Equilibrium Written Response
January 1998
2. Consider the following equilibrium:
CS2 (g)
+ 3Cl2 (g)
4 (g)
+ S2Cl2 (g)

-238 kJ
a) Sketch a potential energy diagram for the reaction above and label H. (2
marks)
b) Some CS2 is added and equilibrium is then reestablished. State the direction of the
equilibrium shift and the resulting change in [Cl2]
(1 mark)
c)The temperature is decreased and equilibrium is then reestablished. What will the
effect be on the value of Keq ? (1 mark)
April 1998
2. State Le Chatelier’s Principle. (2 marks)
3. Consider the following equilibrium:
H2 (g)
2HI (g)
2 (g)
K eq
At equilibrium, the [H 2]
equilibrium
concentration of I2 ? (2 marks)
June 1998
3. Consider the following graph for the reaction:
2O4 (g)
2NO2 (g)
a) What is the stress imposed at time t1 ? (1 mark)
b) What is the stress imposed at time t3 ? (1 mark)
c) Calculate Keq for the equilibrium between t2 and t3 . (2 marks)
2
August 1998
2. Consider the following equilibrium:
H2 (g) + I2 (g)
2HI (g)
K eq
A 1.00 L container is initially filled with 0.180 mol HI. Calculate the concentration of HI
at equilibrium.
January 1999
2. Consider the following diagram for a chemical system containing three substances
represented by A, B and C:
a) What feature of the graph indicates that the system reaches equilibrium? (1 mark)
b) Write a balanced equation for the equilibrium reaction. (2 marks)
c) Calculate K eq at equilibrium. (2 marks)
April 1999
2. Describe how enthalpy and entropy change, in the forward direction, as an exothermic
reaction reaches equilibrium. Explain your reasoning. (2 marks)
3. Consider the graph below representing the following equilibrium:
CH3CH2CH2CH3 (g)
n-butane
CH3CH(CH3)2 (g)
isobutene
Data for the graph was obtained from various equilibrium mixtures.
Calculate the value of K eq for the equilibrium. (2 marks)
June 1999
2. Consider the following equilibrium system:
N2 (g)
2 (g)
3 (g)
A 1.00 L container is filled with 5.0 mol NH3 and the system proceeds to equilibrium as
indicated by the graph.
a) Draw and label the graph for N2 and H2 . (2 marks)
b) Calculate the K eq for N2 (g)
(2 marks)
3. State Le Chatelier’s Principle. (2 marks)
H2 (g)
3 (g)
+
August 1999
2. Consider the following equilibrium:
4HCl (g)
2O(g) +
2 (g)
2 (g)
a) How does the entropy change in the forward direction? Explain your reasoning.
(1 mark)
b) How does the enthalpy change in the forward direction? Explain your reasoning.
(1 mark)
3. Consider the following equilibrium:
H2 (g) +
2 (g)
–2
Keq
(g)
A 2.0 L flask is filled with 0.10 mol HI . Calculate the concentration of H2 at equilibrium.
(3 marks)
January 2000
3. Consider the following equilibrium:
HInd + H2O
(yellow)
H3O
+
+ Ind
(blue)
The system is yellow and turns blue on the addition of NaOH. In terms of the forward
and reverse reaction rates, explain why this shift occurs. (2 marks)
4. Consider the following equilibrium:
Fe
(aq)
+ SCN
-
(aq)
FeSCN
(aq)
Initially, 50.0mL of 0.10M Fe
is added to 30.0mL of 0.20M SCNAt equilibrium, the concentration of FeSCN2+ is found to be 0.050M.
Calculate the Keq for the reaction.
(4 marks)
April 2000
3. Methanol, CH3OH, is produced industrially by the following reaction:
CO(g) + 2H2(g)
CH3OH(g) + heat
a) State two different methods of shifting the equilibrium to the right. (1 mark)
b) In terms of rates, explain why these methods cause the equilibrium to shift to the right.
(1 mark)
4. Consider the following equilibrium:
H2(g)
+ I2(g)
2HI(g)
A 2.0L container is filled with 0.070 mol of H2 and 0.060 mol of I2. Equilibrium is
reached after 15.0 minutes at which time there is 0.060 mol of HI present.
Sketch and label the graphs for the changes in concentrations of H2, I2 and HI for the time
period of 0 to 30.0 minutes. (3 marks)
June 2000
2. Consider the observations for the following equilibrium:
N2O4(g)
2NO2(g)
(colourless)
(brown)
a) Sketch the potential energy curve on the graph below for this equilibrium.
