plumiEnthalpy changes: OCR A‑Level Chemistry A questions
Every OCR A-Level Chemistry A exam question on 3.2.1 Enthalpy changes, 2016–2025, with its mark scheme and examiner’s report.
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Q17(a)(i)
The whole paper ↗![June 2025 Paper 1, question 17(a)(i): [1]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43201-jun25-17-ai-qp.webp?v=3)
Q17(a)(ii)
The whole paper ↗![June 2025 Paper 1, question 17(a)(ii): Calculate the enthalpy change, ΔrH, for the forward reaction in Equilibrium 17.1. ΔH = kJ mol–1 [2]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43201-jun25-17-aii-qp.webp?v=3)
Q23(b)
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Q1(c)(i)
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Q1(c)(ii)
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Q4(b)(ii)
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Q4(b)(iii)
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Q4(b)(iv)
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Q2(a)(i)
The whole paper ↗![June 2024 Paper 3, question 2(a)(i): Why do all average bond enthalpies have a positive value? [1]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43203-jun24-02-ai-qp.webp?v=3)
Q2(a)(ii)
The whole paper ↗![June 2024 Paper 3, question 2(a)(ii): Determine the C–H bond enthalpy, in kJ mol–1, using the information above. C–H bond enthalpy = kJ mol–1 [3]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43203-jun24-02-aii-qp.webp?v=3)
Q2(a)(iii)
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Q23(b)
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Q3(a)
The whole paper ↗![June 2024 AS Paper 2, question 3(a): What is meant by the term enthalpy change of reaction? [1]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h03202-jun24-03-a-qp.webp?v=3)
Q3(b)
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Q3(c)
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All 100 questions on Enthalpy changes, as a list
- June 2025, Paper 1, Q6 (1 mark): Methane reacts with oxygen as shown in the equation: CO2(g) + 2H2O(g) ΔrH = –730 kJ mol–1 CH4(g) + 2O2(g) The table shows some bond enthalpies. Bond O–H O=O C=O
- June 2025, Paper 1, Q17(a)(i) (1 mark): [1]
- 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 2025, AS Paper 1, Q23(b) (3 marks): The displayed formulae of SO2 and SO3 are shown below. O S O O S O O The bond enthalpies for the S=O bond in SO2 and SO3 are not the same. The table shows bond
- June 2025, AS Paper 2, Q1(c)(i) (4 marks): Give your answer to 3 significant figures. Assume that the density of all solutions and the specific heat capacity, c, of the reaction mixture is the same as fo
- June 2025, AS Paper 2, Q1(c)(ii) (2 marks): The student repeats the experiment using the same volume and concentration of NaOH but using 20.0 cm3 of 2.80 mol dm–3 HCl instead of 40.0 cm3 of 1.40 mol dm–3
- June 2025, AS Paper 2, Q4(b)(ii) (2 marks): Use the information in Table 4.1 to calculate the mass of carbon dioxide formed when 1.00 kJ of energy is released during the complete combustion of pentane. Ma
- June 2025, AS Paper 2, Q4(b)(iii) (2 marks): Complete the enthalpy profile diagram for the complete combustion of pentane. On your diagram: • Label the enthalpy change as ΔH. • Include the formulae of the
- June 2025, AS Paper 2, Q4(b)(iv) (1 mark): Pentane C5H12 36 –3509 Hexane C6H14 69 –4163 (iv) Use the data in Table 4.1 to estimate a value for the enthalpy change of combustion of butane. Give your answe
- June 2024, Paper 1, Q7 (1 mark): The equation for the reaction of sulfuric acid with potassium hydroxide is shown below. H2SO4(aq) + 2KOH(aq) K2SO4(aq) + 2H2O(l) 25 cm3 of 1.00 mol dm–3 H2SO4 i
- June 2024, Paper 3, Q2(a)(i) (1 mark): Why do all average bond enthalpies have a positive value? [1]
- June 2024, Paper 3, Q2(a)(ii) (3 marks): Determine the C–H bond enthalpy, in kJ mol–1, using the information above. C–H bond enthalpy = kJ mol–1 [3]
- June 2024, Paper 3, Q2(a)(iii) (2 marks): Hydrogen gas is being considered as a household fuel to replace methane. The enthalpy change of formation, ΔfH, for H2O(l) is –285.8 kJ mol–1. Determine the ene
- 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 2024, AS Paper 1, Q23(b) (3 marks): Hydrazine, N2H4, shown below, can be used as a rocket fuel. H H N N H H As a fuel, N2H4 reacts with oxygen as shown below. N2(g) + 2H2O(g) ΔH = –581 kJ mol–1 N2
- June 2024, AS Paper 2, Q3(a) (1 mark): What is meant by the term enthalpy change of reaction? [1]
- June 2024, AS Paper 2, Q3(b) (6 marks): Enthalpy changes of reaction can be determined by experiment. (b)* A student carries out an experiment to determine the enthalpy change for the reaction between
- June 2024, AS Paper 2, Q3(c) (2 marks): The student modifies the experiment using 50 cm3 instead of 100 cm3 of 0.500 mol dm–3 copper(II) nitrate solution. The value of ΔrH for this modified experiment
