remain
unchanged
(0.5)
b) Heat is released in Reaction 2. Describe how the heat can be used within this industrial process.
(1 mark)
Part B
Provide heat for Reaction 1. / Increase the rate of Reaction 1. / Heat the incoming reactants. / Generate electricity.
(1)
Structured questions
69 When nitrogen monoxide reacts with oxygen, a dynamic equilibrium is established.
2NO(g) + O2(g)
2NO2(g)
ΔHO = –115 kJ
a) State TWO features of a system that is in dynamic equilibrium.
(2 marks)
Rate of forward reaction = rate of backward reaction.
(1)
Concentrations of the reactants and products remain constant.
(1)
b) At a certain temperature, the equilibrium constant, Kc, for the reaction is 65.0 dm3 mol–1.
The concentrations of NO(g) and O2(g) in an equilibrium mixture are 0.600 mol dm–3 and 0.300 mol dm–3
respectively. What is the equilibrium concentration of NO2(g)?
(2 marks)
Kc =
[NO2(g)]2
[NO(g)]2[O2(g)]
3
–1
65.0 dm mol =
[NO2(g)]2
(0.600 mol dm–3)2(0.300 mol dm–3)
–3
[NO2(g)] = 2.65 mol dm
36
(1)
(1)
c) In each of the following cases, sketch on the given graph to show the expected variation in the concentration
of NO2(g) in the equilibrium mixture in (b) until the attainment of a new state of equilibrium. Explain your
answer in each case.
i) Some O2(g) is introduced into the equilibrium mixture at time t1, while the volume and the temperature
are both kept constant.
(2 marks)
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>
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5JNF
An increase in the concentration of O2(g) will shift the position of equilibrium to the right.
(1)
Part B
Thus, the concentration of NO2(g) will increase.
(3 marks)
</0H
>
ii) The temperature of the equilibrium mixture is increased at time t2.
U
5JNF
When the temperature is increased, the system will respond by reducing the temperature.
(1)
As the backward reaction is endothermic, the system will undergo a net backward reaction. Thus, the concentration
of NO2(g) will decrease.
d) State and explain how the pressure should be changed to give a higher yield of NO2(g).
Increase the pressure.
(1)
(2 marks)
(1)
An increase in pressure will bring about a net reaction that decreases the number of moles of gas. This helps to reduce
the pressure.
(1)
A net forward reaction will occur.
Thus, the yield of NO2(g) will increase.
37
70 The ester ethyl ethanoate is hydrolyzed when it is heated with water in the presence of an acid catalyst. An
equilibrium is established.
CH3COOCH2CH3(l) + H2O(l)
CH3COOH(l) + CH3CH2OH(l)
a) A 0.600 mole sample of ethyl ethanoate was heated with 4.00 mole of water. At equilibrium, 68.0% of
the ester was hydrolyzed. Calculate the equilibrium constant, Kc, for this reaction.
(4 marks)
Number of moles of ester reacted = 0.600 mol x 68.0% = 0.408 mol
Number of moles of ester in equilibrium mixture = (0.600 – 0.408) mol = 0.192 mol
(0.5)
Number of moles of water in equilibrium mixture = (4.00 – 0.408) mol = 3.59 mol
(0.5)
Number of moles of ethanoic acid in equilibrium mixture = 0.408 mol
(0.5)
Number of moles of ethanol in equilibrium mixture = 0.408 mol
(0.5)
Part B
3
Let V dm be the total volume of the equilibrium mixture.
[CH3COOH(l)][CH3CH2OH(l)]
Kc =
[CH3COOCH2CH3(l)][H2O(l)]
=
(
(
0.408
V
0.192
V
)(
)(
0.408
V
3.59
V
)
)
= 0.242
(1)
(1)
b) A student repeated the experiment using the same initial quantities. During the experiment she noticed
that the water in the condenser had stopped flowing. There was a sweetish smell coming from the top
of the condenser.
What effect, if any, will this problem have on
i) the concentrations of the products in the flask?
(2 marks)
Ester was lost.
(1)
The concentrations of the products would decrease.
(1)
ii) the value of the equilibrium constant, Kc?
38
Explain your answer in each case.
(2 marks)
The value of Kc would remain constant.
(1)
The value of Kc does not change with concentration. / The value of Kc only changes with temperature.
(1)
71 Hydrogen gas can be made from carbon monoxide and steam as shown by the following equation.
CO(g) + H2O(g)
CO2(g) + H2(g)
$PODFOUSBUJPO
The diagram below shows how the concentrations of H2O(g) and H2(g) change with time as equilibrium is
established.
)H
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9
a) On the time axis mark with an X the time at which equilibrium is first established.
(2 marks)
Part B
b) State Le Chatelier’s principle.
(1 mark)
Le Chatelier’s principle states that if the condition of a system in equilibrium is changed, the position of equilibrium
will shift
(1)
so as to reduce that change.
(1)
c) The volume and the temperature of the equilibrium mixture are both kept constant. Predict the effect of
each of the following changes on the position of equilibrium:
i) adding carbon monoxide to the equilibrium mixture; and
(2 marks)
The system responds by reducing the change. A net forward reaction occurs to use up some of the extra carbon
monoxide.
(1)
The position of equilibrium shifts to the right.
(1)
ii) removing steam from the equilibrium mixture.
(2 marks)
The system responds by reducing the change. A net backward reaction occurs to produce more steam.
(1)
The position of equilibrium shifts to the left.
(1)
39
d) This reaction is usually carried out at 450 °C.
i) State and explain the effect on the rate of production of hydrogen if this reaction is carried out at a
temperature above 450 °C.
(3 marks)
The rate of production of hydrogen will increase.
(1)
The reactant particles have more energy and collide more often.
(1)
A larger portion of the reactant particles have energy equal to or greater than the activation energy.
(1)
ii) State and explain the effect on the yield of hydrogen if the reaction is carried out in the presence of
a catalyst.
(2 marks)
Part B
No effect on the yield of hydrogen.
(1)
A catalyst increases the rates of both the forward reaction and the backward reaction to the same extent.
(1)
72 Methanol can be produced by using a reversible reaction between carbon monoxide and hydrogen.
2H2(g) + CO(g)
CH3OH(g)
When 2.00 mole of hydrogen and 1.00 mole of carbon monoxide are mixed and heated to a high temperature
in a container of volume 1.50 dm3, the equilibrium yield of methanol is 0.800 mole.
a) Calculate a value for the equilibrium constant, Kc, for this reaction at this temperature and give its
units.
(4 marks)
2H2(g) + CO(g)
CH3OH(g)
According to the equation, 2 moles of H2(g) react with 1 mole of CO(g) to give 1 mole of CH3OH(g).
As 0.800 mole of CH3OH(g) is produced at equilibrium, the amount of H2(g) would decrease by 2 x 0.800 mole while
that of CO(g) would decrease by 0.800 mole.
2H2(g)
2.00 – 2 x 0.800
Equilibrium concentration
1.50
(
= 0.267 mol dm–3
–3
0.533 mol dm
Kc =
(0.267 mol dm–3)2(0.133 mol dm–3)
6
–2
= 56.3 (1) dm mol (1)
40
)
mol dm
–3
+ CO(g)
1.00 – 0.800
1.50
(
CH3OH(g)
)
–3
mol dm
= 0.133 mol dm–3
0.800
1.50
mol dm–3
= 0.533 mol dm–3 (1)
(1)
b) The pressure of the equilibrium mixture is increased whilst keeping the temperature constant. The position
of equilibrium shifts to the right.
i) Explain why the position of equilibrium shifts to the right.
(1 mark)
An increase in pressure will bring about a net reaction that decreases the number of moles of gas.
(1)
ii) What is the effect, if any, on the value of Kc?
(1 mark)
No effect.
(1)
i) Explain what will happen to the position of equilibrium.
Part B
c) The temperature of the equilibrium mixture is increased whilst keeping the pressure constant. The value
of Kc decreases.
