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Entropy and free energy

Past-paper questions pack

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Questions are numbered; the mark schemes (the answer key) follow on separate pages so the question pages can be handed out alone. www.chemistryaleveltutor.com

OCR AS and A-level Chemistry past-paper questions since 2016 · 25 parts · mark schemes follow the questions · www.chemistryaleveltutor.com · Questions and mark schemes © OCR

1. June 2025 · Paper 1 · Q17(a)(iii) · 2 marks ·
(iii) Explain why the standard entropy of CO2 is greater than the standard entropy of Fe2O3. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [2]
2. June 2025 · Paper 1 · Q17(a)(iv) · 2 marks ·
(iv) The standard entropy change, ΔSɵ, for the forward reaction in Equilibrium 17.1 has a value of +15 J K–1 mol–1. Use data from the table to calculate the standard entropy, S ɵ, of Fe. Sɵ = ....................................... J K–1 mol–1 [2]
3. June 2025 · Paper 1 · Q17(b) · 3 marks ·
(b) Carbon can also reduce Fe2O3(s) to iron as shown in Equilibrium 17.2. Fe2O3(s) + 3C(g) 3CO(g) + 2Fe(s) Equilibrium 17.2 ΔH = +491 kJ mol–1 ΔS = +543 J K–1 mol–1 The forward reaction is only feasible at high temperatures. • Show that the forward reaction is not feasible at 100 °C. • Calculate the minimum temperature, in K, for the forward reaction to be feasible. Give the minimum temperature to the nearest whole number. T = ..................................................... K [3]
4. June 2024 · Paper 1 · Q16(a)(i) · 4 marks ·
SECTION B 16 This question is about energy changes. Hydrogen peroxide decomposes as shown in Reaction 16.1. H2O2(l) H2O(l) + 1 2O2(g) Reaction 16.1 (a) The table shows enthalpy changes of formation and entropies. ∆Hf ө / kJ mol–1 Sө / J K–1 mol–1 H2O2(l) –188 110 H2O(l) –286 70.0 O2(g) 0 205 (i) Calculate the free-energy change, ∆G, in kJ mol–1, of Reaction 16.1 at 25 °C. Give your answer to 3 significant figures. ∆G = ..............................................kJ mol–1 [4]
5. June 2024 · Paper 1 · Q16(a)(ii) · 1 marks ·
(ii) The decomposition of hydrogen peroxide shown in Reaction 16.1 is feasible. Suggest why Reaction 16.1 does not take place at 25 °C despite being feasible. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [1]
6. June 2023 · Paper 1 · Q9 · 1 marks · AO2
9 The table below shows standard entropies, Sө. Substance SO2(g) O2(g) SO3(l) Sө/J K–1 mol–1 248 204 96 What is the standard entropy change, ∆Sө, in J K mol–1, for the formation of 1 mol of SO3(l) from SO2(g) and O2(g)? A –508 B –254 C +254 D +508 Your answer [1]
7. 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]
8. June 2023 · Paper 1 · Q18(c)(i) · 1 marks · AO2
(c) Ethene can be produced from ethanol, as shown in Reaction 2 below. C2H5OH(g) C2H4(g) + H2O(g) Reaction 2 (i) Predict the sign of the entropy change, ∆S, for Reaction 2. Explain your reasoning. ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
9. June 2023 · Paper 1 · Q18(c)(ii) · 5 marks · AO3
(ii) Reaction 2 is repeated: C2H5OH(g) C2H4(g) + H2O(g) Reaction 2 The Gibbs equation is shown below. ∆G = ∆H – T∆S The enthalpy change, ∆H, and the entropy change, ∆S, can be assumed to be constant at different temperatures. Fig. 18.1 shows values of the free energy change, ∆G, in kJ mol–1, at different temperatures, T, in K, for Reaction 2. Fig. 18.1 60 50 40 30 20 10 ∆G 0 / kJ mol–1 –10 –20 –30 –40 –50 –60 0 100 200 300 400 500 600 700 800 900 T / K Use the graph in Fig. 18.1 to answer the following: • Draw the best-fit line on the graph in Fig. 18.1. • Determine ∆S, in J K–1 mol–1, for Reaction 2. • Determine the minimum temperature, T, at which the reaction is feasible. • Determine ∆H for Reaction 2. ∆S = .................................... J K–1 mol–1 minimum T = ....................... K ∆H = .................................... kJ mol–1 [5]
10. June 2022 · Paper 1 · Q10 · 1 marks · AO2
10 For the condensation of ammonia gas, what are the signs of ∆H and ∆S? A ∆H –ve ∆S –ve B ∆H –ve ∆S +ve C ∆H +ve ∆S +ve D ∆H +ve ∆S –ve Your answer [1]
11. June 2022 · Paper 1 · Q17(c)(i) · 1 marks · AO1
(c) Carbon disulfide, CS2, reacts with dinitrogen oxide, N2O, as shown in Reaction 17.2. 4CS2(l) + 8N2O(g) S8(s) + 4CO2(g) + 8N2(g) Reaction 17.2 Standard entropies, S ө, are shown in the table. Substance CS2(l) N2O(g) S8(s) CO2(g) N2(g) S ө/ J K–1 mol–1 151 220 256 214 192 (i) Explain the term entropy. ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
12. June 2022 · Paper 1 · Q17(c)(ii) · 3 marks · AO2
(ii) The free energy change, ∆G, of Reaction 17.2 is –2672 kJ mol–1 at 25 °C. Calculate the enthalpy change, ∆H, of Reaction 17.2, in kJ mol–1. ∆H = ............................................ kJ mol–1 [3]
13. June 2022 · Paper 1 · Q17(c)(iii) · 2 marks · AO3
