CHAPTER 6.0 CHEMICAL EQUILIBRIUM

CHAPTER 6.0

CHEMICAL EQUILIBRIUM

At the end of this topic, students should be able to:

6.1 Dynamic a) Explain the following terms:

equilibrium i. reversible reaction;

ii. dynamic equilibrium; and

iii. law of mass action.
b) State the characteristics of a system in equilibrium.

c) Interpret the curve of concentration of reactants and products against

time for a reversible reaction.

d) Sketch the graph of concentration of reactants and products against
time for a reversible reaction.

6.2 Equilibrium a) Define

constants i. homogeneous equilibria; and

ii. heterogeneous equilibria.

b) Write expressions for equilibrium constants in terms of concentration, Kc
and partial pressure, Kp for homogeneous and heterogeneous systems.
c) Use the equation, Kp = Kc (RT)Δn to solve equilibrium problems.
d) Calculate Kc, Kp and the quantities of species present at equilibrium.

e) Determine the degree of dissociation, .

f) Predict the direction of net reaction by comparing the values of reaction

quotient, Q and K.

6.3 Le Chatelier’s a) State Le Chatelier’s principle.

principle b) Explain the effect of the following factors on a system at equilibrium using
Le Chatelier’s Principle.

i. concentration of reacting species;

ii. pressure by changing volume;
iii. addition of inert gas at constant volume;

iv. addition of inert gas at constant pressure;

v. temperature; and

vi. catalyst.
(Experiment 5: Chemical Equilibrium)

CHEMISTRY UNIT KMNS, SESSION 2022 / 2023 43

HOUR : 1

1. a) Determine whether the following reactions are homogeneous or heterogeneous.

i) 2PCl3(g) + O2(g) 2POCl3(g)
ii) FeO(s) + CO(g) Fe(s) + CO2(g)

iii) Ag+(aq) + Fe2+(aq) Ag(s) + Fe3+(aq)

iv) 3Fe(s) + 4H2O(l) Fe3O4(s) + 4H2(g)
b) Write the equilibrium law for the above reactions in terms of concentration, Kc and partial
pressure, Kp. (CLO1, C2)

2. Nitrogen (IV) dioxide dimerised as follows,

2NO2(g) N2O4(g) H = –58.84 kJ

Sketch a graph showing how the concentrations of the reactant and product of the above reaction

vary during the course of the reaction. PSPM 2006 / 2007 (CLO3, C3)

3. At equilibrium, there are 2.50 moles of SO2, 1.35×10−5 moles of O2 and 8.70 moles of SO3 present
in a 12.0 L flask.

Calculate the equilibrium constant, Kc for the reaction at 700 °C. (1.08 ×107)
2SO2(g) + O2(g) 2SO3(g)
(CLO3, C4)

4. At 25.0 °C, the value of Kp for the reaction
2NO2(g) N2O4(g)
is 7.13. At equilibrium, the partial pressure of NO2 in a container is 0.150 atm.
What is the partial pressure of N2O4 in the mixture? (0.160 atm) (CLO3, C4)

5. a) At 25.0 °C, H2O(l) reached equilibrium according to the equation,
H2O(l) H2O(g)
Calculate Kp and Kc for the reaction.
[vapour pressure of water at 25.0 °C = 23.8 torr] (0.0313, 1.28 ×10–3)

(CLO3, C3)

b) Decomposition of ammonium hydrogen sulphide, NH4HS, on heating in a sealed flask in an
endothermic process.

NH4HS(s) H2S(g) + NH3(g)
If Kp for this reaction is 0.11 at 25 °C, determine the partial pressure of NH3 and total pressure
in the flask (atm) at equilibrium.
Based on the Kp value, calculate Kc for this reaction at the same temperature.
(0.33 atm, 0.66 atm, Kc = 1.84 x 10 -4)
PSPM 2012 / 2013 (CLO3, C4)

6. a) 1.00 mol each of CO and Cl2 are introduced into an evacuated 1.75 L flask at 395 °C.
At equilibrium, the total pressure of the gaseous mixture is 32.40 atm. Calculate Kp.
CO(g) + Cl2(g) COCl2(g) (25.93)

(CLO3, C4)
b) A vessel was filled with NOCl gas until its concentration reached 0.400 mol L−1. The

temperature of the system was then raised to 245 °C and it was allowed to reach equilibrium

according to equation

2NOCl(g) Cl2(g) + 2NO(g)
At equilibrium, the concentration of Cl2 was 0.0225 mol L−1. Calculate the value of Kc at that
temperature. (3.62 x 10-4)

PSPM 2003 / 2004 (CLO3, C4)

