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Chemical equilibrium and Kc

Past-paper questions pack

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OCR AS and A-level Chemistry past-paper questions since 2016 · 57 parts · mark schemes follow the questions · www.chemistryaleveltutor.com · Questions and mark schemes © OCR

1. June 2025 · AS Paper 1 · Q12 · 1 marks ·
12 Hydrogen and iodine react in the reaction below. H2(g) + I2(g) 2HI(g) Which row is correct when pressure is increased? Rate of reaction Equilibrium yield of HI A Increases Increases B Increases No effect C Increases Decreases D Decreases No effect Your answer [1]
2. June 2025 · AS Paper 1 · Q13 · 1 marks ·
13 Which statement is not a sustainability benefit of an industrial process using a catalyst? A Lower temperatures can be used. B The position of equilibrium moves towards the products. C The reaction takes place in less time. D There is reduced energy demand. Your answer [1]
3. June 2025 · AS Paper 1 · Q23(a) · 2 marks ·
23 This question is about equilibrium and bond enthalpies. The chemical industry manufactures sulfur trioxide, SO3, by reacting sulfur dioxide, SO2, and oxygen. This is a reversible reaction which can reach dynamic equilibrium. 2SO2(g) + O2(g) 2SO3(g) ΔH = –196 kJ mol–1 Equilibrium 23.1 (a) State two features of a dynamic equilibrium. 1 ........................................................................................................................................................ .......................................................................................................................................................... 2 ........................................................................................................................................................ .......................................................................................................................................................... [2]
4. June 2025 · AS Paper 1 · Q23(c) · 3 marks ·
(c) The table shows equilibrium concentrations for Equilibrium 23.1. [SO2(g)] / mol dm–3 [O2(g)] / mol dm–3 [SO3(g)] / mol dm–3 25.0 108 42.0 For Equilibrium 23.1 • write the expression for the equilibrium constant, Kc • calculate Kc to 3 significant figures and in standard form. Kc = ............................................... dm3 mol–1 [3]
5. June 2025 · Paper 3 · Q3(a) · 5 marks ·
3 This question is about equilibrium and rates. (a) Methanol, CH3OH, can be made industrially by the reaction of carbon monoxide, CO, with hydrogen, H2, in the presence of a catalyst. This is a reversible reaction. The equilibrium below is set up. CO(g) + 2H2(g) CH3OH(g) ∆H = –91 kJ mol–1 The concentrations of CO(g) and CH3OH(g) in the equilibrium mixture are shown in the table below. CO(g) CH3OH(g) equilibrium concentration / mol dm–3 0.27 0.11 The value of the equilibrium constant, Kc, for this reaction is 1.8 dm6 mol–2. • Explain the conditions of pressure and temperature that would give the maximum equilibrium yield of CH3OH(g). • Calculate the concentration of H2(g) in the equilibrium mixture to an appropriate number of significant figures. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [5]
6. June 2024 · Paper 1 · Q11 · 1 marks ·
11 The reversible reaction between hydrogen and iodine to form hydrogen iodide is H2(g) + I2(g) 2HI(g) The graph shows how the concentrations of the reactants and product change as the reaction reaches a dynamic equilibrium. At which point on the graph is the equilibrium reached? A B C D concentration product reactants time Your answer [1]
7. June 2024 · AS Paper 1 · Q13 · 1 marks ·
13 The reversible reaction below is at equilibrium. 2SO2(g) + O2(g) 2SO3(g) What is the expression for Kc? [SO2(g)]2 [O2(g)] A [SO3(g)]2 [SO3(g)]2 B [SO2(g)]2 [O2(g)] 2[SO2(g)] + [O2(g)] C 2[SO3(g)] 2[SO3(g)] D 2[SO2(g)] + [O2(g)] Your answer [1]
8. June 2024 · AS Paper 1 · Q23(a)(i) · 2 marks ·
