JPJC 2026 Chemical Equilibrium Revision

1 2026 JC1 H2 Chemistry (9476) Topical Revision for Physical Chemistry (Topic 7: Chemical Equilibria) FOUNDATION KIT [2022 JPJC JC1 H2 Chem EOY P1 Q20] 1 1 mol of CH 4, 2 mol of H 2S, 1 mol of CS 2, and 2 mol of H 2 were placed in a 500 cm 3 reaction vessel at 960 C. The mixture was allowed to reach equilibrium according to the following equation. CH4(g) + 2H2S(g) CS2(g) + 4H2(g) At equilibrium, the concentration of CH4 was 1.12 mol dm–3. What is the value of the equilibrium constant, Kc, at 960 C? A 1.64  103 B 1.20 C 0.836 D 6.10  10–4 [J1988/Paper III/Q9] 2 An equilibrium can be represented by the following equation. P(aq) + Q(aq) ⇌ 2R(aq) + S(aq) In a certain mixture, the equilibrium concentration of Q is 10 mol dm-3. What will be the new equilibrium concentration of Q if 5 mol of pure Q is dissolved in the mixture? A 15 mol dm-3 B between 10 mol dm-3 and 15 mol dm-3 C 10 mol dm-3 D between 5 mol dm-3 and 10 mol dm-3 [2022 JC1 H2 Chemistry Paper 1 Q22] 3 The graphs below show how the percentage of gaseous products present at equilibrium vary with temperature and pressure. Which reaction could the graph represent? A N2O4(g)  2NO2(g) H > 0 B H2(g) + I2(g)  2HI(g) H > 0 C N2(g) + 3H2(g)  2NH3(g) H < 0 D C(s) + O2(g)  CO2(g) H < 0 % products at equilibrium pressure (T + 10) C T C

2 [2014 JC2 H2 Chemistry Prelim Paper 1 Q12] 4 A chemist carried out a series of experiments involving the following equilibrium. H2(g) + I2(g) 2HI(g); H = +54 kJ mol–1 He performed the first set of experiment at a temperature T1 K and found that the equilibrium constant, Kc has a value of 64. In the second set of experiment, he added 1.0 mol of H 2 and 1.0 mol of I2 into a 1 dm3 flask, at a temperature of T2 K. When equilibrium was established, the flask was found to contain 1.2 mol of HI. What is the relationship between T1 and T2? A T1 = T2 B T1 > T2 C T1 < T2 D The relationship cannot be determined based on his experiments alone. [2019 JPJC EOY P1 Q16] 5 Gas P decomposes to two other gases, Q and R, according to the equation: 2P(g) ⇌ 3Q(g) + R(g) ∆H > 0 Which of the following correctly describes what will happen if a proposed change is made to this system at equilibrium? Proposed change value of Kc position of equilibrium A add inert solid X no change shift left B increase pressure no change shift right C increase temperature increase shift right D add catalyst increase no change [2018 JPJC EOY P1 Q19] 6 A student wanted to determine the equilibrium constant for the decomposition of calcium carbonate, CaCO 3. He sealed 2.0 g of CaCO 3 in an evacuated 2.0 dm 3 metal flask and connected a pressure gauge to the flask. The flask was placed in an oven and heated to a temperature of 800 °C, at which an equilibrium was reached according to the following equation: CaCO3(s) CaO(s) + CO2(g) When the equilibrium was established, 1.40  10−2 mol of CO2 was present in the flask. What is the value of equilibrium constant, Kc, for this reaction at 800 °C? A 3.27  10−2 B 1.63  10−2 C 1.40  10−2 D 7.00  10−3