CHEMISTRY UNIT KMNS, SESSION 2022 / 2023 44

HOUR : 2

7. The equilibrium between dinitrogen tetraoxide (colourless) and nitrogen dioxide (dark brown) is

represented by the equation below:
N2O4(g) 2NO2(g) ΔH = + 58 kJ

One mole of dinitrogen tetraoxide is placed in a 22.4 dm3 glass container and allowed to equilibrate

at 100 °C under a pressure of 2.46 atm. Calculate the partial pressure of N2O4, NO2 and the
equilibrium constant, Kc. (0.27 atm, 2.19 atm, 0.58)
PSPM 2009 / 2010 (CLO3, C4)

8. At 30 °C, the equilibrium constant, Kc, is 0.25 for the reaction,
2NO2(g) N2O4(g)
If 3.0 moles of N2O4 is placed in a 2.0 L container at this temperature, calculate the equilibrium
concentration for each of the gases. (0.68 mol L-1, 1.65 mol L-1)

PSPM 2010 / 2011 (CLO3, C4)

9. Nitrosyl bromide, NOBr dissociates according to the following equation:
2NOBr(g) 2NO(g) + Br2(g)
A 1.86 mol sample of NOBr is placed in a 5.00 L evacuated flask at 25 °C. Analysis of the mixture

at equilibrium shows that 0.164 mol of NO is present.

Calculate the equilibrium constant, Kc for this reaction. (1.53 x 10 -4)
PSPM 2004 / 2005 (CLO3, C4)

10. Sulphonyl chloride, SO2Cl2 is a very reactive gas. It decomposes when heated, according to the
following equation:
SO2Cl2(g) SO2(g) + Cl2(g)
A sample of SO2Cl2 was placed in a 1 L container. When the system reached equilibrium at 375 K,
the partial pressure of SO2, Cl2 and SO2Cl2 are 0.632, 0.632 and 0.166 atm respectively. Calculate
Kp and the initial pressure of SO2Cl2. What is the mass of SO2Cl2 that was initially placed in the
container? (2.41, 0.798 atm, 3.50 g)

(CLO3, C4)

HOUR : 3

11. Two moles of hydrogen bromide, HBr, is placed in a 4.0 L container. At high temperature, it

decomposes according to the following equation:

2HBr(g) H2(g) + Br2(g)

At equilibrium, the concentration of bromine is 0.10 M. Calculate the equilibrium constant, Kc and
the percentage of dissociation of HBr. (0.11, 40%)

PSPM 2007 / 2008 (CLO3, C4)

12. If 0.024 mol of N2O4 is allowed to reach equilibrium with NO2 in a 0.372 L flask at 25 °C,
N2O4(g) 2NO2(g) Kc = 4.61×10−3
calculate the degree of dissociation of N2O4. (0.125)

(CLO3, C4)

CHEMISTRY UNIT KMNS, SESSION 2022 / 2023 45

13. In the following reaction, 16.85 atm of COCl2 is introduced into an evacuated 1.00 dm3 flask.
Equilibrium is established at 395 °C when 5.0% of COCl2 has dissociated.
COCl2(g) CO(g) + Cl2(g)
a) Calculate the partial pressure of CO, Cl2 and COCl2 at equilibrium.
(0.84 atm, 0.84 atm, 16.01 atm)

b) Calculate the value of the equilibrium constant, Kp. (0.044)

(CLO3, C4)

14. The equilibrium constant, Kp , equals 1.78 at 250 °C for the decomposition reaction:
PCl5(g) PCl3(g) + Cl2(g)
Calculate the percentage of PCl5 that dissociates if 0.05 mol of PCl5 is placed in a closed vessel
(constant volume) at 250 °C and 2.00 atm pressure. (59.80%)
(CLO3, C4)

HOUR : 4

15. At 430 °C, the equilibrium constant, Kc, for the reversible reaction is 4.2×10−3.
H2(g) + I2(g) 2HI(g)

If 0.040 mol of HI, 0.010 mol of H2 and 0.030 mol of I2 are initially placed in a 2.0 L container, is the
system at equilibrium? Explain. (CLO3, C4)

16. Chloromethane is formed by the reaction;

CH4(g) + Cl2(g) CH3Cl(g) + HCl(g) Kp = 1.6 ×104 at 1500 K
In the reaction mixture, the partial pressures of CH4, Cl2, CH3Cl and HCl are 0.13 atm, 0.035 atm,
0.24 atm and 0.47 atm respectively.