(2 marks)
As the value of Kc decreases, it can be deduced that the concentration of the product decreases while the
concentrations of the reactants increase,
(1)
i.e. the position of equilibrium shifts to the left.
(1)
ii) Deduce the sign of the enthalpy change for the reaction between carbon monoxide and hydrogen.
(3 marks)
It can be deduced that when the temperature is increased, the mixture will undergo a net backward reaction so as
to lower the temperature.
(1)
Hence the backward reaction should be endothermic reaction / the reaction between carbon monoxide and
hydrogen should be an exothermic reaction.
(1)
The sign of the enthalpy change for the reaction between carbon monoxide and hydrogen should be negative.
(1)
41
d) Describe and explain what will happen to the rate of the reaction between carbon monoxide and hydrogen
when the pressure is increased by decreasing the volume of the reaction vessel.
(3 marks)
Decreasing the volume of the reaction vessel will cause the concentrations of the reactants to increase.
(1)
The chance of collision between reactant particles increases and the frequency of effective collisions increases.
(1)
Hence the rate of the reaction will increase.
(1)
73 The equilibrium constant, Kc, for the following reaction is 5.00 mol dm–3 at 873 K.
COCl2(g)
CO(g) + Cl2(g)
Part B
2.00 moles of COCl2(g) are introduced into an evacuated vessel of 4.00 dm3 kept at 873 K and allowed to
achieve equilibrium.
a) Calculate the percentage dissociation of COCl2(g) when equilibrium is achieved.
(4 marks)
Let y% be the percentage dissociation of COCl2(g).
COCl2(g)
2.00
Initial concentration
mol dm–3
4.00
y
2.00 – 2.00 x
100
Equilibrium concentration
4.00
–3
Kc = 5.00 mol dm
=
(
y
100
4.00
CO(g)
0 mol dm
y
100
4.00
mol dm
)(
mol dm–3
2.00 – 2.00 x
4.00
y
100
4.00
2.00 x
y
100
mol dm–3
Cl2(g)
0 mol dm–3
y
100
4.00
2.00 x
2.00 x
–3
2.00 x
+
–3
–3
mol dm
)
mol dm–3
(1)
mol dm–3
(1)
Rearranging the equation gives
200 – 2y
2y 2
=
5.00
400
400
(
) (
)
y2 + 1 000 y – 100 000 = 0
Solving the quadratic equation gives two solutions:
y = 91.6 or – 1 092 (rejected)
∴ the percentage dissociation of COCl2(g) is 91.6%.
42
(2)
b) While keeping the temperature of the system at 873 K, the volume of the reaction vessel is decreased.
State and explain the effect on
i) the position of equilibrium;
(3 marks)
The pressure of the system will increase when the volume of the reaction vessel decreases.
(1)
This will bring about a net reaction that decreases the number of moles of gas. This helps to reduce the pressure. (1)
The position of equilibrium will shift to the left.
(1)
ii) the value of Kc.
(2 marks)
No effect.
(1)
The value of Kc only changes with temperature.
(1)
Part B
74 When heated, phosphorus pentachloride dissociates.
PCl5(g)
PCl3(g) + Cl2(g)
O
–1
ΔH = +93 kJ mol
3
At 227 °C, an equilibrium mixture in a 1.00 dm vessel contains 0.400 mole of PCl5(g), 0.108 mole of PCl3(g)
and 0.900 mole of Cl2(g).
a) Calculate the equilibrium constant, Kc, for the reaction at 227 °C.
Kc =
=
(3 marks)
[PCl3(g)][Cl2(g)]
[PCl5(g)]
(
)(
0.108
mol dm–3
1.00
)
0.900
mol dm–3
1.00
0.400
mol dm–3
1.00
(1)
–3
= 0.243(1) mol dm (1)
b) i) State the effect, if any, on the value of Kc of adding more PCl5(g) at a constant temperature. Explain
your answer.
(2 marks)
No effect.
(1)
The value of Kc does not change with concentration. / The value of Kc only changes with temperature.
(1)
43
ii) State the effect, if any, on the value of Kc of increasing the temperature of the reaction vessel. Explain
your answer.
(3 marks)
The value of Kc increases.
(1)
When the temperature is increased, the system will undergo a net reaction so as to lower the temperature.
(1)
As the forward reaction is endothermic, the system will undergo a net forward reaction. More PCl3(g) and Cl2(g)
are produced.
(1)
So, the value of Kc increases.
Part B
c) In another experiment, a 1.00 dm3 reaction vessel is filled with 0.100 mole of PCl5(g), 0.100 mole of
PCl3(g) and 0.100 mole of Cl2(g) at 227 °C.
i) Calculate Qc of the system and decide whether the system is at equilibrium.
Qc =
(
)(
0.100
mol dm–3
1.00
(2 marks)
)
0.100
mol dm–3
1.00
0.100
mol dm–3
1.00
= 0.100 mol dm–3
Qc ≠ Kc, thus the system is not at equilibrium.
(1)
(1)
ii) If the system is not at equilibrium, in which direction will the net reaction proceed? Explain your
answer.
(2 marks)
When Qc < Kc, the concentration of PCl3(g) and Cl2(g) will increase while that of PCl5(g) will decrease until
44
Qc = Kc.
(1)
Thus a net forward reaction will occur.
(1)
75 X(g) reacts with Y(g) reversibly to give Z(g). A mixture of X(g) and Y(g) is allowed to react in a closed container
of volume 1.0 dm3 kept at a constant temperature. The graph below shows the changes in concentrations
of X(g), Y(g) and Z(g) in the container with time.
(X, Y and Z do not represent symbols of elements.)
9H
:H
Part B
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a) With reference to the above graph,
i) deduce the chemical equation for the reversible reaction between X(g), Y(g) and Z(g).
(1 mark)
From the curve, it can be deduced that 3 moles of X(g) react with 1 mole of Y(g) to give 2 moles of Z(g).
The equation for the reaction is 3X(g) + Y(g)
2Z(g).
ii) write an expression for the equilibrium constant, Kc, for the reaction.
Kc =
[Z(g)]2
[X(g)]3[Y(g)]
(1)
(1 mark)
(1)
b) Compare the rate of the forward reaction and that of the backward reaction
i) at the 10th minute after X(g) and Y(g) are mixed.
At the 10th minute, the rate of forward reaction is greater than the rate of backward reaction.
(1 mark)
(1)
45
ii) at the 60th minute after X(g) and Y(g) are mixed.
(1 mark)
(You are not required to perform any calculation.)
At the 60th minute, the rate of forward reaction is equal to the rate of backward reaction.
(1)
c) If the mixture X(g) and Y(g) is allowed to react at the same temperature but in a closed container of
volume 0.5 dm3 instead,
i) will the rate of attainment of equilibrium remain the same? Explain.
(3 marks)
The rate of attainment of equilibrium will increase.
(1)
Decreasing the volume of the container will cause the concentration of the reactants to increase.
(1)
The chance of collision between reactant particles increases and the frequency of effective collisions increases.
(1)
Part B
ii) will the yield of Z(g) be the same? Explain.
(3 marks)
The yield of Z(g) will increase.
(1)
The pressure of the system will increase when the volume of the container decreases.
(1)
The position of equilibrium will shift to the side with a fewer number of moles of gas.
(1)
More Z(g) will form.
76 Methoxymethane, CH3OCH3, is used as an environmentally friendly propellant in spray cans. It can be made
from methanol according to the following equation:
2CH3OH(g)
CH3OCH3(g) + H2O(g)
ΔH = –24 kJ
The equilibrium constant, Kc, for this reaction at 350 °C is 5.74.
a) Write an expression for Kc for this reaction.
Kc =
46
[CH3OCH3(g)][H2O(g)]
[CH3OH(g)]2
(1 mark)
(1)
b) Calculate the value of Kc’, at 350 °C for the following reaction.