(1 mark)
b) Explain the colour change using Le Chatelier’s Principle. (1 mark)
c) Other than changing temperature, what could be done to cause a shift to
the left? (1 mark)
3. Consider the data obtained for the following equilibrium:
Fe3+(aq) + SCN-(aq)
FeSCN2+(aq)
Calculate the [FeSCN2+] in experiment #2. (3 marks)
August 2000
2. Consider the following reaction:
Fe3+(aq)
(yellow)
+
SCN-(aq)
(colourless)
FeSCN2+(aq)
(red)
When a few drops of 6.0M NaOH is added to 25.0mL of the above system, a precipitate
of Fe(OH)3 forms and the solution turns pale yellow.
a) Explain this colour change in terms of Le Chatelier’s Principle. (2 marks)
b) Describe the effect on the rate of the reverse reaction as the color change occurs.
(1 mark)
3. Consider the following equilibrium:
3I2 (g) + 3F2 (g)
2IF2(g) + I4F2 (g)
Initially, 2.00 X 10-1 mol of I2 and 3.00 X 10-1 mol of F2 and put into a 10.00L flask. At
equilibrium, [I4F2] is 2.00 X 10-3 M. Calculate the Keq.
(4 marks)
January 2001
2. Consider the following equilibrium system:
2COF2(g)
CO2 (g) + CF4 (g)
Keq= 2.00
A 2.00 L container is filled with 0.500 mol of COF2. Calculate the [COF2] at
equilibrium. (5 marks)
3. Consider the following equilibrium system:
Cu2+(aq) +
blue
4Br - (aq)
colourless
CuBr42- (aq)
green
Cooling the equilibrium changes the colour from green to blue. What effect will
the decrease in temperature have on K eq ? Explain, using Le Chatelier’s Principle.
(2 marks)
April 2001
2. A flask is initially filled with some HI. At equilibrium, the [HI] = 0.80
mol/L. What is the [H2] at equilibrium? (3 marks)
2HI(g)
H2(g) + I2(g) Keq = 0.25
3. Consider the following equilibrium system:
2NOCl (g)
2NO (g) + Cl2 (g)
Keq= 1.6 X 10-5
A 1.00 L flask is filled with 0.20mol NOCl, 0.10mol NO and 0.10mol Cl2.
State and show by calculation the direction in which the reaction proceeds to reach
equilibrium. (4 marks)
June 2001
2. Write four statements that apply to all chemical equilibrium systems. (2 marks)
3. Consider the following equilibrium system:
H2 (g) + Br2 (g)
2HBr (g)
Keq= 14.8
A closed container was initially filled with equal moles of H2 and Br2.
When equilibrium is reached, the [HBr] is 0.329 mol/L. What was the initial [H2]?
(4 marks)
August 2001
2. Consider the following graph for the reaction:
H2 (g) + I2 (g)
2HI (g)
The temperature is increased at t1 and equilibrium is re-established at t2.
a) On the above graph, sketch the line representing the
(1 mark)
b) Calculate the value of Keq after t2 . (2 marks)
between time t1 and t3.
3. Consider the following equilibrium system:
2SO2 (g) + O2(g)
2SO3 (g)
A 1.00 L container is initially filled with 0.100mol SO2 0.100mol O2.
At equilibrium the O2 concentration is 0.060 mol/L. Calculate the value of Keq.
(4 marks)
January 2002
3. Consider the following equilibrium:
2NF2(g) ⇄ N2F4(g)
Equilibrium shifts to the right when volume is decreased. Describe the changes in
reaction rates that cause this shift to the right.
4. Consider the following:
H2(g) + I2(g) ⇄ 2HI(g)
Initially, 0.200 mol H2 and 0.200 mol I2 are added to an empty 2.00L container.
At equilibrium, the [I2] = 0.020 mol/L. What is the value of Keq?
April 2002
3. State Le Chatelier’s Principle.
4. Consider the following:
CH4(g) + H2O(g) ⇄ CO(g) + 3H2(g)
Initially, 0.060 mol CH4, 0.080 mol H2O, 0.280 mol CO and 0.740 mol H2 are
placed into a 4.00 L container. At equilibrium, the [H2] = 0.200 mol/L. What is
the value of Keq ?
June 2002
3. Chemical reactions tend toward a position of minimum enthalpy and
maximum entropy.
a) What is meant by the term enthalpy?
b) What is meant by the term entropy?
4. Consider the following:
H2(g) + Br2(g) ⇄ 2HBr(g)
Keq = 12.0
Initially, 0.080 mol H2 and 0.080 mol Br2 are placed into a 4.00 L container.
What is the [HBr] at equilibrium?
August 2002
3. Describe the nature of dynamic equilibrium.
4. Consider the following:
N2O4(g) ⇄ 2NO2(g)
Keq = 9.5 × 10-3
Initially, 0.060 mol N2O4 and 0.020 mol NO2 are placed into a 2.00 L container.
Use calculations to determine the direction in which the reaction proceeds in order
To reach equilibrium.
January 2003
3. Consider the following equilibrium:
CH4(g) + H2O(g) ⇄ CO(g) + 3H2(g)
Keq
Temperature
-3
1.78 × 10
800℃
4.68 × 10-2
1000℃
Is the forward reaction in this equilibrium exothermic or endothermic?