23 This question is about covalent compounds of nitrogen. (a) Ammonia, NH3, is manufactured by reacting nitrogen and hydrogen gases. This is a reversible reaction and the equilibrium is shown below. N2(g) + 3H2(g) 2NH3(g) ΔH = –92 kJ mol–1 (i) This is an example of a dynamic equilibrium. State 2 features of a dynamic equilibrium. 1 ....................................................................................................................................................... .......................................................................................................................................................... 2 ....................................................................................................................................................... .......................................................................................................................................................... [2]
9. June 2024 · AS Paper 1 · Q23(a)(ii) · 3 marks ·
(ii) State and explain the conditions of temperature and pressure that would produce a large equilibrium yield of NH3. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [3]
10. June 2023 · Paper 1 · Q15 · 1 marks · AO2
15 Chlorine trifluoride can be decomposed into its elements forming the equilibrium mixture below. 2Cl F3(g) Cl 2(g) + 3F2(g) ∆H = +318 kJ mol–1 colourless green yellow gas gas gas Which statement(s) is/are correct? 1 The decomposition is a redox reaction. 2 When the equilibrium mixture is cooled, the colour fades. 3 The decomposition has a negative entropy change. A 1, 2 and 3 B Only 1 and 2 C Only 2 and 3 D Only 1 Your answer [1]
11. June 2023 · Paper 1 · Q19(a)(ii) · 3 marks · AO2, 3
(ii) The sealed flask in (a)(i) is then heated to a higher temperature at an increased pressure. The system is allowed to reach equilibrium again. Explain why it is difficult to predict how these changes in reaction conditions affect the amount of N2O4(g) formed at equilibrium. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [3]
12. June 2023 · AS Paper 1 · Q24(a) · 1 marks · AO1
24 The reaction between sulfur dioxide, SO2(g) and oxygen, O2(g), to form sulfur trioxide, SO3(g), is a key step in the industrial manufacture of sulfuric acid. This is a reversible reaction, shown in Equilibrium 24.1: 2SO2(g) + O2(g) 2SO3(g) ΔH = –197 kJ mol–1 Equilibrium 24.1 (a) Why is Equilibrium 24.1 a homogeneous equilibrium? ................................................................................................................................................... .............................................................................................................................................. [1]
13. June 2023 · AS Paper 1 · Q24(b) · 5 marks · AO1
(b) Le Chatelier’s principle can be used to predict how different conditions affect the equilibrium position in Equilibrium 24.1. Explain how changing pressure, temperature and using a catalyst affect the equilibrium yield of SO3. In your answer, use le Chatelier’s principle and other chemical concepts, where appropriate. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [5]
14. June 2023 · AS Paper 1 · Q24(c)(i) · 2 marks · AO2
(c) A mixture of SO2(g) and O2(g) is allowed to reach equilibrium at a constant temperature. The equilibrium concentrations are shown in the table. Equilibrium concentration Substance / mol dm–3 SO2(g) 3.0 × 10–3 O2(g) 3.5 × 10–3 SO3(g) 5.0 × 10–2 (i) Write the expression for Kc and calculate the numerical value for Kc in Equilibrium 24.1 at this constant temperature. Give your answer to an appropriate number of significant figures and in standard form. Kc = ......................................... dm3 mol–1 [2]
15. June 2023 · AS Paper 1 · Q24(c)(ii) · 1 marks · AO3
