plumiHess' law and enthalpy cycles: OCR A‑Level Chemistry A questions
OCR A-Level Chemistry A past-paper questions on 3.2.1.5 Hess' law and enthalpy cycles, 2016–2025, with mark schemes and examiner’s reports where available.
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Q17(a)(ii)
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Open the full PDFYour answerQ18(b)
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Open the full PDFQ23(b)(ii)
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Open the full PDFYour answerQ24(a)
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Open the full PDFQ19(a)(iii)
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Open the full PDFYour answerQ3(c)
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All 19 questions on Hess' law and enthalpy cycles, as a list
- June 2025, Paper 1, Q17(a)(ii) (2 marks): Calculate the enthalpy change, ΔrH, for the forward reaction in Equilibrium 17.1. ΔH = kJ mol–1 [2]
- June 2025, Paper 3, Q6 (6 marks): 6* The enthalpy change of Reaction 6.1, ∆H1, shown below, is difficult to measure directly by experiment. 2NaHCO3(s) Na2CO3(s) + H2O(l) + CO2(g) ∆H1 Reaction 6.
- June 2025, AS Paper 1, Q11 (1 mark): The equation for the complete combustion of methanol, CH3OH, is shown below. CH3OH(l) + 11 2O2(g) CO2(g) + 2H2O(l) The table shows standard enthalpy changes of
- June 2024, AS Paper 1, Q11 (1 mark): An enthalpy cycle is shown below. ΔrH C(s) + ½O2(g) CO(g) + ½O2(g) + ½O2(g) ΔcH = –283 kJ mol–1 ΔcH = –393 kJ mol–1 CO2(g) What is ΔrH, in kJ mol–1, shown in th
- June 2023, Paper 1, Q18(b) (2 marks): Nonane, C9H20, can be broken down by heat to form pentane, C5H12, and ethene, C2H4. C5H12(g) + 2C2H4(g) ∆H = +186 kJ mol–1 Reaction 1 C9H20(g) The enthalpy chan
- June 2023, AS Paper 1, Q23(b)(ii) (3 marks): Calculate the standard enthalpy change of formation for CO2(g). Δ f H o (CO2(g)) = kJ mol–1 [3]
- June 2022, Paper 1, Q17(b)(ii) (2 marks): Calculate the standard enthalpy change of formation, ∆fHө, of NO(g). ∆fHө of NO(g) = kJ mol–1 [2]
- June 2022, AS Paper 1, Q11 (1 mark): Combustion of hydrazine, N2H4, produces NO2 and H2O as in the equation below. N2H4(l) + 3O2(g) → 2NO2(g) + 2H2O(l) The table shows standard enthalpy changes of
- November 2021, AS Paper 1, Q10 (1 mark): The equation for the complete combustion of propene, C3H6, is shown below. C3H6(g) + 4½O2(g) → 3CO2(g) + 3H2O(l) Standard enthalpy changes of formation, ∆f Ho,
- November 2020, Paper 1, Q6 (1 mark): The equation for the combustion of C7H8 is shown in the following equation. C7H8(l) + 9O2(g) 7CO2(g) + 4H2O(l) Enthalpy changes of formation are shown in the ta
- November 2020, AS Paper 1, Q24(a) (3 marks): Ammonia is manufactured by reacting nitrogen with hydrogen: N2(g) + 3H2(g) → 2NH3(g) Standard enthalpy changes of combustion, ∆cHo, are given in the table. ∆cHo
- June 2019, Paper 1, Q19(a)(iii) (3 marks): Calculate the standard enthalpy change of formation, ∆fH ө, of hydrogen sulfide using the enthalpy change for Reaction 1, and the standard enthalpy changes of c
- June 2019, AS Paper 1, Q24(b)(ii) (3 marks): Calculate the standard enthalpy change of formation for NO(g). Give your answer to a whole number. ∆fHo for NO(g) = kJ mol−1 [3]
- June 2018, AS Paper 1, Q11 (1 mark): The equation for the reaction of aluminium sulfide, Al 2S3, with oxygen is shown below. 2Al 2S3(s) + 9O2(g) → 2Al 2O3(s) + 6SO2(g) The table shows standard enth
- June 2017, Paper 1, Q18(b)(iv) (3 marks): Another equilibrium involved in the extraction of iron from Fe3O4 is shown below. 3Fe(s) + 4CO2(g) ∆H = –13.5 kJ mol–1 Fe3O4(s) + 4CO(g) Enthalpy changes of for
- June 2017, Paper 3, Q2(a) (6 marks): A student plans to determine the enthalpy change of reaction 3.1 shown below. Na2O(s) + 2HCl(aq) 2NaCl(aq) + H2O(l) reaction 3.1 This enthalpy change can be det
- June 2017, AS Paper 1, Q12 (1 mark): The table shows standard enthalpy changes of combustion, ΔcH. ΔcH / kJ mol–1 Substance C(s) –393.5 H2(g) –285.8 C4H10(g) –2876.5 What is the enthalpy change for
- June 2017, AS Paper 2, Q3(c) (3 marks): The equation for the complete combustion of cyclohexane is shown below. C6H12(l) + 9O2(g) 6CO2(g) + 6H2O(l) Standard enthalpy changes of formation, ∆fH , are sh
- June 2016, AS Paper 1, Q12 (1 mark): The enthalpy change of formation of butane can be calculated using the enthalpy changes of combustion, ΔcH, below. Substance C(s) H2(g) C4H10(g) ΔcH / kJ mol–1
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