CH3OCH3(g) + H2O(g)
(1 mark)
2CH3OH(g)
[CH3OH(g)]2
Kc’ =
[CH3OCH3(g)][H2O(g)]
1
=
Kc
1
=
5.74
= 0.174
(1)
c) A 1.00 dm3 vessel at 350 °C contains 0.500 mole of CH3OH(g), 0.100 mole of CH3OCH3(g) and 0.700
mole of H2O(g).
i) Calculate Qc of the system and decide whether the system is at equilibrium.
Qc =
(2 marks)
[CH3OCH3(g)][H2O(g)]
[CH3OH(g)]2
0.100 mol
1.00 dm3
(
)(
0.700 mol
1.00 dm3
0.500 mol
1.00 dm3
)
)
Part B
=
(
2
= 0.280
Qc ≠ Kc, thus the system is not at equilibrium.
(1)
(1)
ii) If the system is not at equilibrium, in which direction will a net reaction proceed? Explain your
answer.
(2 marks)
When Qc < Kc, the concentrations of CH3OCH3(g) and H2O(g) will increase while that of CH3OH(g) will decrease until
Qc = Kc.
(1)
Thus, a net forward reaction will occur.
(1)
d) State and explain the effect of an increase in temperature on the position of equilibrium of the above
system.
(3 marks)
When the temperature is increased, the system will respond by reducing the temperature.
(1)
As the backward reaction is endothermic, the system will undergo a net backward reaction,
(1)
i.e. the position of equilibrium will shift to the left.
(1)
47
e) State and explain the effect of an increase in pressure on the position of equilibrium of the above
system.
(2 marks)
The number of moles of gas is the same on both sides of the equation,
(1)
so changing the pressure has no effect on the position of equilibrium.
(1)
f) In an experiment, methanol is pumped into an empty 20.0 dm3 reactor vessel. At equilibrium the vessel
contains 0.340 mole of methanol at 350 °C.
i) Calculate the concentration, in mol dm–3, of methanol at equilibrium.
(1 mark)
0.340 mol
Concentration of methanol =
20.0 dm3
= 0.0170 mol dm–3
(1)
Part B
ii) Calculate the number of moles of methoxymethane present at equilibrium.
(2 marks)
Let x mol dm–3 be the equilibrium concentration of CH3OCH3(g).
2CH3OH(g)
CH3OCH3(g)
–3
Equilibrium concentration
0.0170 mol dm
–3 2
(x mol dm )
Kc = 5.74 =
(0.0170 mol dm–3)2
x mol dm–3
+
H2O(g)
x mol dm–3
x = 0.0407
(1)
–3
3
Number of moles of CH3OCH3(g) = 0.0407 mol dm x 20.0 dm
= 0.814 mol
iii) Calculate the number of moles of methanol initially pumped into the reaction vessel.
(1)
(2 marks)
Number of moles of CH3OH(g) reacted = 2 x 0.814 mol
= 1.63 mol
(1)
Number of moles of CH3OH(g) at equilibrium = 0.340 mol
Number of moles of CH3OH(g) initially = (1.63 + 0.340) mol
= 1.97 mol
48
(1)
77 When hydrogen and iodine gases are allowed to react, an equilibrium is established according to the following
equation.
H2(g) +
colourless
I2(g)
purple
2HI(g)
colourless
O
ΔH = –10 kJ
A 2.00 dm3 container is filled with 0.0700 mole of H2(g) and 0.0600 mole of I2(g). Equilibrium is established
after 15.0 minutes when there is 0.0600 mole of HI(g) present.
a) Sketch and label the curves for changes in the concentrations of H2(g), I2(g) and HI(g) for the time interval
of 0 to 30.0 minutes.
(3 marks)
Part B
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b) Calculate the equilibrium constant, Kc, for the reaction.
Kc =
=
[HI(g)]2
[H2(g)][I2(g)]
(0.0300 mol dm–3)2
(0.0200 mol dm–3)(0.0150 mol dm–3)
= 3.00
(1)
(1)
c) Calculate the equilibrium constant, Kc’, for the following reaction.
2HI(g)
(2 marks)
(1 mark)
H2(g) + I2(g)
Kc’ for this reaction =
=
[H2(g)][I2(g)]
[HI(g)]2
1
3.00
= 0.333
(1)
49
d) Explain the following observations when changes are made to an equilibrium mixture of H2(g), I2(g) and
HI(g).
Include reference to the equilibrium position and any other factors.
i) When the temperature is decreased, the purple colour becomes paler.
The purple colour becomes paler because the position of equilibrium shifts to the right.
(2 marks)
(1)
The forward reaction is exothermic.
When the temperature is decreased, the system will undergo a net forward reaction so as to raise the
temperature.
(1)
Part B
ii) When the pressure is increased by decreasing the volume, the purple colour becomes deeper.
(3 marks)
The molecules are pushed closer together. / The concentration of I2(g) increases.
(1)
Increasing the pressure has no effect on the position of equilibrium,
(1)
because the number of moles of gas is the same on both sides of the equation.
(1)
78 At high temperatures, nitrogen is oxidized by oxygen to form nitrogen monoxide in a reversible reaction as
shown in the equation below.
N2(g) + O2(g)
2NO(g)
O
ΔH = +180 kJ
a) The equilibrium constant, Kc, for the above reaction is 5.00 x 10–3 at 3 000 °C.
What is the equilibrium constant, Kc’, for the following reaction at 3 000 °C?
4NO(g)
2N2(g) + 2O2(g)
Kc = 5.00 x 10–3
[NO(g)]2
=
[N2(g)][O2(g)]
2
2
Kc’ = [N2(g)] [O2(g)]
4
[NO(g)]
2
1
=
–3
5.00 x 10
(
= 4.00 x 104
50
)
(2 marks)
(1)
(1)
b) The rate at which equilibrium is reached could be increased by increasing the pressure or temperature.
In each case explain why the rate increases.
i) Increasing the pressure
(2 marks)
Increasing the pressure will cause the concentrations of the reactants to increase.
(1)
The chance of collision between reactant particles increases and the frequency of effective collisions increases.
(1)
ii) Increasing the temperature
(2 marks)
The reactant particles have more energy and collide more often.
(1)
A larger portion of the reactant have energy equal to or greater than the activation energy.
(1)
Part B
c) State and explain the effect of an increase in pressure and the effect of an increase in temperature, on
the yield of nitrogen monoxide.
i) Effect of an increase in pressure.
(3 marks)
The yield of nitrogen monoxide does not change.
(1)
The number of moles of gas is the same on both sides of the equation.
(1)
So, changing the pressure has no effect on the position of equilibrium.
(1)
ii) Effect of an increase in temperature.
(3 marks)
The yield of nitrogen monoxide increases.
(1)
When the temperature is increased, the system will respond by reducing the temperature.
(1)
As the forward reaction is endothermic, the system will undergo a net forward reaction.
(1)
51
d) N2(g), O2(g) and NO(g) were mixed together and allowed to reach equilibrium. The concentrations of the
gases were then measured at various times and the results plotted. At time t, a change was made to
the composition of the mixture.
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i) What change was made to the mixture at time t?
(1 mark)
Nitrogen was added.
ii) Explain the changes that happen to the mixture after time t.
(1)
(2 marks)
The concentration of NO(g) increased while those of N2(g) and O2(g) decreased.
It can be deduced that the position of equilibrium shifted to the right.
(1)
A new state of equilibrium was attained
(1)
when the concentrations of the substances become constant.
79 Chemists were investigating the production of a chemical, XY2(g), that can be formed from X2(g) and Y2(g)
as shown in the reaction below.
X2(g) + 2Y2(g)
2XY2(g)
a) Write an expression for the equilibrium constant, Kc, for the reaction, and deduce its units.
[XY2(g)]2
Kc =
[X2(g)][Y2(g)]2
(2 marks)
(1)
3
–1
Units of Kc is dm mol .
(1)
b) State and explain the effect of a decrease in pressure on
i) the position of equilibrium;
(2 marks)
A decrease in pressure will bring about a net reaction that increases the number of moles of gas. This helps to
52
increase the pressure.
(1)
The position of equilibrium will shift to the left.
(1)
ii) the rate of reaction.