(ii) In the industrial production of SO3, an excess of O2(g) is used rather than a 2:1 proportion of SO2(g) to O2(g) which would match the stochiometry in Equilibrium 24.1. Suggest, in terms of equilibrium, why an excess of O2(g) is used industrially. ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
16. June 2023 · Paper 3 · Q6(d) · 5 marks · AO1, 2
(d) In the UK, most sulfuric acid, H2SO4, is manufactured by the Contact process. One stage in the Contact process involves the equilibrium between sulfur dioxide, oxygen and sulfur trioxide. 2SO2(g) + O2(g) 2SO3(g) This equilibrium is investigated: Step 1 5.82 × 10–2 mol of SO2 is mixed with 7.40 × 10–2 mol of O2 in a 2.00 dm3 container. Step 2 The container is sealed and allowed to reach equilibrium at constant temperature. Step 3 At equilibrium, 5.20 × 10–2 mol of SO3 is formed. Determine the equilibrium concentrations and calculate Kc, including units. Kc = ........................ units ........................ [5]
17. June 2022 · AS Paper 1 · Q12 · 1 marks · AO1
12 Which prediction can be made using le Chatelier’s principle? A The effect of a catalyst on the reaction rate. B The effect of a catalyst on the equilibrium position. C The effect of temperature on the reaction rate. D The effect of concentration on the equilibrium position. Your answer [1]
18. June 2022 · AS Paper 1 · Q13 · 1 marks · AO2
13 Four equilibrium reactions are set up. The concentration of each gas in the equilibrium mixtures is 0.1 mol dm–3. Which equilibrium has a numerical Kc value of 0.01? A CH4(g) + 2H2O(g) CO2(g) + 4H2(g) B N2(g) + 3H2(g) 2NH3(g) C H2(g) + I2(g) 2HI(g) D 2NO2(g) N2O4(g) Your answer [1]
19. June 2022 · Paper 1 · Q20(a) · 6 marks · AO1, 2
20 This question is about equilibria involving hydrogen. (a)* Hydrogen is used industrially to manufacture ammonia. The equilibrium is shown below. N2(g) + 3H2(g) 2NH3(g) ∆H = –92 kJ mol–1 Equilibrium 20.1 1.20 mol N2(g) is mixed with 3.60 mol H2(g) in a 8.00 dm3 container. The mixture is heated to 550 °C with an iron catalyst and allowed to reach equilibrium. The equilibrium mixture contains 0.160 mol of NH3. Determine the equilibrium constant Kc for Equilibrium 20.1, and explain why the operational conditions used by industry may be different from those required for a maximum equilibrium yield of ammonia. [6] ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Additional answer space if required. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ...................................................................................................................................................
20. June 2022 · Paper 1 · Q20(b)(ii) · 1 marks · AO3
(ii) Two students are discussing the effect of pressure on the equilibrium position of Equilibrium 20.2. Student 1 says: “There are more moles of products than reactants, so increasing the pressure will shift the equilibrium to the left hand side.” Student 2 disagrees. Determine which student is correct. Justify your answer. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
21. June 2022 · AS Paper 2 · Q4(a)(i) · 6 marks · AO1, 2
4 This question is about the manufacture of hydrogen, H2. (a) In industry, hydrogen is manufactured from methane, as shown in Equilibrium 4.1. CH4(g) + H2O(g) CO(g) + 3H2(g) ΔH = +206 kJ mol–1 Equilibrium 4.1 The industrial process is carried out at 15 atmospheres pressure and at a temperature of 800 °C using an excess of steam. A nickel catalyst is used. (i)* Explain why these conditions are used industrially. [6] ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Additional answer space if required. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
22. June 2022 · AS Paper 2 · Q4(a)(ii) · 2 marks · AO2
(ii) A chemist mixes CH4(g) and H2O(g) and leaves the mixture to reach equilibrium. CH4(g) + H2O(g) CO(g) + 3H2(g) ΔH = +206 kJ mol–1 Equilibrium 4.1 The equilibrium mixture contains the following concentrations. Substance Concentration / mol dm–3 CH4(g) 0.111 H2O(g) 0.682 CO(g) 0.510 H2(g) 1.530 Write an expression for the equilibrium constant, Kc, for Equilibrium 4.1 and calculate the numerical value of Kc. Give your answer to 3 significant figures. Kc = ......................................... mol2 dm–6 [2]
