CHEMICAL EQUILIBRIUM UNIT REVIEW
(Chemistry 12)
I.
Multiple Choice (60 marks)
1. 6. Consider the following reversible reaction:
Fe3+ (aq) + SCN-(aq)  FeSCN2+ (aa)
A solution of Fe(NO3)3 (aq) is added to a solution of KSCN. Which one of the following statements
describes the changes in forward and reverse reaction rates as the reaction moves towards
equilibrium?
A. Forward and reverse rates increase.
B. Forward and reverse rates decrease.
C. Forward rate increases and reverse rate decreases.
D. Forward rate decreases and reverse rate increases.
2. A system at equilibrium is said to be dynamic because at equilibrium the:
A. temperature does not change.
B. macroscopic properties are constant.
C. forward and reverse reactions continue to occur.
D. concentrations of reactants and products are constant
3. Consider the following equilibrium system:
CaCO3 (s)  CaO (s) + CO2 (g)
Which one of the following changes would cause the above system to shift left?
A. Add more CaO
B. Remove CaCO3 .
C. Decrease volume
D. Increase surface area of CaO
4. Consider the above concentration versus time graph for the equilibrium:
At time = “ t ”, which one of the following stresses occurred?
A. Catalyst was added.
B. Pressure was changed.
C. Temperature was changed.
D. Concentration of NO2 was changed.
5.
6. Hydrogen gas dissociates into atomic hydrogen as follows:
H2 (g)  2 H (g) ; Keq = 1.2 x 10-71
The value of the equilibrium constant for the above system indicates that:
A. the reaction rate is very slow.
B. the equilibrium is exothermic.
C. reactants are favoured at equilibrium.
D. a catalyst is necessary to establish equilibrium.
7. Consider the following equilibrium system: CO (g) + Cl2 (g)  COCl2 (g)
At equilibrium, a 2.0 litre sample was found to contain 1.00 mol CO, 0.500 mol Cl2 and 0.100 mol
COCl 2. The Keq value for the above system is:
A. 0.40
B. 0.20
8.
C. 2.5
D. 5.0
9.
10.
11.
12.
13.
14.
15. Macroscopic properties become constant in an equilibrium system when:
A. all reactions have stopped.
B. the reactants are completely used up.
C. maximum enthalpy has been reached.
D. forward and reverse reaction rates are equal.
16. In which of the following systems would the tendencies toward minimum enthalpy and maximum
entropy be in opposition to each other?
17.
18. Which of the following reactions will shift left when pressure is increased and when temperature is
decreased?
19. The value of the equilibrium constant will change when:
A. a catalyst is used.
B. temperature changes.
C. product concentrations change.
D. the volume of a gaseous system changes.
20. Products are favoured in an equilibrium reaction when the:
A. reaction is endothermic.
B. equilibrium constant is large.
C. macroscopic properties are constant.
D. activation energy of the forward reaction is high.
21. Consider the following equilibrium system at 900C:
Initially 5.0 moles of H2O and 4.0 moles of CO were reacted. At equilibrium, it is found that
2.0 moles of H2 are present. How many moles of H2O remain in the mixture?
A. 1.0 moles
B. 2.0 moles
C. 3.0 moles
D. 4.0 moles
22. In which of the following reactions does the tendency towards minimum enthalpy and maximum
entropy oppose each other?
23. Consider the following equilibrium system at 25C:
2 SO2 (g) O2 (g)2 SO3 (g)
At equilibrium, [SO2]is 4.00 10-3 mol / L, [O2]is 4.00 10 mol / L and [SO3]is
2.33 10 mol / L. From this data, the Keq value for the above system is:
A. 6.85 10
B. 1.18 10
C. 84.8
D. 146
24. Consider the following equilibrium:
Which of the following will increase the solubility of ammonium chloride?
C. adding more NH4Cl
D. increasing the temperature
A. stirring the solution
B. adding more water
25. A saturated NaCl
nature of:
(aq)solution
A. solidifying and melting.
B. crystallizing and dissolving.
is an example of an equilibrium system because of the reversible
C. evaporating and condensing.
D. crystal structure and bond energy.
26. In which of the following systems will the factors of entropy and enthalpy both favour the
reactants?