Determine whether more CH3Cl or CH4 is formed. (CLO3, C4)

17. For the following reaction, the value of Kp is 6.4 × 10-6 at 227 °C.
2Cl2(g) + 2H2O(g) 4HCl(g) + O2(g)
In A 2.0 L container, the initial quantity of Cl2 is 0.50 mol, H2O is 0.40 mol, HCl is 0.50 mol and O2 is
0.015 mol. Determine whether the reaction is at equilibrium at this temperature. If not, predict to which
direction will the reaction proceed? PSPM 2016 / 2017 (CLO3, C4)

18. Consider the following reaction system;

2NO(g) + O2(g) 2NO2(g)

This system is made up from 2.0  10−3 mol of each gaseous component in a 2.0 L closed vessel. If

the equilibrium constant, Kc, for the system is 7.7  107 at 600 K, prove that the system is not in
equilibrium at this temperature.

When the system is cooled to 450 K and left to reach equilibrium, the final amount of O2 detected in

the vessel is 9.6  10−4 M. Calculate the equilibrium constant, Kc, of the system at 450 K. (1435.49)
(CLO3, C4)

19. Some hydrogen and iodine are mixed at 229 °C in a 1.00 liter container. When equilibrium is

established, the following concentrations are present: [HI] = 0.490 M, [H2] = 0.080 M, and [I2] = 0.060
M. If an additional 0.300 M of HI is then added, calculate concentration will be present when the new

equilibrium is established?

H2(g) + I2(g) 2HI(g)

([H2] = 0.113 M, [I2] = 0.093 M, [HI] = 0.724 M)
(CLO3, C4)

CHEMISTRY UNIT KMNS, SESSION 2022 / 2023 46

20. Sulphuryl chloride gas, SO2Cl2, dissociates reversibly into sulphur dioxide gas, SO2 and chlorine gas,
Cl2. The reaction is endothermic.
An amount of 0.20 mol of SO2Cl2 was placed into a 2.0 L vessel and allowed to reach equilibrium.
Calculate the equilibrium constant, Kc for the reaction if the final concentration of Cl2 is 0.080 M.
The system in equilibrium is disturbed by adding 0.14 mol of SO2Cl2 at the same temperature.
Calculate the concentrations of sulphuryl chloride and chlorine at the new equilibrium.
(Kc = 0.32, [SO2Cl2] = 0.047 M, [Cl2] = 0.123 M)
PSPM 2008 /2009 (CLO3, C4)

HOUR : 5

21. State Le Chatelier’s principle. (CLO1, C1)
Consider the reaction:

N2(g) + 3H2(g) 2NH3(g)

Complete the diagram below to show the changes in concentration of nitrogen, hydrogen and
ammonia in chemical equilibrium when ammonia is added to the mixture. Explain your answer.

Initial equilibrium Final equilibrium
state state

Concentration H2

NH3

N2

Time (CLO3, C4)

22. Decomposition of ethane produces ethene and hydrogen gas.

C2H6(g) C2H4(g) + H2(g) H = + ve

By using Le Chatelier’s principle, explain the shift in equilibrium position (if any) of the reaction if
i) the concentration of hydrogen gas is decreased.

ii) the temperature is lowered.

iii) a catalyst is added.

iv) C2H6 is removed from the system.
v) the volume of the container is increased.

vi) the pressure is increased.

vii) an inert gas is added at constant pressure.

viii) an inert gas is added at constant volume. (CLO3, C4)

CHEMISTRY UNIT KMNS, SESSION 2022 / 2023 47

23. Sulphur trioxide, SO3, in a 1 L closed container was left to dissociate according to the equation below:

2SO3(g) 2SO2(g) + O2(g) H° = -197.8KJ

Predict the equilibrium position of the reaction under the following separate conditions; temperature

is increased, pressure is decreased, and SO2 is removed. PSPM 2013 / 2014 (CLO3, C4)

24. In an experiment 0.10 mol of N2O4 and 0.55 mol of NO2 are mixed in a closed vessel at 350 K with
a total pressure of 2.0 atm. At this temperature, the equilibrium constant, Kp is 3.89. The reaction is
given as follows:

N2O4(g) 2NO2 (g)
In which direction will the reaction proceed to reach equilibrium? If the above experiment is carried
out at 500 K, the new Kp is 1700. Is the reaction exothermic or endothermic? Explain.
PSPM 2017 / 2018 (CLO3, C4)

25. Nitrogen(IV) oxide dimerised as follows;

2NO2(g) N2O4(g) H = −ve

Based on Le Chatelier’s principle, explain how the product could be increased. (CLO3, C4)

26. A 10.00L vessel contains 0.0681 mol phosphorus trichloride, PCl3 and 0.2056 mol chlorine, Cl2 at
250 °C. If 0.0316 mol phosphorus pentachloride, PCl5 is produced at equlibrium, calculate Kp for this
reaction. (1.16)

PCl3(g) + Cl2(g) PCl5(g) H = +ve

Discuss four factors that can increase the amount of PCl5 produced. PSPM 2014 / 2015 (CLO3, C4)

CHEMISTRY UNIT KMNS, SESSION 2022 / 2023 48