23. June 2022 · Paper 3 · Q4(a) · 4 marks · AO1, 2
4 Two students plan to investigate Equilibrium 4.1, shown below. CoCl 4 2−(aq) + 6H2O(l) [Co(H2O)6]2+(aq) + 4Cl −(aq) Equilibrium 4.1 blue pink (a) The students are supplied with the equilibrium mixture in Equilibrium 4.1 at room temperature. • One student heats 20 cm3 of the mixture to 50 °C. • The other student heats 20 cm3 of the mixture to 90 °C. The students use colorimetry to observe how the colour of the equilibrium mixture changes over time. • The colorimeter is set up so that the greater the absorbance, the greater the concentration of [Co(H2O)6]2+. • The initial absorbance is set to zero. • The absorbance is recorded every 30 seconds. The students plot the graph below from the results of the experiment. 50 °C 90 °C Absorbance 0,0 Time / s Use the graph and relevant chemical theory to answer the following. Include all reasoning: • Explain the different initial rates at 50 °C and 90 °C. • Predict the sign of ∆H for the forward reaction in Equilibrium 4.1. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [4]
24. June 2022 · Paper 3 · Q4(b)(ii) · 3 marks · AO3
(ii) Explain the changes in concentration of CoCl 4 2–, Cl – and [Co(H2O)6]2+ after time = t1. Refer to Equilibrium 4.1 in your answer. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [3]
25. November 2021 · Paper 1 · Q9 · 1 marks ·
9 An equilibrium system is shown below. CO(g) + NO2(g) CO2(g) + NO(g) Different amounts of CO, NO2, CO2 and NO are added to four containers, as shown below. Container CO / mol NO2 / mol CO2 / mol NO / mol A 1 1 0 0 B 1 0 1 1 C 1 1 1 0 D 0 1 1 1 Which container would have the largest concentration of CO(g) at equilibrium? Your answer [1]
26. November 2021 · AS Paper 1 · Q12 · 1 marks · AO1
12 Which statement about dynamic equilibrium is not correct? A A catalyst increases the rate of both forward and reverse reactions by the same amount. B Dynamic equilibrium exists only in a closed system. C The concentrations of the reactants and products are equal. D The rate of the forward reaction is equal to the rate of the reverse reaction. Your answer [1]
27. November 2021 · Paper 1 · Q20(a) · 1 marks · AO1
20 This question is about chemical equilibrium. Nitrogen monoxide, NO, and oxygen, O2, react to form nitrogen dioxide, NO2, in the reversible reaction shown in Equilibrium 20.1. Equilibrium 20.1 2NO(g) + O2(g) 2NO2(g) ΔH = –114 kJ mol–1 ΔS = –147 J mol–1 K–1 (a) A dynamic equilibrium exists in a closed system. State one other feature of a dynamic equilibrium. ................................................................................................................................................... .............................................................................................................................................. [1]
28. November 2021 · Paper 1 · Q20(c)(ii) · 3 marks · AO1
(ii) The chemist then repeats the experiment three times. In each experiment, the chemist makes one change but uses the same initial amounts of NO and O2. Complete the table to show the predicted effect of each change compared with the original experiment. Only use the words greater, smaller or same. Equilibrium amount Change Kp Initial rate of NO2(g) Temperature increase Pressure increase Catalyst added [3]
29. November 2021 · AS Paper 1 · Q22(b) · 4 marks · AO1, 2
(b) Explain how le Chatelier’s principle can be used to predict the conditions of pressure and temperature for a maximum equilibrium yield of hydrogen in Equilibrium 22.1. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [4]
30. November 2021 · AS Paper 2 · Q5(a) · 1 marks · AO1
