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Enthalpy and entropy: OCR A‑Level Chemistry A questions

Every OCR A-Level Chemistry A exam question on 5.2.2 Enthalpy and entropy, 2016–2025, with its mark scheme and examiner’s report.

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1,820 questions

  1. Q17(a)(iii)

    June 2025 · Paper 1

    2 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2025 Paper 1, question 17(a)(iii): Explain why the standard entropy of CO2 is greater than the standard entropy of Fe2O3. [2]

  2. Q17(a)(iv)

    June 2025 · Paper 1

    2 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2025 Paper 1, question 17(a)(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

  3. Q17(b)

    June 2025 · Paper 1

    3 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2025 Paper 1, question 17(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

  4. Q16(a)(i)

    June 2024 · Paper 1

    4 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2024 Paper 1, question 16(a)(i): Give your answer to 3 significant figures. ∆G = kJ mol–1 [4]

  5. Q16(a)(ii)

    June 2024 · Paper 1

    1 mark5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2024 Paper 1, question 16(a)(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. Q9

    June 2023 · Paper 1

    1 mark5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2023 Paper 1, question 9: The table below shows standard entropies, Sө. 9 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,

  7. Q18(c)(i)

    June 2023 · Paper 1

    1 mark5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2023 Paper 1, question 18(c)(i): Predict the sign of the entropy change, ∆S, for Reaction 2. Explain your reasoning. [1]

  8. Q18(c)(ii)

    June 2023 · Paper 1

    5 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2023 Paper 1, question 18(c)(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,

  9. Q10

    June 2022 · Paper 1

    1 mark5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2022 Paper 1, question 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]

  10. Q17(c)(i)

    June 2022 · Paper 1

    1 mark5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2022 Paper 1, question 17(c)(i): Explain the term entropy. [1]

  11. Q17(c)(ii)

    June 2022 · Paper 1

    3 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2022 Paper 1, question 17(c)(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]

  12. Q17(c)(iii)

    June 2022 · Paper 1

    2 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    June 2022 Paper 1, question 17(c)(iii): A student concludes that Reaction 17.2 is feasible at all temperatures. Explain whether the student is correct or not. [2]

  13. Q11

    November 2021 · Paper 1

    1 mark5.2.2 Enthalpy and entropy
    The whole paper ↗
    November 2021 Paper 1, question 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

  14. Q20(b)(i)

    November 2021 · Paper 1

    2 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    November 2021 Paper 1, question 20(b)(i): Show that the formation of NO2 in Equilibrium 20.1 is feasible at 25 °C. [2]

  15. Q20(b)(ii)

    November 2021 · Paper 1

    1 mark5.2.2 Enthalpy and entropy
    The whole paper ↗
    November 2021 Paper 1, question 20(b)(ii): Determine the maximum temperature, in K, for feasibility. Give your answer to an appropriate number of significant figures. maximum temperature = K [1]

  16. Q17(c)

    November 2020 · Paper 1

    5 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    November 2020 Paper 1, question 17(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 equilib

  17. Q17(d)

    November 2020 · Paper 1

    3 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    November 2020 Paper 1, question 17(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 l

  18. Q5(a)

    November 2020 · Paper 3

    2 marks5.1.2 How far?5.2.2 Enthalpy and entropy
    The whole paper ↗
    November 2020 Paper 3, question 5(a): The table shows the values of Kp at different temperatures for an equilibrium. Complete the table by adding the missing values of 1 T and ln Kp. Temperature, T

  19. Q5(c)

    November 2020 · Paper 3

    4 marks5.1.2 How far?5.2.2 Enthalpy and entropy
    The whole paper ↗
    November 2020 Paper 3, question 5(c): Plot a graph of ln Kp against 1 T using the axes provided on the opposite page. Use your graph and equation 5.1 to determine ∆H, in kJ mol−1, for this equilibri

  20. Q5(d)

    November 2020 · Paper 3

    2 marks5.2.2 Enthalpy and entropy
    The whole paper ↗
    November 2020 Paper 3, question 5(d): Explain how ∆S could be calculated from a graph of ln Kp against 1 T . [2]
All 27 questions on Enthalpy and entropy, as a list
  1. June 2025, Paper 1, Q17(a)(iii) (2 marks): 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): 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
  3. June 2025, Paper 1, Q17(b) (3 marks): 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
  4. June 2024, Paper 1, Q16(a)(i) (4 marks): Give your answer to 3 significant figures. ∆G = kJ mol–1 [4]
  5. June 2024, Paper 1, Q16(a)(ii) (1 mark): 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 mark): The table below shows standard entropies, Sө. 9 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,
  7. June 2023, Paper 1, Q18(c)(i) (1 mark): Predict the sign of the entropy change, ∆S, for Reaction 2. Explain your reasoning. [1]
  8. June 2023, Paper 1, Q18(c)(ii) (5 marks): 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,
  9. June 2022, Paper 1, Q10 (1 mark): 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]
  10. June 2022, Paper 1, Q17(c)(i) (1 mark): Explain the term entropy. [1]
  11. June 2022, Paper 1, Q17(c)(ii) (3 marks): 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]
  12. June 2022, Paper 1, Q17(c)(iii) (2 marks): A student concludes that Reaction 17.2 is feasible at all temperatures. Explain whether the student is correct or not. [2]
  13. November 2021, Paper 1, Q11 (1 mark): 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
  14. November 2021, Paper 1, Q20(b)(i) (2 marks): Show that the formation of NO2 in Equilibrium 20.1 is feasible at 25 °C. [2]
  15. November 2021, Paper 1, Q20(b)(ii) (1 mark): Determine the maximum temperature, in K, for feasibility. Give your answer to an appropriate number of significant figures. maximum temperature = K [1]
  16. November 2020, Paper 1, Q17(c) (5 marks): 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 equilib
  17. November 2020, Paper 1, Q17(d) (3 marks): 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 l
  18. November 2020, Paper 3, Q5(a) (2 marks): The table shows the values of Kp at different temperatures for an equilibrium. Complete the table by adding the missing values of 1 T and ln Kp. Temperature, T
  19. November 2020, Paper 3, Q5(c) (4 marks): Plot a graph of ln Kp against 1 T using the axes provided on the opposite page. Use your graph and equation 5.1 to determine ∆H, in kJ mol−1, for this equilibri
  20. November 2020, Paper 3, Q5(d) (2 marks): Explain how ∆S could be calculated from a graph of ln Kp against 1 T . [2]
  21. June 2019, Paper 1, Q19(a)(ii) (4 marks): 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
  22. June 2019, Paper 3, Q5(b) (4 marks): The standard enthalpy change of reaction, ∆rH o , and the standard free energy change, ∆G o , for converting anhydrous sodium thiosulfate to hydrated sodium thi
  23. June 2018, Paper 1, Q11 (1 mark): 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,
  24. June 2017, Paper 1, Q17(d)(i) (2 marks): The reaction of aqueous barium nitrate with aqueous sodium sulfate. Full equation with state symbols Explanation of entropy change [2]
  25. June 2017, Paper 1, Q17(d)(ii) (2 marks): The change that accompanies the standard enthalpy change of atomisation of iodine. Equation with state symbols Explanation of entropy change [2]
  26. June 2017, Paper 1, Q18(a) (4 marks): 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
  27. June 2017, Paper 1, Q18(b)(iii) (3 marks): 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 minim

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