Chemical Kinetics Chemistry Test Section Answers
Multiple Choice (1 mark each K/U)
K/U
/18
1. The rate of a chemical reaction between substances A and B is found to follow the rate
equation rate=k[A]2[B] where k is constant. If the concentration of A is halved, what should
be done to the concentration of B to make the reaction go at the same rate as before?
a. The concentration of B should be kept constant
b. The concentration of B should be doubled
c. The concentration of B should be halved
d. The concentration of B should be quadrupled
Rate= k [1/2A]2 [4B]
Rate= k (1/2)2 [A]2 [4B]
Rate= k (1/4) [A]2[B]4
Rate= k [A]2[B]
2. The rate expression for an overall second-order reaction could be
a. rate=k[A] b. rate=k[A]2[B] c. rate=k[A][B]
d. rate=k[A]2[B]2
3. For a first-order reaction of half-life, 150 min, what is the rate constant in min -1?
a. 0.00104
b. 0.00462
c. 69.3
d. 216
kt1/2=0.693
k (150 min)= 0.693
k= 0.693 / 150 min
k= 0.00462 min-1
4. The half-life of 14C is 5570 years. How many years will it take for 90% of the sample to
decompose?
a. 5,570 years b. 17,740 years
c. 18, 570 years
d. 50, 150 years
kt1/2=0.693
Half-life for first order reactions
k(5570 years) =0.693
ln [A]t /[A]o = -kt
k= 0.693/ 5570 years
ln [10] / [100]= -(1.24x10-4 years-1) t
-1
k=0.000124 years
-2.30= -(1.24x10-4 years-1) t
k= 1.24 x 10 -4 years-1
t= -2.30 /-(1.24x10-4 years-1)
t= 18, 570 years
2
5. For a certain reaction the rate law is, rate=k[C] . If the rate of the reaction is 0.020M•s-1 when
[C]=1.0 M, what is the rate when [C] = 0.60 M?
a. 0.0093 M•s-1 b. 0.020 M•s-1
c. 0.0072 M•s-1
d. 0.012 M•s-1
2
2
rate=k[C]
rate =k[C]
-1
2
0.020 M•s =k[1.0 M]
rate= (0.020M-1 s-1)[0.60M]2
-1
2
k=0.020 M•s / 1.0 M
rate= (0.020M-1 s-1)(0.36 M2)
-1 -1
k= 0.020 M •s
rate= 0.0072 M•s-1
6. The rate constant of a reaction is generally expected to
a. Be independent of temperature
b. Decrease with increasing temperature
c. Increase with increasing temperature
d. Increase with increasing temperature, only if the reaction is endothermic
The rate of a reaction increases with increasing temperature. Therefore the rate constant must
increase with increasing temperature if all other factors are held constant.
7. The rate of a second order decomposition of a substance, A, is 2.7 M-1•s-1 when the initial
concentration of the reactant A is 0.60 M. What is the initial rate of the reaction when the
initial concentration of the reactant is 0.20 M?
a. 0.30 M•s-1
b. 0.60 M•s-1
c. 0.90 M•s-1
d. 2.7 M•s-1
2
rate=k [A]
rate= k [A]2
-1 -1
2.7 M •s =k [0.60 M]2
rate= 7.5 M-3•s-1 [0.2 M]2
2.7 M-1•s-1=k (0.60 M2)
rate= 7.5 M-3•s-1[0.04 M2]
-1 -1
2
k= 2.7 M •s / (0.36 M )
rate= 0.3 M•s-1
-3 -1
k= 7.5 M •s
8. The slowest step of a reaction is called the
a. Elementary process
c. Order
b. Molecularity
d. Rate-determining step
9. During an exothermic reaction, heat passes to the surrounding environment because
a. the potential energy of the reaction increases
b. the kinetic energy of the reaction decreases
c. the potential energy is changed into heat
d. the kinetic energy is converted into heat
c.