Explain your answer.
4. Consider the following equilibrium:
CO(g) + Cl2(g) ⇄ COCl2(g)
At equilibrium, the system contains 2.00 mol CO, 1.00 mol Cl2 and 0.200 mol COCl2
in a 2.0 L container. Calculate the value of Keq.
April 2003
3. Consider the following reaction for the Haber Process for ammonia production:
N2(g) + 3H2(g) ⇄ 2NH3(g)
∆H=-92 kJ
The system is normally maintained at a temperature of approximately 500℃.
a) Explain why 1000℃ is not used.
b) Explain why 100℃ is not used.
4. Consider the following equilibrium:
SO3(g) + NO(g) ⇄ NO2(g) + SO2(g)
In an experiment, 0.100 moles of SO3 and 0.100 moles of NO are placed in a 1.00L
container. When equilibrium is achieved, [NO2] = 0.0414 mol/L.
Calculate the Keq value.
June 2003
3. Consider the following exothermic reaction:
?
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
Explain, in terms of increasing or decreasing entropy and enthalpy, whether or not the
reaction will reach equilibrium.
4. Given the reacting system:
H2(g) + I2(g) ⇄ 2HI(g)
Keq = 64
Equal moles of H2, I2 and HI are placed in a 1.0L container. Use calculations to
determine the direction the reaction will proceed in order to reach equilibrium.
August 2003
3. Consider the following exothermic reaction:
H2(g) + I2 ⇄ 2HI(g)
The system is said to “shift right” as the result of the addition of extra H2(g).
Describe the sequence of changes in both forward and reverse reaction rates as the
system goes from the original equilibrium to the new equilibrium.
4. Consider the following equilibrium system:
2NO(g) + Cl2(g) ⇄ 2NOCl(g)
Keq = 8.5
A closed flask is found to contain 0.40 M NO(g), 0.32 M Cl2(g) and 5.6 M NOCl(g).
Use appropriate calculations to determine the direction the reaction proceeds to reach
equilibrium.
January 2004
2. Consider the following equilibrium system:
C(s) + 2H2(g) ⇄ CH4(g)
State three different ways to make more C(s) react.
∆H = -75 kJ
Written Response Answers
January 1998
April 1998
3.
June 1998
2.
August 1998
January 1999
2.
April 1999
3.
June 1999
August 1999
2. Solution
a) Entropy is decreasing. Five particles of gas (reactants) have more entropy than four
particles of gas (products).
(1 mark)
b) Enthalpy is decreasing. The reaction is exothermic, so the enthalpy of the
products is less than the enthalpy of the reactants. (1 mark)
3.
January 2000
3. Solution:
Addition of OH- decreases [H3O+ ], decreasing the reverse rate. Since the forward rate
is greater than the reverse rate, the system shifts to the right. (2 marks)
4.
April 2000
3. Solution:
a) Any two of the following: (1 mark)
~ adding reactant
~ removing methanol
~ decreasing the temperature
~ increasing the pressure by decreasing the volume
b) The shift occurs because rate (f) must be greater than rate (r) as a result of the stress.
(1 mark)










4.
June 2000
2.
3.
August 2000
2.
3.
January 2001
2.
3.
April 2001
3.
June 2001
August 2001
January 2002
3. Both forward and reverse rates increase as a result of increased concentration.
The forward rate increases more than the reverse rate, so the equilibrium shifts
to the right.
4.
April 2002
3. Le Chatelier’s Principle states that when a stress is placed on an equilibrium system,
the system will shift to offset this stress until a new equilibrium is reached.
4.
June 2002
3. a) Enthalpy is a measure of heat content.
b) Entropy is a measure of randomness.
4.
August 2002
3. In a dynamic equilibrium, the forward reaction and reverse reaction continue to
proceed at equal rates.
4.
January 2003
3. This equilibrium is endothermic.
Since Keq increases as a result of a temperature increase,
equilibrium has shifted to the right.
4.
April 2003
3. a) Equilibrium will be shifted to the left, reducing the yield of NH3.
b) The rate of the reaction would be too low.
4.
June 2003
3. Entropy increases in the forward reaction.
Enthalpy decreases in the forward reaction.
Since both favour products, equilibrium will not be attained; or the reaction will go to
completion.
4.
August 2003
3. As [H2] is increased the forward rate increases. The forward rate will be greater than
the reverse rate, resulting in more HI being produced.
The [H2] is consumed as the shift occurs and the forward rate starts to decrease.
The increasing [HI] results in an increasing reverse rate.
At the new equilibrium the forward and reverse rates will be equal.
4.
January 2004
2. Any three of the following:
- add H2
- remove CH4
- decrease temperature
- increase pressure/decrease volume