5 Methanol, CH3OH, is manufactured by the reaction of carbon monoxide, CO, with hydrogen, H2. CO(g) + 2H2(g) CH3OH(g) ∆H = –91 kJ mol–1 (a) Write the expression for the equilibrium constant, Kc, for this equilibrium. [1]
31. November 2021 · AS Paper 2 · Q5(b)(i) · 2 marks · AO2
(b) A chemist mixes CO and H2 in a container. The mixture is heated to 200 °C and left to reach equilibrium. The equilibrium concentrations of CO and H2 are shown in the table. Equilibrium concentration Compound / mol dm–3 CO(g) 0.57 H2(g) 0.40 The numerical value of Kc for this equilibrium is 15.4. (i) Calculate the equilibrium concentration of CH3OH(g). concentration = .......................................... mol dm–3 [2]
32. November 2021 · AS Paper 2 · Q5(b)(ii) · 1 marks · AO1
(ii) What does the numerical value of Kc tell you about the position of equilibrium? ........................................................................................................................................... ...................................................................................................................................... [1]
33. November 2021 · Paper 3 · Q1(b) · 2 marks · AO1, 2
(b) Pure PCl5 is placed in a sealed container which is heated to 200 ºC. An equilibrium is set up: PCl5(g) PCl3(g) + Cl2(g) Under these conditions: • Kc = 8.00 × 10–3 mol dm–3 • equilibrium concentration of PCl5 = 0.0500 mol dm–3. Calculate the concentrations of PCl3 and Cl2 at equilibrium. concentration of PCl3 = ................................................ mol dm–3 concentration of Cl2 = ................................................ mol dm–3 [2]
34. November 2020 · AS Paper 1 · Q12 · 1 marks · AO1
12 Hydrogen gas can be produced as shown below. CH4(g) + H2O(g) CO(g) + 3H2(g) ∆H = +206 kJ mol−1 Which conditions produce the greatest equilibrium yield of hydrogen? A Low temperature and high pressure B Low temperature and low pressure C High temperature and high pressure D High temperature and low pressure Your answer [1]
35. November 2020 · AS Paper 1 · Q13 · 1 marks · AO1
13 The reversible reaction below is in equilibrium. 2SO2(g) + O2(g) 2SO3(g) The equilibrium concentrations are shown in the table. Substance SO2(g) O2(g) SO3(g) Equilibrium concentration 4.00 2.40 1.44 / mol dm−3 What is the numerical value of Kc? A 0.0375 B 0.0540 C 0.150 D 18.5 Your answer [1]
36. November 2020 · Paper 1 · Q13 · 1 marks · AO1
13 Which statement(s) is/are correct when a catalyst is added to a system in dynamic equilibrium? 1 The rates of the forward and reverse reactions increase by the same amount. 2 The concentrations of the reactants and products do not change. 3 The value of Kc increases. A 1, 2 and 3 B Only 1 and 2 C Only 2 and 3 D Only 1 Your answer [1]
37. November 2020 · Paper 1 · Q17(a) · 3 marks · AO1
17 Methanol, CH3OH, can be made industrially by the reaction of carbon monoxide with hydrogen, as shown in equilibrium 1. CO(g) + 2H2(g) CH3OH(g) ∆H = −91 kJ mol−1 Equilibrium 1 (a) Predict the conditions of pressure and temperature that would give the maximum equilibrium yield of CH3OH in equilibrium 1. Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [3]
38. November 2020 · AS Paper 2 · Q4(b)(i) · 1 marks · AO1
(b) Nitrosyl chloride, NOCl , dissociates into nitrogen monoxide and chlorine as in the equilibrium below. 2NOCl (g) 2NO(g) + Cl 2(g) ΔH = +77.0 kJ mol–1 Nitrosyl chloride is added to a container, which is then sealed. The container is heated to 400 °C, and equilibrium is allowed to be reached. (i) Write the expression for the equilibrium constant, Kc, for this equilibrium. [1]
39. November 2020 · AS Paper 2 · Q4(b)(iii) · 2 marks · AO2
(iii) At 400 °C, Kc = 0.015 mol dm–3. Calculate the equilibrium concentration of NOCl (g) at 400 °C. equilibrium concentration of NOCl (g) = .......................................... mol dm−3 [2]
40. November 2020 · AS Paper 2 · Q4(b)(iv) · 2 marks · AO2