(3 marks)
Decreasing the pressure will cause the concentrations of the reactants to decrease.
(1)
The chance of collision between reactant particles decreases and the frequency of effective collisions decreases.
(1)
Hence the rate of reaction will decrease.
(1)
Part B
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c) The chemists measured the percentage conversion of X2(g) at various temperatures. The results are shown
in the graph below.
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i) Use the graph to predict the percentage conversion of X2(g) at 350 °C.
13.5%
(1 mark)
(1)
ii) Deduce from the graph whether the production of XY2(g) from X2(g) and Y2(g) is an exothermic or
an endothermic reaction. Explain your answer.
(3 marks)
The production of XY2(g) from X2(g) and Y2(g) is an endothermic reaction.
(1)
The percentage conversion of X2(g) increases with increasing temperature.
(1)
It can be deduced that when the temperature is increased, the system will undergo a net forward reaction so as to
lower the temperature.
(1)
Hence the production of XY2(g) (the forward reaction) should be an endothermic reaction.
53
d) The chemists decided to use a catalyst in the process. State the effect of using a catalyst on:
i) the rate of conversion of X2(g) and Y2(g) into XY2(g);
(1 mark)
The rate of conversion increases.
(1)
ii) the percentage conversion of X2(g) and Y2(g) into XY2(g) at equilibrium.
(1 mark)
No change.
(1)
80 Fe3+(aq) ions and SCN–(aq) ions react in solution to give a deep red [Fe(SCN)]2+(aq) ions.
Fe3+(aq) +
yellow-brown
SCN–(aq)
colourless
[Fe(SCN)]2+(aq)
deep red
Part B
a) A student makes changes to separate samples of the equilibrium mixture. The changes are listed in the
table below.
Complete the table by placing ticks in the appropriate boxes to indicate the effect of each change on
(1) the intensity of the red colour of the mixture; and
(2) the concentration of Fe3+(aq) ions
once a new state of equilibrium has been attained.
Change to the
equilibrium mixture
(1) Intensity of the red colour of
mixture compared with initial
equilibrium
less intense
(i) Adding 1 drop of FeCl3(aq)
54
(3 marks)
more intense
✔
(ii) Adding 1 drop of AgNO3(aq)
+
(Ag (aq) ions will form a
AgSCN precipitate)
✔
(0.5)
(iii) Adding NH4Cl(s) (NH4Cl(s)
3+
reacts with Fe (aq) ions to
form complex ions)
✔
(0.5)
3+
(2) Concentration of Fe (aq)
ions compared with initial
equilibrium
decreases
(0.5)
✔
(0.5)
increases
✔
(0.5)
✔
(0.5)
b) The forward reaction is exothermic. The graph below shows the initial equilibrium concentration of the
ions. The temperature of the system is increased at t1.
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i) Sketch on the graph to show the expected variation in the concentrations until the attainment of a
new state of equilibrium at t2.
(2 marks)
(2 marks)
When the temperature is increased, the system will respond by lowering the temperature.
(1)
As the forward reaction is exothermic, the system will undergo a net backward reaction.
(1)
Part B
ii) Explain your answer.
3+
–
2+
Thus, the concentrations of Fe (aq) ions and SCN (aq) ions increase while that of [Fe(SCN)] (aq) ions decreases.
c) The following table lists the compositions of two equilibrium mixtures.
Fe3+(aq)
SCN–(aq)
Concentration in equilibrium mixture 1 (mol dm–3)
3.91 x 10
–2
8.02 x 10
–5
Concentration in equilibrium mixture 2 (mol dm–3)
6.27 x 10–3
3.65 x 10–4
Calculate the concentration of [Fe(SCN)]2+(aq) ions in equilibrium mixture 2.
Kc =
=
9.22 x 10–4 mol dm–3
–2
(3.91 x 10 mol dm–3)(8.02 x 10–5 mol dm–3)
[[Fe(SCN)]2+(aq)]
(6.27 x 10–3 mol dm–3)(3.65 x 10–4 mol dm–3)
2+
–4
–3
[[Fe(SCN)] (aq)] = 6.73 x 10 mol dm
[Fe(SCN)]2+(aq)
–4
9.22 x 10
?
(2 marks)
(1)
(1)
55
81 Under suitable conditions, the equilibrium represented below was established.
2CH4(g)
3H2(g) + C2H2(g)
ΔHO = +377 kJ
The following graph shows the concentrations of the substances in the equilibrium mixture at a certain
temperature and pressure.
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a) i) Write an expression for the equilibrium constant, Kc, for the reaction.
Kc =
[H2(g)]3[C2H2(g)]
[CH4(g)]2
ii) With reference to the above graph, calculate Kc under the experimental conditions.
(1.20 mol dm–3)3(0.400 mol dm–3)
Kc =
(1.60 mol dm–3)2
= 0.270 (1) mol2 dm–6 (1)
56
(1 mark)
(1)
(2 marks)
b) At time t1, the concentration of CH4(g) was increased to 2.00 mol dm–3 and a new state of equilibrium
was established at t2.
i) Calculate the equilibrium concentration of each gas at time t2.
(6 marks)
Let x mol dm–3 be the increase in concentration of C2H2(g) when a new state of equilibrium was established.
Initial concentration
2CH4(g)
3H2(g)
2.00 mol dm–3
1.20 mol dm–3
+
New equilibrium concentration (2.00 – 2x) mol dm–3
(1.20 + 3x) mol dm–3
–3 3
mol dm–3)
Kc = 0.270 mol2 dm–6 = ((1.20 + 3x) mol dm ) ((0.400 +–3 x)
2
((2.00 – 2x) mol dm )
27(0.400 + x)4
0.270 =
4(1.00 – x)2
(0.400 + x)4
0.0400 =
(1.00 – x)2
C2H2(g)
0.400 mol dm–3
(0.400 + x) mol dm–3
(1)
Rearranging the equation gives
Part B
0.200 (1.00 – x) = (0.400 + x)
2
x2 + x – 0.0400 = 0
Solving the quadratic equation gives two solutions:
x = 0.0385 or –1.04 (rejected)
(2)
∴ the equilibrium concentrations at time t2 are as follows:
–3
[CH4(g)] = (2.00 – 2 x 0.0385) mol dm
= 1.92 mol dm–3
(1)
–3
[H2(g)] = (1.20 + 3 x 0.0385) mol dm
= 1.32 mol dm–3
(1)
–3
[C2H2(g)] = (0.400 + 0.0385) mol dm
= 0.439 mol dm–3
ii) Sketch on the graph to show the changes in concentration of CH4(g) and H2(g).
(1)
(2 marks)
57
c) Describe the conditions that would produce a high yield of hydrogen at equilibrium. Explain your
answers.
i) Temperature
(3 marks)
A high temperature is required.
(1)
When a high temperature is used, the system will respond by reducing the temperature.
(1)
As the production of hydrogen is endothermic, the system will undergo a net forward reaction.
(1)
Thus, the yield of hydrogen will increase.
ii) Pressure
(2 marks)
Part B
A low pressure is required.
(1)
A decrease in pressure will bring about a net reaction that increases the number of moles of gas. This helps to
increase the pressure.
(1)
A net forward reaction will occur.
Thus, the yield of hydrogen will increase.
d) A different equilibrium mixture was produced, starting from CH4(g) alone.
When 1.00 moles of CH4(g) were charged into a container of volume 0.250 dm3, it was found that the
equilibrium mixture formed contained 0.730 moles of CH4(g). Calculate the number of moles of H2(g)
and C2H2(g) present in this equilibrium mixture.