27. Consider the following equilibrium system:
Which of the following will not shift the equilibrium to the right?
A. adding more O2
B. adding a catalyst
C. increasing the pressure
D. lowering the temperature
28. For an exothermic reaction at equilibrium, an increase in temperature will cause the equilibrium
to shift:
A. left and Keq increases.
B. left and Keq decreases.
C. right and Keq increases
D. right and Keq decreases.
29. Consider the following equilibrium system:
Which equation compares the concentration of oxygen and ozone?
30. Identify the equilibrium system that least favours the formation of products.
31. Consider the following equilibrium:
How will the forward and reverse equilibrium reaction rates change when additional H2 is added to the
system?
32. Consider the following system at equilibrium:
This equilibrium will shift right as the result of the addition of some extra H2O.
How will this shift affect the concentrations of the other gases?
33. Consider the following equ ilibrium:
Which of the following graphs shows the result of increasing the
temperature at time t1 ?
34. Consider the following equilibrium:
What is the final result of adding some NH3 gas to the system at constant volume?
A. Keq increases.
B. [H2] decreases.
C. [NH3] decreases.
D. Keq remains unchanged.
35. Consider the following equilibrium:
Which of the factors below would increase the concentration of CH3OH at equilibrium?
A. an addition of CO
B. an increase in the volume
C. a decrease in the pressure
D. an increase in the temperature
36. Consider the following equilibrium:
If the volume of the system is decreased, how will the reaction rates in the new equilibrium compare with the
rates in the original equilibrium?
37. Consider the following equilibrium:
Which of the following would allow you to conclude that the system has reached equilibrium?
A. The pressure remains constant.
B. The reaction rates become zero.
C. The colour intensity remains constant.
D. The system shifts completely to the right.
38. Consider the following equilibrium system:
In which direction will the equilibrium shift and what happens to the value of Keq when the temperature of
the system is increased?
39. Consider the following equilibrium:
Identify the equilibrium constant expression.
40. Consider the following:
Initially, NH3 is added to an empty flask. How do the rates of the forward and
reverse reactions change as the system proceeds to equilibrium?
41. Consider the following:
What positions do minimum enthalpy and maximum entropy tend toward?
42.
Which of the following stresses will not cause a shift in equilibrium?
A. decrease [I2]
C. decrease volume
B. increase [H2]
D. increase temperature

43. Referring to the equilm in question 42, which of the following shows the reverse rate of reaction when
the volume is decreased at time t 1 ?
44. Consider the following:
Initially, some SO3 is placed into a 3.0 L container. At equilibrium there
is 0.030 mol SO present. What is the [O2]at equilibrium?
A. 0.0050 mol/ L
B. 0.010 mol /L
C. 0.015 . mol/ L
D. 0.030 . mol/ L
45. Which reaction has the following equilibrium expression?
46. What will cause the value of Keq for an exothermic reaction to decrease?
A. increasing the pressure
B. increasing the temperature
47.
C. decreasing the temperature
D. decreasing the surface area
48.
49.
Consider the following equilibrium:
The temperature of the equilibrium system is increased and a new equilibrium is established.
The rates of the forward and reverse reactions for the new equilibrium compared to the original
equilibrium have
50. Consider the following reaction:
The equilibrium constant expression for the reaction is
51. Consider the following equilibrium:
52. Consider the following equilibrium:
When a small amount of solid C is added to the system,
A. [H2]decreases.
B.[CH4]increases.
C. the temperature increases.
D. all concentrations remain constant.
53. Consider the following equilibrium:
When 2.0 mol of O2 and 3.0 mol of N2 were placed in a 10.0 L container at 25C, the value
of Keq 0.90 . If the same number of moles of reactant were placed in a 5.0 L container at 25C,
the equilibrium constant would be
A. 0.011
B. 0.45
C. 0.90
D. 1.80
54. Consider the following equilibrium:
Predict what will occur when 2.0 mol of PCl5, 3.0 mol of PCl3 and 4.0 mol of Cl2 are placed
in a 1.0 L container and allowed to establish equilibrium.