10. The term “specific heat capacity” refers to
a. the amount of heat needed to raise the temperature of one mole of a substance by 1°
b. the amount of heat needed to raise the temperature of one gram of a substance by 1°
Celsius or Kelvin.
c. the amount of heat needed to raise the temperature of one molecule of a substance by 1°
Celsius or Kelvin.
d. the amount of heat needed to raise the temperature of a substance by 1° Celsius or Kelvin.
b.
11. The rate of reaction is typically determined by measuring
a. the rate that products are formed
b. the rate that reactant are produced
c. the rate that reactants are consumed
d. the volume and density of the solution
c.
12. TYPICALLY raising the temperature affects the rate of reaction by
a. reducing the rate of reaction
b. increasing the rate of reaction
c. leveling the pressure
d. does not affect the rate of reaction
b.
13. Which of the following is NOT a factor that affects the rate of reaction?
a. concentration of reactants
b. number of products formed
c. surface area
d. temperature
b.
14. A catalyst increases the rate of reaction which of the following ways
a. changing the products formed in the reaction
b. increasing the enthalpy change
c. increasing the activation energy
d. decreasing the activation energy
d.
15. A catalyst can only be effective on a reaction if
a. it is added in large quantities
b. is added before reactants come into contact
c. it is specific to the particular reaction
d. is heated beforehand
c.
16. The energy required for a reaction to start is referred to as
a. enthalpy change
b. reaction energy
c. activation energy
d. kinetic energy
c.
17. The “rate determining step” is the
a. first step in a reaction mechanism
b. last step in a reaction mechanism
c. slowest step in a reaction mechanism
d. fastest step in a reaction mechanism
c.
18. Endothermic reactions occur when
a. heat is transferred from surrounding environment into a system
b. heat is transferred from a system into the surrounding environment
c. no heat is transferred
d. all potential energy is transferred into kinetic
a.
Short Answer and Calculation Questions:
19. Explain why ΔHsystem = -qsurroundings
COMM
:
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Total amount of energy in the system has to remain constant, therefore the enthalpy change
must be equal, but possess an opposite sign to the energy released/absorbed from the
surroundings.
20. Name two ways in which the rate of change of the product is related to the rate of change of
the reactant.
COMM
:
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- rates of changes have opposite signs / reactant decreases and product increases
- magnitude of change not necessarily equal – depends on the ratio of the coefficients
(determined in a balanced reaction)
21. If 335g of water at 24.5°C absorbed 2.66 kJ of energy, what is the final temperature of the
water?
APP:
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m = 335g
q = 2.66 kJ
= 2660 J
c = 4.18 J/ (g/°C)
T1= 24.5°C
ΔT = q / mc
= 2660 / (355)(4.18)
= 1.90°C
T2 = T1 + ΔT
= 24.5 + 1.9
= 26.4°C
22. Determine the molar heat of vaporization of water.
Hint: Make sure you show how you change any reactions.
APP:
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H2(g) + ½ O2 (g)  H2O(g) + 242.8 kJ
H2(g) + ½ O2 (g)  H2O(l) + 286.9 kJ
H2(g) + ½ O2 (g)  H2O(g) + 242.8 kJ
ΔHf = -242.8 kJ/mol
H2(g) + ½ O2 (g)  H2O(l) + 286.9 kJ
ΔHf = +286.9 kJ/mol
------------------------------------------------------------------------------------H2O(l)  H2O(g)
ΔHvap = 44.1 kJ/mol
23. The following initial rate data was obtained for the reaction
2ClO2 (aq) + 2OH-(aq)  ClO3-(aq) + ClO2-(aq) + H2O
[ClO2]
[OH-]
Initial Rate
M•s-1
0.024
0.018
0.024
APP:
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APP:
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0.011
0.011
0.014
7.27 x 10-4
4.09 x 10-4
9.25 x 10-4
a. Write the rate law for this reaction. Show calculations.