(iv) The temperature of the equilibrium mixture is increased above 400 °C while keeping the pressure constant. State and explain the effect on the equilibrium concentration of nitrogen monoxide, NO(g), with these new conditions. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2]
41. November 2020 · Paper 3 · Q5(b) · 1 marks · AO2
(b) State and explain how increasing the temperature affects the position of this equilibrium and whether the forward reaction is exothermic or endothermic. ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [1]
42. June 2019 · AS Paper 1 · Q13 · 1 marks · AO1
13 A catalyst is added to a system in equilibrium. What is the effect on the rates of the forward and reverse reactions? A There is no effect on the rate in either direction. B Both rates increase by the same factor. C The rate in the forward direction increases by a greater factor than the reverse direction. D The rate in the reverse direction increases by a greater factor than the forward direction. Your answer [1]
43. June 2019 · Paper 1 · Q19(b)(iv) · 1 marks · AO3
(iv) Explain the significance of the expression: Kp & 1. ........................................................................................................................................... ...................................................................................................................................... [1]
44. June 2019 · AS Paper 1 · Q24(a)(i) · 4 marks · AO1, 2
24 This question is about ammonia, NH3. (a) In industry, ammonia is made from nitrogen and hydrogen. This is a reversible reaction, as shown in equilibrium 24.1 below. N2(g) + 3H2(g) 2NH3(g) ∆H = −92 kJ mol−1 Equilibrium 24.1 (i) Explain how le Chatelier’s principle can be used to predict the conditions of temperature and pressure for a maximum equilibrium yield of ammonia. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [4]
45. June 2019 · AS Paper 1 · Q24(a)(ii) · 2 marks · AO2
(ii) Using certain conditions, equilibrium 24.1 has the equilibrium concentrations in the table. Equilibrium concentration Substance / mol dm−3 N2(g) 1.25 H2(g) 2.75 NH3(g) 0.862 Calculate the numerical value for Kc for equilibrium 24.1 under these conditions. Give your answer to an appropriate number of significant figures and in standard form. Kc = ......................................................... [2]
46. June 2018 · AS Paper 1 · Q13 · 1 marks ·
13 The reversible reaction below is at equilibrium. 2SO2(g) + O2(g) 2SO3(g) ∆H = −197 kJ mol−1 Which changes in pressure and temperature would shift the equilibrium position towards the products? Pressure Temperature A Decrease Decrease B Decrease Increase C Increase Decrease D Increase Increase Your answer [1]
47. June 2018 · AS Paper 1 · Q14 · 1 marks ·
14 The reversible reaction below is at equilibrium. N2(g) + 3H2(g) 2NH3(g) What is the expression for Kc? [N2(g)] [H2(g)]3 A [NH3(g)]2 [NH3(g)]2 B [N2(g)] [H2(g)]3 [N2(g)] + 3[H2(g)] C 2[NH3(g)] 2[NH3(g)] D [N2(g)] + 3[H2(g)] Your answer [1]
48. June 2018 · Paper 1 · Q18(a) · 2 marks ·
18 Nitrogen monoxide, NO, and oxygen, O2, react to form nitrogen dioxide, NO2, in the reversible reaction shown in equilibrium 18.1. 2NO(g) + O2(g) 2NO2(g) Equilibrium 18.1 (a) Write an expression for Kc for this equilibrium and state the units. Kc = Units = ........................ [2]
49. June 2018 · AS Paper 2 · Q4(b) · 1 marks ·
(b) Write an expression for the equilibrium constant, Kc, in equilibrium 4.1. [1]
50. June 2018 · AS Paper 2 · Q4(c) · 5 marks ·
(c) Predict the conditions of temperature and pressure for a maximum equilibrium yield of nitrogen monoxide in equilibrium 4.1. • Explain your prediction in terms of le Chatelier’s principle. • State and explain how these conditions could be changed to achieve a compromise between equilibrium yield, rate and other operational factors. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [5]
51. June 2018 · Paper 3 · Q2(c)(ii) · 2 marks ·