(3 marks)
Number of moles of CH4(g) reacted = (1.00 – 0.730) mol
= 0.270 mol
(1)
3
x 0.270 mol
Number of moles of H2(g) in the equilibrium mixture =
2
= 0.405 mol
0.270
mol
Number of moles of C2H2(g) in the equilibrium mixture =
2
= 0.135 mol
58
(1)
(1)
82 The hydrogen used in the Haber process is made by the following reaction:
CH4(g) + H2O(g)
ΔHO = +206 kJ
CO(g) + 3H2(g)
a) Initially, 0.0600 mole of CH4(g), 0.0800 mole of H2O(g), 0.280 mole of CO(g) and 0.740 mole of H2(g) are
placed in a 4.00 dm3 container. At equilibrium, the concentration of H2(g) is 0.200 mol dm–3. Calculate
the equilibrium constant, Kc, for the reaction.
(5 marks)
CH4(g)
0.0600
mol dm–3
Initial concentration
4.00
+
= 0.0150 mol dm–3
H2O(g)
0.0800
mol dm–3
4.00
CO(g)
0.280
4.00
+
= 0.0200 mol dm–3
= 0.0700 mol dm–3
–3
mol dm
3H2(g)
0.740
4.00
mol dm–3
= 0.185 mol dm–3
As the concentration of H2(g) increased by 0.0150 mol dm–3 when equilibrium is reached, that of CO(g) would increase
–3
by 5.00 x 10
mol dm–3 while that of both CH4(g) and H2O(g) would decrease by 5.00 x 10–3 mol dm–3.
Species
H2O(g)
CO(g)
H2(g)
0.0150
0.0200
0.0700
0.185
–3
Change in concentration (mol dm )
–0.00500
–0.00500
+0.00500
+0.0150
Equilibrium concentration (mol dm–3)
0.0100
0.0150
0.0750
0.200
Initial concentration (mol dm–3)
Kc =
=
3
[CO(g)][H2(g)]
[CH4(g)][H2O(g)]
–3
–3 3
(0.0750 mol dm )(0.200 mol dm )
–3
(0.0100 mol dm )(0.0150 mol dm–3)
Part B
CH4(g)
(2)
(1)
2
–6
= 4.00 (1) mol dm (1)
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b) In another experiment, some methane and steam are placed in a closed container and allowed to react
at a fixed temperature. The following graph shows the changes in concentrations of methane and carbon
monoxide as the reaction proceeds.
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59
i) On the graph above, draw a line to show the change in concentration of hydrogen as the reaction
proceeds. Label this line.
(1 mark)
ii) On the graph above, draw a line to show how the formation of carbon monoxide would differ over
time in the presence of a catalyst. Label this line.
(1 mark)
c) A high temperature of over 1 000 °C is used in the production of hydrogen by this reaction.
i) Explain why a high temperature is needed to produce a high yield of hydrogen.
(2 marks)
When a high temperature is used, the system will respond by reducing the temperature.
(1)
As the production of hydrogen is endothermic, the system will undergo a net forward reaction.
(1)
Thus, the yield of hydrogen will increase.
ii) Give one disadvantage of using temperatures much higher than 1 000 °C.
Part B
The cost of energy is high. / The amount of energy used is high.
(1 mark)
(1)
d) State and explain how the overall pressure must be changed to increase the yield of hydrogen.
(2 marks)
Decrease the pressure.
(1)
A decrease in pressure will bring about a net reaction that increases the number of moles of gas. This helps to increase
the pressure.
(1)
A net forward reaction will occur.
Thus, the yield of hydrogen will increase.
83 Dinitrogen tetroxide is a pale yellow gas. It exists in equilibrium with nitrogen dioxide, a dark brown gas.
N2O4(g)
pale yellow
2NO2(g)
dark brown
a) 0.400 mole of N2O4(g) is heated in a closed reaction vessel of volume 16.0 dm3. A state of equilibrium
is established at a constant temperature. The equilibrium mixture is found to contain 0.180 mole of
N2O4(g).
i) What is the number of moles of NO2(g) in the equilibrium mixture?
(2 marks)
Number of moles of N2O4(g) reacted = (0.400 – 0.180) mol
= 0.220 mol
(1)
Number of moles of NO2(g) in the equilibrium mixture = 2 x 0.220 mol
= 0.440 mol
60
(1)
ii) Calculate the equilibrium constant, Kc, for the reaction.
Kc =
=
[NO2(g)]2
[N2O4(g)]
(
(3 marks)
)
0.440
mol dm–3 2
16.0
(1)
0.180
mol dm–3
16.0
= 0.0672 (1) mol dm–3 (1)
b) The diagram below shows a gas syringe containing a pale brown mixture of N2O4(g) and NO2(g) at
equilibrium at 25 °C.
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i) Keeping the volume constant, the temperature is then raised to 35 °C. The brown colour becomes
more intense.
Is the forward reaction below exothermic or endothermic? Explain your answer.
N2O4(g)
2NO2(g)
(3 marks)
The forward reaction is endothermic.
(1)
When the temperature is increased, the brown colour becomes more intense because more NO2(g) is produced.
(1)
It can be deduced that when the temperature is increased, the system will undergo a net forward reaction so as to
lower the temperature.
(1)
Hence the forward reaction should be an endothermic reaction.
ii) Keeping the temperature at 35 °C, the plunger of the syringe is then pushed in so as to halve the
volume at time t.
(1) State and explain your expected observations.
(5 marks)
The brown colour of the mixture becomes more intense for a moment
(1)
and then gets paler gradually.
(1)
When the volume of the syringe is halved, the concentration of NO2(g) doubles.
(1)
A decrease in volume will bring about a net reaction that decreases the number of moles of gas.
The position of equilibrium will shift to the left.
(1)
61
The concentration of NO2(g) decreases, but not back to its concentration before time t.
(1)
(2) Sketch on the given graph to show the expected variation in the concentration of NO2(g) in the
mixture until the attainment of a new state of equilibrium.
(1 mark)
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84 The dissociation of NH4HS(s) can be represented by the following equation:
NH4HS(s)
NH3(g) + H2S(g)
ΔH > 0
0.700 mole of NH4HS(s) is introduced into an evacuated vessel of 2.00 dm3 kept at 310 K. When equilibrium
is attained, 1.00% of the NH4HS(s) is found to have dissociated.
a) Write an expression for the equilibrium constant, Kc, for the dissociation of NH4HS(s).
Kc = [NH3(g)][H2S(g)]
(1 mark)
(1)
b) Calculate the equilibrium constant, Kc, for the dissociation at 310 K.
(3 marks)
Number of moles of NH4HS(s) dissociated = 0.700 mol x 1.00%
–3
= 7.00 x 10
mol
According to the equation, 1 mole of NH4HS(s) dissociates to give 1 mole of NH3(g) and 1 mole of H2S(g),
–3
7.00 x 10
mole of NH4HS(s) has dissociated.
∴ number of mole of NH3(g) formed = number of moles of H2S(g) formed
–3
= 7.00 x 10
Kc = [NH3(g)][H2S(g)]
7.00 x 10–3
–3
=
mol dm
2.00
(
)(
)
7.00 x 10–3
–3
mol dm
2.00
= 1.23 x 10–5 (1) mol2 dm–6 (1)
62
mol
(1)
c) If more NH4HS(s) is added to the equilibrium system at 310 K, will the equilibrium concentration of NH3(g)
change? Explain your answer.
(2 marks)
No effect.
(1)
The equilibrium constant depends only on temperature. Adding NH4HS(s) will not affect the equilibrium concentration of
NH3(g).
(1)
d) What will happen to the percentage dissociation of NH4HS(s) if the temperature of the above equilibrium
system is increased? Explain your answer.
(3 marks)
(1)
As the dissociation of NH4HS(s) is endothermic, the system will undergo a net forward reaction.
(1)
Thus, the percentage dissociation of NH4HS(s) will increase.
(1)
Part B
When the temperature is increased, the system will respond by lowering the temperature.
85 Part of the process by which coal can be converted into a combustible mixture of gases involves passing
steam over white hot coke.
H2O(g) + C(s)
H2(g) + CO(g)
ΔH = +131 kJ
a) Write an expression for the equilibrium constant, Kc, for the reaction. State the units of Kc.
Kc =
(2 marks)
[H2(g)][CO(g)]
[H2O(g)]
(1)
–3
Units of Kc : mol dm
(1)
b) At 1 000 K, the value of Kc for the reaction is 3.00 x 10–2.