Rate=k[ClO2]m[OH-]n
Run 1 7.27 x 10-4= k[0.024]m[0.011]n
Run 2 ÷ 4.09 x 10-4= k[0.018]m[0.011]n
1.77= 1.33m
m= 2
Therefore Rate=k[ClO2]2[OH-]
Run 3
9.25 x 10-4 =k[0.024]2[0.014]n
Run 1 ÷ 7.27 x 10-4 =k[0.024]2[0.011]n
1.27= 1.27n n= 1
b. Calculate the specific rate constant. Specify units.
Rate=k[ClO2]2[OH-]
7.27 x 10-4 M•s-1=k[0.024 M]2[0.011 M]
7.27 x 10-4 M•s-1=k [6.336 x 10-6 M3]
k = 7.27 x 10-4 M•s-1 / 6.336 x 10-6 M3
k= 115 M-2•s-1
c. Calculate the rate when [ClO2]= 0.75 M and [OH-]= 0.0250 M
APP
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Rate=k[ClO2]2[OH-]
Rate = 115 M-2•s-1[0.75 M]2[0.0250 M]
Rate = 115 M-2•s-1[0.0.0141 M3]
Rate = 1.62 M•s-1
24. Answer the following four questions relating to the Collision Theory (2 marks each
Communication)
a. Why is initial instantaneous rate critical for determining the rate law expression?At any
time other than t=0, both reverse and forward reactions are occurring and the calculated
rate will be the net rate, or the difference between forward and reverse reactions, not the
actual rate of the forward reaction.
b.
How does temperature affect the reaction rate (in terms of collision theory)?Particles
COMM
have more kinetic energy, so they will collide more often and with greater kinetic energy.
/2
c. How does concentration affect the reaction rate (in terms of collision theory)?It increases
the collision frequency.
d. Explain reaction rate in terms of bond energy. It is the rate at which collisions occur that
have sufficient energy to break the bonds that must be broken to form the activated
complex.
25. The oxidation of SO2 to SO3 is catalyzed by NO2. The reaction proceeds as follows:
NO2 (g) + SO2 (g)  NO(g) + SO3 (g)
2NO (g) + O2(g)  2NO2 (g)
a. Show tat the two reactions can be summed to give the overall oxidation of SO 2 by O2 to
APP
give SO3.
/2
2SO2 + O2  2SO3
COMM
/2
b. Why do we consider NO2 as a catalyst and not an intermediate?
It is present at the start and at the end. It is not used up in the reaction or formed in
the middle of the reaction.
c. Is this an example of a homogeneous catalysis or a heterogeneous catalysis?
Homogeneous
K/U
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26. The following mechanism has been proposed:
NO(g) + NO(g)  N2O2 (g)
N2O2(g) + H2 (g)  N2O(g) + H2O(g)
a.
Show
that the elementary steps of the proposed mechanism provide a balance equation
APP
for the reaction.
/2
2NO2 (g) + H2(g)  N2O (g) + H2O (g)
COMM
/2
K/U
/2
b. Identify any intermediates.
N2O2
c. The observed rate law is rate=k[NO]2[H2]. If the proposed mechanism is correct, what
can we conclude about the relative speeds of the first and second steps? Explain your
answer.
The second step is the slow step. The proposed rate law for the first equation would be
rate=k[NO]2 and then for the second reaction it would be rate=k[N2O2][H2]. However, N2O2
is an intermediate that was formed by combining 2 NO molecules so the rate law for the
second equation would now become rate=k[NO]2[H2]. As mentioned in the question that the
observed rate for the overall proposed mechanism is rate=k[NO]2[H2] which is the same rate
as the second step, we can determine that the second step is the rate limiting step.
27. What is the enthalpy change for the formation of one mole of butane (C4H10) gas from its
elements? The reaction is:
�ΔH° �= ?