(ii) Explain the observations and predict whether the formation of the blue colour is exothermic or endothermic. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2]
52. June 2017 · AS Paper 1 · Q13 · 1 marks ·
13 The reversible reaction below is allowed to reach equilibrium. H2(g) + I2(g) 2HI(g) ΔH = –9.4 kJ mol–1 Which change in conditions would be expected to shift the equilibrium position towards the products? A decrease the pressure B decrease the temperature C increase the pressure D increase the temperature Your answer [1]
53. June 2017 · Paper 1 · Q20(a) · 4 marks ·
20 This question is about equilibrium reactions. (a) Hydrogen gas is manufactured by the chemical industry using the reaction of methane and steam. This is a reversible reaction, shown in equilibrium 20.1 below. equilibrium 20.1 CH4(g) + H2O(g) 3H2(g) + CO(g) ∆H = +210 kJ mol–1 Explain, in terms of le Chatelier’s principle, the conditions of pressure and temperature for a maximum yield of hydrogen from equilibrium 20.1, and explain why the operational conditions used by the chemical industry may be different. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [4]
54. June 2017 · Paper 1 · Q20(b)(i) · 2 marks ·
(b) A chemist investigates the equilibrium reaction between sulfur dioxide, oxygen, and sulfur trioxide, shown below. 2SO2(g) + O2(g) 2SO3(g) • The chemist mixes together SO2 and O2 with a catalyst. • The chemist compresses the gas mixture to a volume of 400 cm3. • The mixture is heated to a constant temperature and is allowed to reach equilibrium without changing the total gas volume. The equilibrium mixture contains 0.0540 mol SO2 and 0.0270 mol O2. At the temperature used, the numerical value for Kc is 3.045 × 104 dm3 mol–1. (i) Write the expression for Kc and the units of Kc for this equilibrium. [2]
55. June 2017 · AS Paper 1 · Q23(c) · 2 marks ·
(c) A chemist investigates the equilibrium that produces methanol: CO(g) + 2H2(g) CH3OH(g) The chemist mixes CO(g) with H2(g) and leaves the mixture to react until equilibrium is reached. The equilibrium mixture is analysed and found to contain the following concentrations. Concentration Substance /mol dm–3 CO(g) 0.310 H2(g) 0.240 CH3OH(g) 0.260 Calculate the numerical value of Kc for this equilibrium. Give your answer to an appropriate number of significant figures. Kc = ........................................ dm2 mol−6 [2]
56. June 2016 · AS Paper 1 · Q25(a) · 5 marks · AO1, 3
20 25 Sulfur trioxide, SO3, is used for the industrial manufacture of sulfuric acid. SO3 is produced by reacting sulfur dioxide, SO2, and oxygen, O2, as shown in equilibrium 25.1 below. Equilibrium 25.1 2SO2(g) + O2(g) 2SO3(g) ΔH = –197 kJ mol–1 (a) Le Chatelier’s principle can be used to predict how different conditions affect the equilibrium position. • Using le Chatelier’s principle, show that a low temperature and a high pressure should be used to obtain a maximum equilibrium yield of SO3. • Explain why the actual conditions used in industry may be different from the conditions needed for a maximum equilibrium yield. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [5]
57. June 2016 · AS Paper 1 · Q25(b) · 4 marks · AO1, 2, 3
21 (b) Under certain conditions, Kc for equilibrium 25.1 is 0.160 dm3 mol–1. The equilibrium mixture under these conditions has the following concentrations of SO2 and O2. Species Equilibrium concentration / mol dm–3 SO2 2.00 O2 1.20 • Using the value of Kc, explain whether the equilibrium position will be towards the right or towards the left under these conditions. • Calculate the concentration of SO3 in the equilibrium mixture. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [4]

Mark schemes

1. June 2025 · AS Paper 1 · Q12
Mark scheme June 2025 Paper 1 Q12
2. June 2025 · AS Paper 1 · Q13
Mark scheme June 2025 Paper 1 Q13
3. June 2025 · AS Paper 1 · Q23(a)
Mark scheme June 2025 Paper 1 Q23(a)
4. June 2025 · AS Paper 1 · Q23(c)
Mark scheme June 2025 Paper 1 Q23(c)
5. June 2025 · Paper 3 · Q3(a)