In an experiment, 6.00 moles of steam and an excess of solid carbon are heated in a 5.00 dm3 container.
Calculate the equilibrium concentration of H2O(g), H2(g) and CO(g).
(5 marks)
Let x mol dm–3 be the decrease in the concentration of H2O(g) when equilibrium is reached.
Initial concentration
H2O(g)
6.00
mol dm–3
5.00
+
C(s)
H2(g)
+
CO(g)
–3
0 mol dm
0 mol dm–3
x mol dm–3
x mol dm–3
= 1.20 mol dm–3
Equilibrium concentration
Kc = 3.00 x 10–2 mol dm–3
(1.20 – x) mol dm–3
–3 2
(x mol dm )
=
(1.20 – x) mol dm–3
(1)
63
Rearranging the equation gives
3.00 x 10–2 (1.20 – x) = x2
x2 + 0.0300 x – 0.0360 = 0
Solving the quadratic equation gives two solutions:
x = 0.175 or –2.05 (rejected)
(2)
–3
i.e. the decrease in the concentration of H2O(g) is 0.175 mol dm .
The equilibrium concentrations are as follows:
[H2O(g)] = (1.20 – 0.175) mol dm–3
= 1.03 mol dm–3
(1)
[H2(g)] = [CO(g)]
–3
(1)
= 0.175 mol dm
Part B
c) State and explain how the amount of steam in the equilibrium mixture would change if there is an
increase in
i) the pressure;
(3 marks)
The amount of steam will increase.
(1)
An increase in pressure will bring about a net reaction that decrease the number of moles of gas. This helps to
reduce the pressure.
(1)
The position of equilibrium will shift to the left.
(1)
ii) the temperature.
64
(3 marks)
The amount of steam will decrease.
(1)
When the temperature is increased, the system will respond by reducing the temperature.
(1)
As the forward reaction is endothermic, the system will undergo a net forward reaction.
(1)
86 A crucial reaction in the manufacture of sulphuric acid involves the oxidation of sulphur dioxide to sulphur
trioxide by oxygen in the air.
2SO2(g) + O2(g)
2SO3(g)
ΔH = –197 kJ
A catalyst is used in the process.
a) Name the catalyst used in the process.
(1 mark)
Vanadium(V) oxide
(1)
b) The high yield is only achieved under certain conditions. After each condition, explain why it leads to an
increased yield of sulphur trioxide.
i) There needs to be an excess of air in the reacting gas mixture.
(2 marks)
An excess of air is needed to provide excess oxygen
(1)
to drive the position of equilibrium to the right
(1)
Part B
so that a high yield of sulphur trioxide can be obtained.
ii) The catalyst needs to be cooled.
(3 marks)
When the temperature is decreased, the system will respond by raising the temperature.
(1)
As the forward reaction is exothermic, the system will undergo a net forward reaction.
(1)
Thus, a high yield of sulphur trioxide can be obtained.
Also the catalyst gets heated up and a high temperature may denature the catalyst.
(1)
c) In an experiment, a mixture of 1.59 moles of sulphur dioxide and 0.855 mole of oxygen is introduced to
a 30.0 dm3 container at 800 K. When equilibrium is reached, it is found that 94.3% of sulphur dioxide
is converted to sulphur trioxide.
Calculate the equilibrium constant, Kc, at 800 K.
2SO2(g) + O2(g)
(4 marks)
2SO3(g)
According to the equation, 2 moles of SO2(g) react with 1 mole of O2(g) to give 2 moles of SO3(g). As 94.3% of SO2(g)
is converted to SO3(g), the amount of SO2(g) would decrease by (1.59 x 0.943) mole and that of O2(g) would decrease
1.59 x 0.943
2
mole; the amount of SO3(g) would increase by (1.59 x 0.943) mole.
by
(
)
2SO2(g)
+
Equilibrium
concentration
(
)
1.59 – 1.59 x 0.943
mol dm–3
30.0
= 3.02 x 10–3 mol dm–3
O2(g)
(
0.855 –
1.59 x 0.943
2
30.0
2SO3(g)
)
mol dm–3
= 3.51 x 10–3 mol dm–3
(
1.59 x 0.943
30.0
)
mol dm–3
= 0.0500 mol dm–3
(1)
65
Kc =
–3
(3.02 x 10
4
(0.0500 mol dm–3)2
mol dm–3)2(3.51 x 10–3 mol dm–3)
3
–1
= 7.81 x 10 (1) dm mol
(1)
(1)
Part B
87 Consider the following equations which show reversible reactions.
Reaction 1
4NH3(g) + 5O2(g)
Reaction 2
CO(g) + 2H2(g)
4NO(g) + 6H2O(g)
CH3OH(g)
ΔHO = –900 kJ
ΔHO = –91 kJ
a) In industry these reactions are carried out in the presence of catalysts. A platinum catalyst is used in
Reaction 1 and copper catalyst is used in Reaction 2.
State and explain the effect on the yield of a reaction when a catalyst is used.
(2 marks)
No effect.
(1)
A catalyst increases the rates of both the forward reaction and the backward reaction to the same extent.
(1)
b) State and explain which of the above reactions will give an increase in the yield of product(s) when the
pressure is increased at a constant temperature.
(3 marks)
Reaction 2
(1)
An increase in pressure will bring about a net reaction that decreases the number of moles of gas. This helps to reduce
the pressure.
(1)
The position of equilibrium will shift to the side of the equation with a fewer number of moles of gas, i.e. the product
side of Reaction 2.
Thus, the yield of the product in Reaction 2 will increase.
66
(1)
c) State and explain the effect on the yield of NO(g) when the temperature is increased in Reaction 1 at a
constant pressure.
(3 marks)
When the temperature is increased, the system will respond by reducing the temperature.
(1)
As the backward reaction is endothermic, the system will undergo a net backward reaction.
(1)
Thus, the yield of NO(g) will decrease.
(1)
d) i) Write an expression for the equilibrium constant, Kc, for Reaction 2. State the units of Kc. (2 marks)
Kc =
[CH3OH(g)]
[CO(g)][H2(g)]2
(1)
6
–2
Units of Kc : dm mol
(1)
ii) The numerical value for the equilibrium constant, Kc, of Reaction 2 is 14.5 at 500 K. At equilibrium
the concentrations of CO(g) and H2(g) are 0.120 and 0.0953 mol dm–3 respectively. Calculate the
equilibrium concentration of CH3OH(g).
(2 marks)
[CH3OH(g)]
(0.120 mol dm–3)(0.0953 mol dm–3)2
Part B
Kc = 14.5 dm6 mol–2 =
(1)
–3
[CH3OH(g)] = 0.0158 mol dm
(1)
iii) What does the size of the equilibrium constant, Kc, tell you about the position of equilibrium for
Reaction 2?
(1 mark)
The position of equilibrium lies to the product side.
(1)
88 In the Haber process, ammonia is synthesized by the exothermic reaction of nitrogen and hydrogen.
N2(g) + 3H2(g)
2NH3(g)
The table shows the percentage yield of ammonia, under different conditions of pressure and temperature,
when the reaction has reached equilibrium.
Temperature (K)
600
800
1 000
% yield of ammonia at 100 atm
50
10
2
% yield of ammonia at 200 atm
60
16
4
% yield of ammonia at 500 atm
75
25
7
a) i) Explain why, at a given temperature, the percentage yield of ammonia increases with an increase in
pressure.
(2 marks)
An increase in pressure will bring about a net reaction that decreases the number of moles of gas. This helps to
reduce the pressure.
(1)
A net forward reaction occurs.
(1)
Thus, the percentage yield of ammonia increases.
67
ii) Give a reason why a high pressure of 500 atmospheres is NOT normally used in the Haber process.
(1 mark)
Too expensive to generate. / The cost of building and running the plant is too high.
(1)
b) Many industrial ammonia plants operate at a compromise temperature of about 723 K.
i) State one advantage, other than cost, of using a temperature lower than 723 K. Explain your
answer.