(iii) A student concludes that Reaction 17.2 is feasible at all temperatures. Explain whether the student is correct or not. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2]
14. November 2021 · Paper 1 · Q11 · 1 marks ·
11 The feasibility of a chemical reaction depends on the temperature and the signs of ΔH and ΔS. Which row has signs for ΔH and ΔS for a reaction that is feasible at high temperatures but is not feasible at low temperatures? ΔH ΔS A + + B + – C – + D – – Your answer [1]
15. November 2021 · Paper 1 · Q20(b)(i) · 2 marks · AO2
(b) (i) Show that the formation of NO2 in Equilibrium 20.1 is feasible at 25 °C. [2]
16. November 2021 · Paper 1 · Q20(b)(ii) · 1 marks · AO2
(ii) Determine the maximum temperature, in K, for feasibility. Give your answer to an appropriate number of significant figures. maximum temperature = .................... K [1]
17. November 2020 · Paper 1 · Q17(c) · 5 marks · AO2, 3
(c) Standard entropy values are given below. Substance CO(g) H2(g) CH3OH(g) Sө / J K−1 mol−1 198 131 238 A chemist proposed producing methanol at 525 K using equilibrium 1. Explain, with a calculation, whether the production of methanol is feasible at 525 K. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [5]
18. November 2020 · Paper 1 · Q17(d) · 3 marks · AO1, 3
(d) At 298 K, the free energy change, ∆G, for the production of methanol in equilibrium 1 is −2.48 × 104 J mol−1. ∆G is linked to Kp by the relationship: ∆G = −RT lnKp. R = gas constant T = temperature in K. Calculate Kp for equilibrium 1 at 298 K. Give your answer to 3 significant figures. Kp = .......................... units ...................... [3]
19. June 2019 · Paper 1 · Q19(a)(ii) · 4 marks · AO2, 3
(ii) Some standard entropy values are given below. Substance H2S(g) O2(g) SO2(g) H2O(l) Sө / J K−1 mol−1 206 205 248 70 Using calculations, explain whether Reaction 1 is feasible at 20 °C. Calculations Explanation for feasible or non feasible ....................................................................... ...................................................................................................................................... [4]
20. June 2019 · Paper 3 · Q5(b) · 4 marks · AO2
(b) The standard enthalpy change of reaction, ∆rH o , and the standard free energy change, ∆G o , for converting anhydrous sodium thiosulfate to hydrated sodium thiosulfate are shown below. Na2S2O3(s) + 5H2O(l) → Na2S2O3•5H2O(s) ∆rH o = –55.8 kJ mol−1 ∆G o = –16.1 kJ mol−1 Standard entropies are given in the table. Compound S o / J K−1 mol−1 Na2S2O3•5H2O(s) 372.4 H2O(l) 69.9 Determine the standard entropy, S o , of anhydrous sodium thiosulfate, Na2S2O3(s). Give your answer to 3 significant figures. S o = ....................................... J K−1 mol−1 [4]
21. June 2018 · Paper 1 · Q11 · 1 marks · AO1
11 The table below shows standard entropies, Sө. Substance CO(g) H2(g) CH3OH(l) Sө / J mol–1 K–1 197.6 130.6 239.7 What is the entropy change, ∆Sө, in J mol–1 K–1, for the following reaction? CO(g) + 2H2(g) CH3OH(l) A –219.1 B –88.5 C +88.5 D +219.1 Your answer [1]
22. June 2017 · Paper 1 · Q17(d)(i) · 2 marks ·
(d) Two changes are described below. For each change, • write an equation, including state symbols, • state and explain how the entropy changes. (i) The reaction of aqueous barium nitrate with aqueous sodium sulfate. Full equation with state symbols ........................................................................................................................................... Explanation of entropy change .......................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2]
23. June 2017 · Paper 1 · Q17(d)(ii) · 2 marks ·
(ii) The change that accompanies the standard enthalpy change of atomisation of iodine. Equation with state symbols ........................................................................................................................................... Explanation of entropy change .......................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2]
24. June 2017 · Paper 1 · Q18(a) · 4 marks ·
18 This question is about free energy changes, ∆G, enthalpy changes, ∆H, and temperature, T. (a) The Gibbs’ equation is shown below. ∆G = ∆H – T∆S A chemist investigates a reaction to determine how ∆G varies with T. The results are shown in Fig. 18.1. + ΔG Q / kJ mol–1 0,0 T/K – P Fig. 18.1 What is significant about the gradient of the line and the values P and Q shown in Fig. 18.1? Explain your reasoning. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [4]
25. June 2017 · Paper 1 · Q18(b)(iii) · 3 marks ·
(iii) The forward reaction in equilibrium 18.1 is only feasible at high temperatures. • Show that the forward reaction is not feasible at 25 °C. • Calculate the minimum temperature, in K, for the forward reaction to be feasible. minimum temperature = ...................................................... K [3]