Mark scheme June 2025 Paper 3 Q3(a)
6. June 2024 · Paper 1 · Q11
Mark scheme June 2024 Paper 1 Q11
7. June 2024 · AS Paper 1 · Q13
Mark scheme June 2024 Paper 1 Q13
8. June 2024 · AS Paper 1 · Q23(a)(i)
Mark scheme June 2024 Paper 1 Q23(a)(i)
9. June 2024 · AS Paper 1 · Q23(a)(ii)
Mark scheme June 2024 Paper 1 Q23(a)(ii)
10. June 2023 · Paper 1 · Q15
Mark scheme June 2023 Paper 1 Q15
11. June 2023 · Paper 1 · Q19(a)(ii)
Mark scheme June 2023 Paper 1 Q19(a)(ii)
12. June 2023 · AS Paper 1 · Q24(a)
Mark scheme June 2023 Paper 1 Q24(a)
13. June 2023 · AS Paper 1 · Q24(b)
Mark scheme June 2023 Paper 1 Q24(b)
14. June 2023 · AS Paper 1 · Q24(c)(i)
Mark scheme June 2023 Paper 1 Q24(c)(i)
15. June 2023 · AS Paper 1 · Q24(c)(ii)
Mark scheme June 2023 Paper 1 Q24(c)(ii)
16. June 2023 · Paper 3 · Q6(d)
Mark scheme June 2023 Paper 3 Q6(d)
17. June 2022 · AS Paper 1 · Q12
Mark scheme June 2022 Paper 1 Q12
18. June 2022 · AS Paper 1 · Q13
Mark scheme June 2022 Paper 1 Q13
19. June 2022 · Paper 1 · Q20(a)
Mark scheme June 2022 Paper 1 Q20(a)
20. June 2022 · Paper 1 · Q20(b)(ii)
Mark scheme June 2022 Paper 1 Q20(b)(ii)
21. June 2022 · AS Paper 2 · Q4(a)(i)
Mark scheme June 2022 Paper 2 Q4(a)(i)
22. June 2022 · AS Paper 2 · Q4(a)(ii)
Mark scheme June 2022 Paper 2 Q4(a)(ii)
23. June 2022 · Paper 3 · Q4(a)
Mark scheme June 2022 Paper 3 Q4(a)
24. June 2022 · Paper 3 · Q4(b)(ii)
Mark scheme June 2022 Paper 3 Q4(b)(ii)
25. November 2021 · Paper 1 · Q9
Mark scheme November 2021 Paper 1 Q9
26. November 2021 · AS Paper 1 · Q12
Mark scheme November 2021 Paper 1 Q12
27. November 2021 · Paper 1 · Q20(a)
Mark scheme November 2021 Paper 1 Q20(a)
28. November 2021 · Paper 1 · Q20(c)(ii)
Mark scheme November 2021 Paper 1 Q20(c)(ii)
29. November 2021 · AS Paper 1 · Q22(b)
Mark scheme November 2021 Paper 1 Q22(b)
30. November 2021 · AS Paper 2 · Q5(a)
Mark scheme November 2021 Paper 2 Q5(a)
31. November 2021 · AS Paper 2 · Q5(b)(i)
Mark scheme November 2021 Paper 2 Q5(b)(i)
32. November 2021 · AS Paper 2 · Q5(b)(ii)
Mark scheme November 2021 Paper 2 Q5(b)(ii)
33. November 2021 · Paper 3 · Q1(b)
Mark scheme November 2021 Paper 3 Q1(b)
34. November 2020 · AS Paper 1 · Q12
Mark scheme November 2020 Paper 1 Q12
35. November 2020 · AS Paper 1 · Q13
Mark scheme November 2020 Paper 1 Q13
36. November 2020 · Paper 1 · Q13
Mark scheme November 2020 Paper 1 Q13
37. November 2020 · Paper 1 · Q17(a)
Mark scheme November 2020 Paper 1 Q17(a)
38. November 2020 · AS Paper 2 · Q4(b)(i)
Mark scheme November 2020 Paper 2 Q4(b)(i)
39. November 2020 · AS Paper 2 · Q4(b)(iii)
Mark scheme November 2020 Paper 2 Q4(b)(iii)
40. November 2020 · AS Paper 2 · Q4(b)(iv)
Mark scheme November 2020 Paper 2 Q4(b)(iv)
41. November 2020 · Paper 3 · Q5(b)
Mark scheme November 2020 Paper 3 Q5(b)
42. June 2019 · AS Paper 1 · Q13
Mark scheme June 2019 Paper 1 Q13
43. June 2019 · Paper 1 · Q19(b)(iv)
Mark scheme June 2019 Paper 1 Q19(b)(iv)
44. June 2019 · AS Paper 1 · Q24(a)(i)
Mark scheme June 2019 Paper 1 Q24(a)(i)
45. June 2019 · AS Paper 1 · Q24(a)(ii)
Mark scheme June 2019 Paper 1 Q24(a)(ii)
46. June 2018 · AS Paper 1 · Q13
Mark scheme June 2018 Paper 1 Q13
47. June 2018 · AS Paper 1 · Q14
Mark scheme June 2018 Paper 1 Q14
48. June 2018 · Paper 1 · Q18(a)
Mark scheme June 2018 Paper 1 Q18(a)
49. June 2018 · AS Paper 2 · Q4(b)
Mark scheme June 2018 Paper 2 Q4(b)
50. June 2018 · AS Paper 2 · Q4(c)
Mark scheme June 2018 Paper 2 Q4(c)
51. June 2018 · Paper 3 · Q2(c)(ii)
Mark scheme June 2018 Paper 3 Q2(c)(ii)
52. June 2017 · AS Paper 1 · Q13
Mark scheme June 2017 Paper 1 Q13
53. June 2017 · Paper 1 · Q20(a)
Mark scheme June 2017 Paper 1 Q20(a)
54. June 2017 · Paper 1 · Q20(b)(i)
Mark scheme June 2017 Paper 1 Q20(b)(i)
55. June 2017 · AS Paper 1 · Q23(c)
Mark scheme June 2017 Paper 1 Q23(c)
56. June 2016 · AS Paper 1 · Q25(a)
Mark scheme June 2016 Paper 1 Q25(a)
57. June 2016 · AS Paper 1 · Q25(b)
Mark scheme June 2016 Paper 1 Q25(b)