(3 marks)
The yield of ammonia increases.
(1)
When the temperature is decreased, the system will respond by raising the temperature.
(1)
As the forward reaction is exothermic, the system will undergo a net forward reaction.
(1)
Part B
ii) State the major advanatge of using a temperature higher than 723 K.
The rate of production of ammonia is higher.
iii) Hence explain why 723 K is referred to as a compromise temperature.
723 K is a balance between rate and yield.
(1 mark)
(1)
(1 mark)
(1)
c) In a simulation of the process, a mixture of nitrogen and hydrogen is placed in a closed container. The
initial concentrations of nitrogen and hydrogen are 0.500 mol dm–3 and 1.50 mol dm–3 respectively. When
the equilibrium is attained at 723 K, 25.0% of the original nitrogen are consumed.
i) Calculate the respective concentrations of nitrogen, hydrogen and ammonia in the equilibrium
mixture.
(3 marks)
N2(g) + 3H2(g)
2NH3(g)
According to the equation, 1 mole of N2(g) reacts with 3 moles of H2(g) to give 2 moles of NH3(g).
–3
As 25.0% of N2(g) are consumed, therefore the concentration of N2(g) decreases by (0.500 x 0.250) mol dm , that
–3
–3
of H2(g) decreases by (3 x 0.500 x 0.250) mol dm and that of NH3(g) increases by (2 x 0.500 x 0.250) mol dm .
–3
Equilibrium concentration of N2(g) = (0.500 – 0.500 x 0.250) mol dm
= 0.375 mol dm–3
(1)
–3
Equilibrium concentration of H2(g) = (1.50 – 3 x 0.500 x 0.250) mol dm
= 1.13 mol dm–3
(1)
–3
Equilibrium concentration of NH3(g) = (2 x 0.500 x 0.250) mol dm
= 0.250 mol dm–3
68
(1)
ii) Calculate Kc for the reaction at 723 K.
Kc =
(3 marks)
(0.250 mol dm–3)2
(0.375 mol dm–3)(1.13 mol dm–3)3
(1)
6
–2
= 0.116 (1) dm mol (1)
89 In the Contact process, sulphur dioxide is catalytically oxidized to sulphur trioxide according to the
equation:
2SO2(g) + O2(g)
2SO3(g)
ΔH < 0
The following table summarizes several possible conditions for the preparation of sulphur trioxide:
Conditions
Reactants
Pressure (atm)
Temperature (°C)
A
SO2(g) + excess O2(g)
500
400
B
excess SO2(g) + O2(g)
500
1 000
C
excess SO2(g) + air
1
1 000
D
SO2(g) + excess air
1
400
Part B
Case
Which of the cases would represent
a) the theoretical conditions for obtaining the maximum yield of sulphur trioxide? Explain your answer.
(5 marks)
Case A
(1)
An increase in pressure will bring about a net reaction that decreases the number of moles of gas. This helps to reduce
the pressure.
(1)
A net forward reaction occurs.
(1)
Thus, a high pressure will increase the yield of SO3(g).
When the temperature is decreased, the system will respond by raising the temperature.
(1)
As the forward reaction is exothermic, the system will undergo a net forward reaction.
(1)
Thus, a low temperature will increase the yield of SO3(g).
69
b) the most economical conditions for the industrial preparation of sulphur trioxide? Explain your answer.
(4 marks)
Case D
(1)
Air is readily available and cheap.
(1)
At lower pressure, the cost of building and running the plant is lower.
(1)
At lower temperature, the cost of energy is lower / less fuel is required.
(1)
90 Ethanol is manufactured by catalytic hydration of ethene:
CH2=CH2(g) + H2O(g)
CH3CH2OH(g)
Part B
a) The reaction represented by the above equation can reach a position of dynamic equilibrium. State TWO
features of a system that is in dynamic equilibrium.
(2 marks)
Rate of forward reaction = rate of backward reaction
(1)
Concentration of the reactants and product remain constant.
(1)
b) In a simulation study of the manufacturing process, 0.900 mole of ethene and 0.600 mole of steam are
mixed in a closed container of volume 1.20 dm3. When equilibrium is attained, 5.00% of the ethene are
converted into ethanol.
Calculate the equilibrium constant, Kc, under the above conditions.
CH2=CH2(g) + H2O(g)
(4 marks)
CH3CH2OH(g)
According to the equation, 1 mole of CH2=CH2(g) reacts with 1 mole of H2O(g) to give 1 mole of CH3CH2OH(g).
As 5.00% of CH2=CH2(g) are converted to CH3CH2OH(g), the amounts of both CH2=CH2(g) and H2O(g) would decrease
by (0.900 x 0.0500) mole while that of CH3CH2OH(g) would increase by (0.900 x 0.0500) mole.
CH2=CH2(g)
+
H2O(g)
CH3CH2OH(g)
Equilibrium
concentration
Kc =
(
0.900 – 0.900 x 0.0500
1.20
= 0.713 mol dm–3
–3
(0.0375 mol dm )
–3
(0.713 mol dm )(0.463 mol dm–3)
3
–1
= 0.114 (1) dm mol (1)
70
)
mol dm–3
(
0.600 – 0.900 x 0.0500
1.20
= 0.463 mol dm–3
)
mol dm–3
(
0.900 x 0.0500
1.20
)
= 0.0375 mol dm–3
mol dm–3
(1)
(1)
c) The following table lists the percentage conversion of ethene using excess steam at various reaction
conditions used in industry.
Temperature (°C)
Percentage conversion (%)
50
200
45
50
320
30
80
200
60
80
320
45
i) State and explain the effect of increasing the pressure on the percentage conversion.
Part B
Pressure (atm)
(3 marks)
The percentage conversion increases with pressure.
(1)
An increase in pressure will bring about a net reaction that decreases the number of moles of gas. This helps to
reduce the pressure.
(1)
The position of equilibrium will shift to the side of the equation with a fewer number of moles of gas, i.e. shift to
the right.
ii) Deduce the sign of the enthalpy change for the forward reaction. Explain your answer.
(1)
(3 marks)
When the temperature is increased, the percentage conversion decreases.
(1)
It can be deduced that when the temperature is increased, the system will undergo a net backward reaction so as
to lower the temperature.
(1)
Hence the backward reaction should be an endothermic reaction / the forward reaction should be an exothermic
reaction.
The sign of the enthalpy change for the forward reaction should be negative.
(1)
71
iii) The equation for the formation of ethanol shows that equal numbers of moles of ethene and steam
are required. In industry however excess steam is used.
Suggest why excess steam in used.
(1 mark)
Using excess steam will shift the position of equilibrium to the right.
(1)
91 Read the following passage and answer the questions that follow.
Chlorine for disinfection
Chlorine is used in water treatment for disinfection.
When chlorine is added to pure water, hypochlorous acid (HOCl) and hydrochloric acid (HCl) are formed.
Part B
HOCl(aq) + H+(aq) + Cl–(aq)
Cl2(aq) + H2O(l)
The principal disinfecting action of aqueous chlorine is due to the hypochlorous acid formed.
Hypochlorous acid dissociated into hydrogen ions and hypochlorite ions.
HOCl(aq)
+
–
H (aq) + OCl (aq)
The concentrations of hypochlorous acid and hypochlorite ions in chlorinated water depend on the pH of the
water.
Instead of using chlorine gas, some plants apply sodium hypochlorite or calcium hypochlorite to water. Sodium
hypochlorite completely dissociates in water to form sodium ions and hypochlorite ions. In solution, the
hypochlorite ions hydrolyze to form the disinfectant hypochlorous acid according to the following equation:
–
OCl (aq) + H2O(l)
HOCl(aq) + OH–(aq)
a) State Le Chatelier’s principle.
(2 marks)
Le Chatelier’s principle states that if the conditions of a system in equilibrium is changed, the position of equilibrium
72
will shift
(1)
so as to reduce that change.
