09 Chemical Equilibria, Set B To print: Ctrl + P, paper A4, margins Default.
Learners' AlleyDesigned by Khow Willard
09
Chemical Equilibria
Set BExam practice
Name
Time 1 hour
Total 27 marks
A Data Booklet may be used.
A
Multiple choice
4 marks
1
CommonConsider the reaction W + X ⇌ Y + Z. Which of the following statements is incorrect when a catalyst is added to the reaction mixture?
AThe catalyst alters the mechanism of the reaction.
BThe addition of a catalyst lowers the activation energy of the forward reaction but not that of the reverse reaction.
CThe enthalpy change of reaction remains unchanged.
DThe catalyst does not alter the proportion of W, X, Y or Z at equilibrium.
Answer [1]
Source: AJC 2017 Promo, via Chemical Equilibria Topical Revision Booklet, Q1
2
A 0.0113 mol sample of ammonium nitrate is introduced into an evacuated vessel with a volume of 0.83 dm3. The vessel, of constant volume, is heated to a constant temperature such that the equilibrium below is established. NH4NO3(s) ⇌ NH3(g) + HNO3(g)
Given that the percentage dissociation of ammonium nitrate at equilibrium is 66.8%, what is the value of the equilibrium constant, Kc?
A5.70 × 10−5
B8.27 × 10−5
C0.0152
D0.0183
Answer [1]
Source: MJC 2017 Promo, via Chemical Equilibria Topical Revision Booklet, Q7
3
The key stage in the manufacture of sulfuric acid is 2SO2(g) + O2(g) ⇌ 2SO3(g), ΔH = −197 kJ mol−1. Which of the following statements are correct for an increase in temperature?
1The equilibrium constant increases.2The rate constant for the forward reaction increases.3The rate constant for the backward reaction increases.4The activation energy for the forward reaction decreases.
A1, 2, 3 and 4
B2, 3 and 4 only
C2 and 3 only
D3 only
Answer [1]
Source: MJC 2017 Promo, via Chemical Equilibria Topical Revision Booklet, Q8
4
In an experiment, 2 mol of compound P were placed in a vessel and heated. Compound P decomposed partially to give compounds Q and R. At equilibrium, the mixture was found to contain x mol of R and the total amount present was (2 + x). Which equilibrium represents this equilibrium? All compounds are gases.
AP ⇌ 2Q + R
B2P ⇌ 2Q + R
C2P ⇌ Q + R
D2P ⇌ Q + 2R
Answer [1]
Source: EJC 2022 JC1 Chemical Equilibria Tutorial, A2
B
Structured questions
23 marks
5
Common0.200 mol of carbon monoxide and 0.200 mol of steam were placed in a reaction vessel and allowed to react at a temperature of 800 K. The percentage of carbon monoxide reacted was found to be 77.5%. CO(g) + H2O(g) ⇌ CO2(g) + H2(g)
(a)
Write the expression for the equilibrium constant, Kc, for this reaction.
[1]
(b)
Calculate the value of the equilibrium constant, Kc, at 800 K.
[2]
(c)
The Gibbs free energy change of this reaction is −18.4 kJ mol−1 at 600 K. The relationship between the equilibrium constant and the Gibbs free energy change of this reaction can be expressed as ΔG = −RT ln Kc. Calculate the value of Kc at 600 K.
[1]
Source: 2025 NJC J2 Prelim P2 Q4(c)
6
In the gaseous state, NO2 can form a dimer. 2NO2(g) ⇌ N2O4(g). NO2 is brown and N2O4 is colourless.
(a)
Calculate the enthalpy change, ΔH, for the dimerisation, using ΔHf(NO2) = +33.2 kJ mol−1 and ΔHf(N2O4) = +9.2 kJ mol−1.
[1]
(b)
Predict, with reasons, whether the dimerisation is favoured by high or low pressure, and by high or low temperature.
[2]
(c)
Two sealed tubes each contain a mixture of NO2 and N2O4. One is placed in iced water and the other in hot water. Deduce the colour expected in each tube. Explain your answers.
[2]
Source: EJC 2022 JC1 Chemical Equilibria Tutorial, A10(a) and (b)
7
Steam can react with carbon monoxide under appropriate conditions according to the following reversible reaction. H2O(g) + CO(g) ⇌ CO2(g) + H2(g) ΔH = −40 kJ mol−1
(a)
Calculate the amount of CO in the equilibrium mixture if 4 moles each of CO(g) and H2O(g) were placed in a vessel of constant volume at a temperature at which Kc is 9.0.
[2]
(b)
What are the mole fractions of all the reactants and products?
[1]
(c)
The reaction is repeated as before, except that (i) the temperature is lowered, (ii) the volume of the vessel is decreased, (iii) a catalyst is added, with other factors remaining constant. For each change, describe and explain the effect on the amount of CO2 at equilibrium and on the time taken to reach equilibrium.
In the Haber process, a mixture of H2 and N2 reacts to form ammonia.
3H2(g) + N2(g) ⇌ 2NH3(g) ΔH = −92 kJ mol−1
CommonThe spontaneity of the reaction would be higher if the temperature were reduced. Yet the temperature used in the Haber process is still fairly high, between 400 and 500 °C. Give one advantage and one disadvantage of using a temperature much lower than this range.
[2]
Source: 2023 CJC J2 Prelim P3 Q2(b)(iii)
9
ChallengeIn the Contact process, SO2 is oxidised to SO3 using a vanadium(V) oxide catalyst. SO2(g) + ½O2(g) ⇌ SO3(g) ΔS = −93.5 J K−1 mol−1
(a)(i)
Given that at 298 K, ΔHf(SO2) = −297 kJ mol−1 and ΔHf(SO3) = −396 kJ mol−1, calculate ΔH for this reaction. Include the units.
[1]
(ii)
Hence calculate ΔG for this reaction at 298 K. Include the units.
[1]
(b)(i)
For any reaction, ΔG is related to the equilibrium constant by ΔG = −2.303RT log10 Kc. Calculate a value of Kc for the oxidation of SO2 at 298 K.
[2]
(ii)
What is the significance of the magnitude of your answer in (b)(i)?
[1]
(iii)
Suggest why, when sulfur is heated in air, SO2 is formed but not SO3.
[1]
Source: EJC 2022 JC1 Chemical Equilibria Tutorial, A11