The following known equations, determined by calorimetry, are provided:
APP
/8
( 1 ) C4 H10(g)
+ 13/2� O2  4 CO2(G) + 5H2O (g)
�
ΔH ° = -2657.4kJ
ΔH°3 = -483.6kJ
(Hint: Be sure to show the ΔH for each reaction equation)
-1 x (1): 4CO2(g) + 5H2O(g)  C4H10(g) + 13/2O2(g) ΔH ° = -1(-657.4)kJ
4 x (2): 4C(s) + 4O2(g)  4 CO2(g)
ΔH ° = 4(-393.5)kJ
5/2 x (3) 5H2(g) + 5/2O2(g)  5 H2O(g)
ΔH ° = 5/2 (-483.6)kJ
------------------------------------------------------------------------------------------4CO2(g) + 5H2O(g) +4C(s) = 13/2O2(g) +5H2(g)  C14H10(g) = 13/2O2(g) + 4CO2(g) + 5H2O(g)
or
4C(s) + 5H2(g)  C4H10(g)
ΔH °total = (+2657.4) + (-1574.0) + (-1209.0)kJ
ΔH °total = -125.6KJ
Chemical Kinetics Chemistry Test Section
Multiple Choice (1 mark each K/U)
1. The rate of a chemical reaction between substances A and B is found to follow the rate
equation rate=k[A]2[B] where k is constant. If the concentration of A is halved, what
K/U
should be done to the concentration of B to make the reaction go at the same rate as
/8
before?
a. The concentration of B should be kept constant
b. The concentration of B should be doubled
c. The concentration of B should be halved
d. The concentration of B should be quadrupled
2. The rate expression for an overall second-order reaction could be
a. rate=k[A]
b. rate=k[A]2[B]
c. rate=k[A][B]
d. rate=k[A]2[B]2
3. For a first-order reaction of half-life, 150 min, what is the rate constant in min -1?
a. 0.00104
b. 0.00462
c. 69.3
d. 216
4. The half-life of 14C is 5570 years. How many years will it take for 90% of the sample to
decompose?
a. 5,570 years b. 17,740 years
c. 18, 570 years
d. 50, 150 years
5. For a certain reaction the rate law is, rate=k[C]2. If the rate of the reaction is 0.020M•s-1
when [C]=1.0 M, what is the rate when [C] = 0.60 M?
a. 0.0093 M•s-1 b. 0.020 M•s-1
c. 0.0072 M•s-1
d. 0.012 M•s-1
6. The rate constant of a reaction is generally expected to
a. Be independent of temperature
b. Decrease with increasing temperature
c. Increase with increasing temperature
d. Increase with increasing temperature, only if the reaction is endothermic
7. The rate of a second order decomposition of a substance, A, is 2.7 M-1•s-1 when the initial
concentration of the reactant A is 0.60 M. What is the initial rate of the reaction when the
initial concentration of the reactant is 0.20 M?
a. 0.30 M•s-1
b. 0.60 M•s-1
c. 0.90 M•s-1
d. 2.7 M•s-1
8. The slowest step of a reaction is called the
a. Elementary process
b. Molecularity
c. Order
d. Rate-determining step
9. During an exothermic reaction, heat passes to the surrounding environment because
a. the potential energy of the reaction increases
b. the kinetic energy of the reaction decreases
c. the potential energy is changed into heat
d. the kinetic energy is converted into heat
10. The term “specific heat capacity” refers to
a. the amount of heat needed to raise the temperature of one mole of a substance by 1°
b. the amount of heat needed to raise the temperature of one gram of a substance by 1°
Celsius or Kelvin.
c. the amount of heat needed to raise the temperature of one molecule of a substance by 1°
Celsius or Kelvin.
d. the amount of heat needed to raise the temperature of a substance by 1° Celsius or Kelvin.