(1)
b) Use Le Chatelier’s principle to explain how the pH of the chlorinated water will affect the concentrations
of hypochlorous acid and hypochlorite ions in the water.
(4 marks)
Decreasing the pH of the chlorinated water means increasing the concentration of hydrogen ions.
(1)
The system will respond by reducing this change.
(1)
The position of equilibrium will shift to the left.
(1)
More hypochlorous acid will form.
(1)
OR
Increasing the pH of the chlorinated water means decreasing the concentration of hydrogen ions.
(1)
The system will respond by reducing this change.
(1)
The position of equilibrium will shift to the right.
(1)
More hypochlorite ions will form.
(1)
OCl–(aq) + H2O(l)
Part B
c) Suppose the equilibrium constant for the following reaction is K1.
HOCl(aq) + OH–(aq)
What is the equilibrium constant K2 for the following reaction?
2HOCl(aq) + 2OH–(aq)
–
2OCl–(aq) + 2H2O(l)
[HOCl(aq)][OH (aq)]
K1 =
[OCl–(aq)][H2O(l)]
[OCl–(aq)]2[H2O(l)]2
K2 =
[HOCl(aq)]2[OH–(aq)]2
1
=
2
K1
(2 marks)
(1)
(1)
d) The hypochlorous acid produced in a solution of sodium hypochlorite can react further to produce small
amount of chlorine according to the following equation:
HOCl(aq) + H+(aq) + Cl–(aq)
Cl2(g) + H2O(l)
What will happen to the concentration of chlorine if a little sodium hydroxide solution is added to a
sodium hypochlorite solution? Explain your answer.
(4 marks)
When sodium hydroxide solution is added, the hydroxide ions react with the hydrogen ions to form water.
(1)
Thus, the concentration of hydrogen ions decreases.
(1)
The system responds by reducing this change.
A net backward reaction occurs to produce more hydrogen ions.
(1)
Thus, the concentration of chlorine will decrease.
(1)
73
92 Read the following passage and answer the questions that follow.
Carbon dioxide and formation of stalactites and stalagmites in limestone caves
In addition to being a component of the atmosphere, carbon dioxide also dissolves in the water of the oceans.
The dissolving process can be described by the following equations:
CO2(g)
CO2(aq) ...... equation (1)
CO2(aq) + H2O(l)
+
H (aq) + HCO3–(aq) ...... equation (2)
In nature, surface water often becomes acidic because atmospheric carbon dioxide dissolves in it. This acidic
solution can dissolve limestone:
CaCO3(s) + CO2(aq) + H2O(l)
2+
Ca (aq) + 2HCO3–(aq) ...... equation (3)
Openings formed in the limestone as the calcium carbonate dissolves.
Part B
Slight cooling of the water saturated with carbon dioxide can reduce the solubility of carbon dioxide. The
position of equilibrium shifts, resulting in the precipitation of calcium carbonate. This precipitate, the stalactite,
is formed immediately when the seeping water comes into contact with air currents in a cave. Stalagmites
are formed on the floors of caves in the same way.
a) Suggest why the balance between CO2(g) in the atmosphere and CO2(aq) in the oceans CANNOT be
regarded as a true dynamic equilibrium.
(1 mark)
The system is not closed.
(1)
b) Based on equations (1) and (2), explain the likely effect of the increasing concentration of atmospheric
carbon dioxide on the pH of water at the ocean surface.
(5 marks)
When the concentration of CO2(g) increases, the system responds by reducing the change.
(1)
The position of equilibrium of the first system will shift to the right.
(1)
The concentration of CO2(aq) will increase.
(1)
When the concentration of CO2(aq) increases, the position of equilibrium of the second system will shift to the right.
74
The concentration of hydrogen ions will increase.
(1)
Thus, the pH of the water at the ocean surface will decrease.
(1)
c) Use Le Chatelier’s principle to explain why slight cooling of the water saturated with carbon dioxide will
result in the precipitation of calcium carbonate.
(3 marks)
Slight cooling of the water saturated with carbon dioxide can reduce the solubility of carbon dioxide.
Some carbon dioxide bubbles out of the solution and the concentration of carbon dioxide decreases.
(1)
The system responds by reducing the change.
(1)
The position of equilibrium of the system represented by equation (3) shifts to the left,
(1)
Part B
resulting in the precipitation of calcium carbonate.
75
93 Ethanoic acid and ethanol react to form ethyl ethanoate and water in the presence of concentrated sulphuric
acid.
CH3COOH(l) + CH3CH2OH(l)
CH3COOCH2CH3(l) + H2O(l)
Describe briefly how you can carry out an experiment to determine the equilibrium constant, Kc, for the
esterification reation. Suggest also how you can calculate Kc from the experimental results obtained. (No
actual calculation is required.)
(For this question, you are required to give answers in paragraph form.)
(9 marks)
Mix equal number of moles of ethanoic acid and ethanol in a pear-shaped flask.
3
3
Withdraw 1.00 cm of this mixture and add to a conical flask containing 25 cm of distilled water.
(0.5)
3
Titrate the contents of the conical flask against standard sodium hydroxide solution. V1 cm of the alkali are required to
reach the end point.
(0.5)
Add a few drops of concentrated sulphuric acid to the acid-alcohol mixture in the pear-shaped flask.
(0.5)
Part B
3
3
Titrate 1.00 cm of the resulting mixture against standard sodium hydroxide solution. V2 cm of the alkali are required to
reach the end point.
(0.5)
Heat the mixture in the pear-shaped flask under reflux for 2 hours. After cooling, withdraw 1.00 cm3 of the mixture
and
(0.5)
3
titrate against standard sodium hydroxide solution. Vf cm of the alkali are required to reach the end point.
(0.5)
The concentration of CH3COOH(l) in the original mixture (Xi) can be obtained from V1 and the concentration of
NaOH(aq).
(0.5)
The concentration of CH3COOH(l) in the mixture after reflux (Xf) can be obtained from [Vf – (V2 – V1)] and the concentration
of NaOH(aq).
(0.5)
Change in concentration of CH3COOH(l) = Xf – Xi
According to the equation, 1 mole of CH3COOH(l) reacts with 1 mole of CH3CH2OH(l) to give 1 mole of CH3COOCH2CH3(l)
and H2O(l).
The concentrations of substances in the mixture after reflux are as follows:
[CH3CH2OH(l)] = Xf
[CH3COOCH2CH3(l)] = [H2O(l)] = Xf – Xi
[CH3COOCH2CH3(l)][H2O(l)]
Equilibrium constant (Kc) =
[CH3COOH(l)][CH3CH2OH(l)]
2
(Xf – Xi)
=
2
Xf
(1)
(1)
(3 marks for organization and presentation)
76
94 The hydrogen used in the Haber process is made by the following reaction:
CH4(g) + H2O(g)
CO(g) + 3H2(g)
ΔHO = +206 kJ
Discuss how the yield of hydrogen in the process is affected by changing the pressure, changing the temperature
and using a catalyst.
(For this question, you are required to give answers in paragraph form.)
•
(9 marks)
An increase in pressure will bring about a net reaction that decreases the number of moles of gas. This helps to reduce
the pressure.
(1)
A net backward reaction will occur. Thus, the yield of hydrogen will decrease.
(1)
OR
A decrease in pressure will bring about a net reaction that increases the number of moles of gas. This helps to increase
(1)
A net forward reaction will occur. Thus, the yield of hydrogen will increase.
(1)
When the temperature is increased, the system will respond by reducing the temperature.
(1)
Part B
•
the pressure.
As the production of hydrogen is endothermic, the system will undergo a net forward reaction. Thus, the yield of
hydrogen will increase.
(1)
OR
When the temperature is decreased, the system will respond by raising the temperature.
(1)
As the production of hydrogen is endothermic, the system will undergo a net backward reaction. Thus, the yield of
•
hydrogen will decrease.
(1)
The yield of hydrogen is not affected by a catalyst.
(1)
A catalyst increases the rates of both the forward reaction and the backward reaction to the same extent.
(1)
(3 marks for organization and presentation)
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