3 [2018 JPJC EOY P1 Q21] 7 The graph below shows how the percentage of product present at equilibrium varies with temperature and pressure for a reaction. Which reactions could the graph represent? 1 4Fe(s) + 3º2(g)  2Fe2O3(s) H = −1644 kJ mol−1 2 N2O4(g)  2NO2(g) H = +57 kJ mol−1 3 CO(g) + Cl2(g)  COCl2(s) H = +86 kJ mol−1 A 1 only B 2 only C 3 only D 1, 2 and 3 Structured Questions 8 [PJC MYE 2011/P2/Q2(a)] (a) Part of the process by which coal can be converted to a combustible mixture of gases involves passing steam over white hot coke (a form of carbon obtained from coal) according to the following equation. C(s) + H2O(g) CO(g) + H2(g) The resultant mixture of gases that results is an important industrial fuel called water gas. When a 4:1 ratio of steam and hydrogen at a total initial pressure of 10 atm was passed over coke at 1000 C, in a 2 dm3 reaction vessel, the partial pressure of steam at equilibrium was found to be 2.4 atm. (i) Calculate the equilibrium partial pressures of CO and H2, in atm, in the mixture at this temperature. (ii) Write an expression for Kp and calculate a value for Kp, giving its units. (iii) What is the minimum amount of carbon required to achieve the equilibrium under these conditions? (iv) State and explain the effect on the composition of the equilibrium mixture if pressure of the system is reduced. Percentage of product (T + 20) °C T °C Pressure

4 10 [PJC Prelim 2012/P2/Q4] In the vapour phase, ethanoic acid associates to form dimers and an equilibrium is established as shown. 2CH3COOH(g)  (CH3COOH)2(g) At 177 °C and 1 atm, 0.084 g of the equilibrium mixture occupies 37.8 cm3. (a) (i) Calculate the average relative molecular mass of the gaseous mixture, giving your answer to 1 decimal place. (ii) Calculate the mole fraction (proportion) of ethanoic acid and its dimer in the equilibrium mixture. (iii) Hence, calculate the value of Kp for the 4imerization of ethanoic acid at 177 °C, giving its units. (b) State and explain the effect of an increase in temperature on the average Mr of the equilibrium mixture. 9 [PJC Prelim 2010/P3/Q1(a)] (a) Sulfuric acid, H2SO4, is an important chemical used in industry and in the school laboratory. One step in the manufacture of sulfuric acid by the Contact Process involves the reaction between sulfur dioxide and oxygen. 2SO2(g) + O2(g)  2SO3(g) H = −197 kJ mol−1 The reaction is carried out at a temperature of 450 C in the presence of a catalyst. (i) Explain why a temperature of 450 C is usually employed for the reaction. (ii) When a 2:1 ratio of SO 2 and O2 at a total initial pressure of 1.5 atm is passed over a catalyst at 450 C, the percentage conversion of SO2 into SO3 was found to be 94%. Calculate the equilibrium partial pressures of SO 3, SO 2 and O 2 and hence, find the value for Kp. (iii) State and explain the effect on the percentage conversion of SO 2 into SO3 when the volume of the reaction vessel is reduced. 11 [PJC Prelim 2013/P2/Q4(a)] (a) On heating, a solid sample of phosphorus pentachloride is completely vaporised in a 2 dm3 vessel. At 227 °C, the following equilibrium is set up: PCl5(g)  PCl3(g) + Cl2(g) The partial pressure of Cl2 at equilibrium is found to be 2.00 atm. Given that the equilibrium constant, Kp, is 1.01 atm, (i) Write an expression for the equilibrium constant, Kp, for this reaction. (ii) Calculate the total pressure of the system at equilibrium (iii) Calculate the mass of the solid PCl5 placed in the vessel initially.

5 12 [MI Prelim 2011/ P3/ Q2d – modified] 0.500 mol of ethanoic acid, CH 3COOH, and 1.00 mol of ethanol, CH 3CH2OH, were shaken together and the following equilibrium is established. CH3COOH(l) + CH3CH2OH(l)  CH3COOCH2CH3(l) + H2O(l) The total volume of the mixture is 0.5 dm 3. The equilibrium mixture required 80 cm 3 of 1.00 mol dm−3 of sodium hydroxide for complete reaction. (i) Explain why the reaction mixture has to be titrated with sodium hydroxide rapidly. (ii) Write the Kc expression for the reaction formed between ethanoic acid and ethanol. (iii) Calculate the equilibrium constant for the equilibrium. 13 [CJC Prelim 2011/ P2/ Q3(a)] Hydrobromic acid, HBr, is one of the strongest mineral acids known and is even stronger than hydrochloric acid. Both the industrial and laboratory syntheses of HBr