55. Consider the following equilibrium:
A 1.0 L container is initially filled with 2.0 mol of SO2Cl2 . As the reaction proceeds towards
equilibrium, the rate of the forward reaction
56. When the temperature of an equilibrium system is increased, the equilibrium always shifts to
favour the:
A. exothermic reaction.
B. endothermic reaction.
C. formation of products.
D. formation of reactants.
57. Consider the following equilibrium:
58. Consider the following:
A 1.00 L flask is initially filled with 2.00 mol C and 0.500 mol O2 . At equilibrium, the [O2]
is 0.250 mol/ L. The Keq value is:
A. 0.444
B. 1.00
C. 2.00
D. 2.25
59.
60. Which of the following best describes the relationship between Keq and temperature
for an endothermic reaction?
II.
Problem Solving (31 marks)
1. 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 Keq at equilibrium. (2 marks)
2. Consider the following equilibrium system: H2 (g) + I2 (g)
2 HI (g)
The system is said to “shift right” as the result of the addition of extra H2.
Describe the sequence of changes in both forward and reverse reaction rates as the system goes from the
original equilibrium to the new equilibrium.
(3 marks)
3. Consider the following equilibrium system:
A closed flask is found to contain 0.40 M NO, 0.32 M Cl2 and 5.6 M NOCl. Use appropriate calculations to
determine the direction the reaction proceeds to reach equilibrium.
(3 marks)
4. In an experiment, 0.200 mol of CO g and 0.400 mol of O2 g are placed in a 1.00 L container and the
following equilibrium is achieved:
At equilibrium, the [CO2]is found to be 0.160 mol /L. Calculate the value of Keq .
(3 marks)
5. Consider the observations for the following equilibrium:
a) Sketch the potential energy curve 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)
6. Consider the data obtained for the following equilibrium:
Calculate the [FeSCN2+] in experiment #2. (3 marks)
7. The production of ammonia by the Haber process involves the following equilibrium:
The table below indicates the percentage of ammonia in equilibrium mixtures at various
temperatures.
a) Explain why the lower temperature results in a higher percentage of ammonia in the
equilibrium mixture. (1 mark)
b) Explain why a temperature of 500C is used in the Haber process rather than a lower
temperature. (1 mark)
8. Consider the graph below representing the following equilibrium:
Data for the graph was obtained from various equilibrium mixtures.
Calculate the value of Keq for the equilibrium. (2 marks)
9. Consider the following graph for the reaction:
The temperature is increased at t1 and equilibrium is re-established at t2.
a) On the above graph, sketch the line representing the [HI] between time t1 and t3. (1 mark)
b) Calculate the value of Keq after t2. (2 marks)
10. Consider the following graph for the reaction:
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)
Answers:
I. Multiple Choice
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
III.
D
C
C
C
C
C
A
A
A
A
A
B
C
A
D
16. A
17. A
18. C
19. B
20. B
21. C
22.C
23.C
24.D
25.B
26.A
27.B
28.B
29.A
30.A
31.A
32.C
33.A
34.D
35.A
36.A
37.C
38.D
39.A
40.C
41.A
42.C
43.C
44.A
45.A
46.B
47.B
48.B
49.A
50.A
51.B
52.D
53.C
54.B
55.C
56.B
57.C
58.B
59.C
60.C
Problem Solving
1.
a) [ ] becomes constant
b) A
2B + C
c) Keq = 0.053
2. [H2] increases, so rate forward increases. But as H2 getting used up forward rate begins to decline and shift occurs. [HI] begins
to climb and therefore, reverse rate begins to speed up. When equi m is reached, reverse and forward rates are identical.
3. Shift to reactants.
4. Keq = 5.0 x 101
5.
a)
b) As heat increases, rxn. shifts away from introduced stress, and moves to products side.
c) Add more NO2. Or increase pressure.
6.
[FeSCN2+] = 6.73 x 10-4 M
7.
a) Le Chatelier’s Principle states that lowering temp. on an endo. Rxn. causes the equil m to shift towards products.
b) A colder temp. would slow down rate of rxn. too much.
8.
Keq = 1.5
9.
a) [HI] equilm = 1.0 M
b) Keq = 20
10. a) Increase temp.
b) Increase [NO2]
c) Keq = 1.8