Mark schemes

1. June 2025 · Paper 1 · Q17(a)(iii)
Mark scheme June 2025 Paper 1 Q17(a)(iii)
2. June 2025 · Paper 1 · Q17(a)(iv)
Mark scheme June 2025 Paper 1 Q17(a)(iv)
3. June 2025 · Paper 1 · Q17(b)
Mark scheme June 2025 Paper 1 Q17(b)
4. June 2024 · Paper 1 · Q16(a)(i)
Mark scheme June 2024 Paper 1 Q16(a)(i)
5. June 2024 · Paper 1 · Q16(a)(ii)
Mark scheme June 2024 Paper 1 Q16(a)(ii)
6. June 2023 · Paper 1 · Q9
Mark scheme June 2023 Paper 1 Q9
7. June 2023 · Paper 1 · Q15
Mark scheme June 2023 Paper 1 Q15
8. June 2023 · Paper 1 · Q18(c)(i)
Mark scheme June 2023 Paper 1 Q18(c)(i)
9. June 2023 · Paper 1 · Q18(c)(ii)
Mark scheme June 2023 Paper 1 Q18(c)(ii)
10. June 2022 · Paper 1 · Q10
Mark scheme June 2022 Paper 1 Q10
11. June 2022 · Paper 1 · Q17(c)(i)
Mark scheme June 2022 Paper 1 Q17(c)(i)
12. June 2022 · Paper 1 · Q17(c)(ii)
Mark scheme June 2022 Paper 1 Q17(c)(ii)
13. June 2022 · Paper 1 · Q17(c)(iii)
Mark scheme June 2022 Paper 1 Q17(c)(iii)
14. November 2021 · Paper 1 · Q11
Mark scheme November 2021 Paper 1 Q11
15. November 2021 · Paper 1 · Q20(b)(i)
Mark scheme November 2021 Paper 1 Q20(b)(i)
16. November 2021 · Paper 1 · Q20(b)(ii)
Mark scheme November 2021 Paper 1 Q20(b)(ii)
17. November 2020 · Paper 1 · Q17(c)
Mark scheme November 2020 Paper 1 Q17(c)
18. November 2020 · Paper 1 · Q17(d)
Mark scheme November 2020 Paper 1 Q17(d)
19. June 2019 · Paper 1 · Q19(a)(ii)
Mark scheme June 2019 Paper 1 Q19(a)(ii)
20. June 2019 · Paper 3 · Q5(b)
Mark scheme June 2019 Paper 3 Q5(b)
21. June 2018 · Paper 1 · Q11
Mark scheme June 2018 Paper 1 Q11
22. June 2017 · Paper 1 · Q17(d)(i)
Mark scheme June 2017 Paper 1 Q17(d)(i)
23. June 2017 · Paper 1 · Q17(d)(ii)
Mark scheme June 2017 Paper 1 Q17(d)(ii)
24. June 2017 · Paper 1 · Q18(a)
Mark scheme June 2017 Paper 1 Q18(a)
25. June 2017 · Paper 1 · Q18(b)(iii)
Mark scheme June 2017 Paper 1 Q18(b)(iii)