11. The rate of reaction is typically determined by measuring
a. the rate that products are formed
b. the rate that reactant are produced
c. the rate that reactants are consumed
d. the volume and density of the solution
12. TYPICALLY raising the temperature affects the rate of reaction by
a. reducing the rate of reaction
b. increasing the rate of reaction
c. leveling the pressure
d. does not affect the rate of reaction
13. Which of the following is NOT a factor that affects the rate of reaction?
a. concentration of reactants
b. number of products formed
c. surface area
d. temperature
14. A catalyst increases the rate of reaction which of the following ways
a. changing the products formed in the reaction
b. increasing the enthalpy change
c. increasing the activation energy
d. decreasing the activation energy
15. A catalyst can only be effective on a reaction if
a. it is added in large quantities
b. is added before reactants come into contact
c. it is specific to the particular reaction
d. is heated beforehand
16. The energy required for a reaction to start is referred to as
a. enthalpy change
b. reaction energy
c. activation energy
d. kinetic energy
17. The “rate determining step” is the
a. first step in a reaction mechanism
b. last step in a reaction mechanism
c. slowest step in a reaction mechanism
d. fastest step in a reaction mechanism
18. Endothermic reactions occur when
a. heat is transferred from surrounding environment into a system
b. heat is transferred from a system into the surrounding environment
c. no heat is transferred
d. all potential energy is transferred into kinetic
Short Answer and Calculations:
COMM
:
/1
19. Explain why ΔHsystem = -qsurroundings
20. Name two ways in which the rate of change of the product is related to the rate of change of
the reactant.
COMM
:
/2
21. If 335g of water at 24.5°C absorbed 2.66 kJ of energy, what is the final temperature of the
water?
APP:
/3
22. Determine the molar heat of vaporization of water.
Hint: Make sure you show how you change any reactions.
H2(g) + ½ O2 (g)  H2O(g) + 242.8 kJ
H2(g) + ½ O2 (g)  H2O(l) + 286.9 kJ
APP:
/2
23. The following initial rate data was obtained for the reaction
2ClO2 (aq) + 2OH-(aq)  ClO3-(aq) + ClO2-(aq) + H2O
[ClO2]
[OH-]
Initial Rate M•s-1
0.024
0.018
0.024
0.011
0.011
0.014
7.27 x 10-4
4.09 x 10-4
9.25 x 10-4
a. Write the rate law for this reaction. Show calculations.
APP:
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b. Calculate the specific rate constant. Specify units.
APP:
/5
APP:
c. Calculate the rate when [ClO2]= 0.75 M and [OH-]= 0.0250 M
/2
24. Answer the following four questions relating to the Collision Theory (2 marks each)
a. Why is initial instantaneous rate critical for determining the rate law expression?
COMM
b. How does temperature affect the reaction rate (in terms of collision theory)?
/8
c. How does concentration affect the reaction rate (in terms of collision theory)?
d. Explain reaction rate in terms of bond energy.
25. The oxidation of SO2 to SO3 is catalyzed by NO2. The reaction proceeds as follows:
NO2 (g) + SO2 (g)  NO(g) + SO3 (g)
2NO (g) + O2(g)  2NO2 (g)
a. Show that the two reactions can be summed to give the overall oxidation of SO2 by
O2 to give SO3.
APP
/2
b. Why do we consider NO2 as a catalyst and not an intermediate?
COMM
/2
c. Is this an example of a homogeneous catalysis or a heterogeneous catalysis?
K/U
/2
26. The following mechanism has been proposed:
NO(g) + NO(g)  N2O2 (g)
N2O2(g) + H2 (g)  N2O(g) + H2O(g)
a. Show that the elementary steps of the proposed mechanism provide a balance
equation for the reaction.
APP
/2
b. Identify any intermediates.
K/U
/1
c. The observed rate law is rate=k[NO]2[H2]. If the proposed mechanism is correct, what
can we conclude about the relative speeds of the first and second steps? Explain your
answer.
COMM
/5
APP
/8
27. What is the enthalpy change for the formation of one mole of butane (C4H10) gas from its
elements? The reaction is:
�ΔH° �= ?
The following known equations, determined by calorimetry, are provided:
( 1 ) C4 H10(g)
+ 13/2� O2  4 CO2(G) + 5H2O (g)
�
(Hint: Be sure to show the ΔH for each reaction equation)
ΔH ° = -2657.4kJ
ΔH°3 = -483.6kJ