- June 2023, Paper 1, Q18(a) (3 marks): A student determines the enthalpy change of combustion, ∆cH, of heptane, C7H16, using the method outlined below. • Add 150 g of water to a beaker and measure it
- 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, Paper 3, Q2(a)(ii) (3 marks): A student carries out an experiment to determine the enthalpy change of solution of RbCl O3 using the method below. • A 2.00 g sample of solid RbCl O3 is added
- June 2023, AS Paper 1, Q11 (1 mark): The standard enthalpy change of formation of water is –286 kJ mol–1. Which statement or equation is correct? ΔH o = –143 kJ mol–1 A H2(g) + ½O2(g) H2O(l) ΔH o =
- June 2023, AS Paper 1, Q12 (1 mark): Which statement about energy changes is correct? A Combustion of an alkane is endothermic. B In an exothermic reaction, more energy is needed to break bonds tha
- June 2023, AS Paper 1, Q23(a)(ii) (2 marks): In a petrol engine, polluting gases such as CO and NO are formed. These are mostly removed before being emitted from the exhaust. The equation for the removal o
- June 2023, AS Paper 1, Q23(b)(i) (1 mark): State the conditions of temperature and pressure for standard enthalpy changes. Temperature Pressure [1]
- 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 2023, AS Paper 2, Q4(a) (2 marks): Explain the term enthalpy change of combustion. [2]
- June 2023, AS Paper 2, Q4(b)(i) (1 mark): The thermometer had an uncertainty of ± 0.25 °C in each temperature reading. Calculate the percentage uncertainty in the temperature change. percentage uncertai
- June 2023, AS Paper 2, Q4(b)(ii) (6 marks): This question is about the enthalpy change of combustion of alcohols. (ii)* Use the student’s results to determine ∆cH of butan-1-ol in kJ mol–1. Explain why th
- June 2023, AS Paper 2, Q4(b)(iii) (1 mark): Another student carries out the experiment in 4(b) using 150 g of water in the beaker instead of 200 g. Calculate the mass of butan-1-ol that would produce the
- June 2023, AS Paper 2, Q4(c)(i) (4 marks): Use this value of ΔcH and the average bond enthalpies below to calculate the average bond enthalpy of C=O. Average bond enthalpy / Bond kJ mol–1 C–H +413 C–C +3
- June 2023, AS Paper 2, Q4(c)(ii) (2 marks): Methoxymethane, CH3OCH3, is an isomer of ethanol. On combustion, methoxymethane, in the gaseous state, produces carbon dioxide and steam. H H H C O C H(g) + 3 O
- June 2022, Paper 1, Q7 (1 mark): Bond enthalpies are shown in the table. Bond C–C C–H O–H C–O C=O O–O O=O Bond enthalpy 347 435 464 358 805 144 498 / kJ mol–1 What is the enthalpy change, in kJ
- June 2022, Paper 1, Q17(a)(i) (4 marks): Determine the maximum temperature reached in this reaction. Give your answer to 3 significant figures. Assume that the specific heat capacity and density of the
- June 2022, Paper 1, Q17(a)(ii) (1 mark): The student wants to repeat the experiment, but there is not enough AgNO3(aq) left to use another 100.0 cm3 portion. The student decides to modify the method by
- June 2022, Paper 1, Q17(b)(i) (1 mark): Explain the term enthalpy change of formation. [1]
- 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, Paper 3, Q5(a) (6 marks): This question is about energy changes. (a)* A student plans to determine the enthalpy change of hydration of calcium ions. The student finds the information bel
- June 2022, Paper 3, Q5(b)(ii) (3 marks): Calculate the energy released, in kJ, by the complete combustion of 8.00 kg of this fuel. ∆cH(C8H18) = –5470 kJ mol–1; ∆cH(C2H5OH) = –1367 kJ mol–1. energy rele
- 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
- June 2022, AS Paper 1, Q25(a)(i) (4 marks): Calculate ∆rH, in kJ mol–1, for the reaction shown in Equation 25.1. Give your answer to 3 significant figures. Assume that the density and specific heat capaci
- June 2022, AS Paper 1, Q25(a)(ii) (2 marks): The student looked back at Equation 25.1 and noticed that the reaction was a neutralisation. The student concluded that ∆rH is the enthalpy change of neutralisa
- June 2022, AS Paper 2, Q4(b) (3 marks): Hydrogen can also be manufactured by reacting ethanol with steam, as shown in Equilibrium 4.2. H H H C C O H + 3 H O H 6 H H + 2 O C O Equilibrium 4.2 H H Avera
- November 2021, Paper 1, Q8 (1 mark): Bond enthalpies are given in the table. Bond H–S O=O S=O O–H Bond enthalpy +346 +498 +531 +464 / kJ mol–1 What is the enthalpy change, in kJ mol–1, for the reac
- November 2021, Paper 1, Q22(b)(ii) (4 marks): Calculate the enthalpy change, ΔH1, for Equation 22.1, and deduce the value for the enthalpy change of neutralisation, ∆neutH, in kJ mol–1. Assume that the dens
- November 2021, Paper 1, Q22(b)(iii) (2 marks): The student repeats the experiment using 50.0 cm3 of 1.60 mol dm–3 H2SO4 and 110.0 cm3 of 1.50 mol dm–3 NaOH. Predict the increase in temperature. Explain your
- 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 2021, AS Paper 1, Q22(c) (3 marks): The reaction for the production of hydrogen is repeated below. 3H2(g) + CO(g) ∆H = +206 kJ mol–1 CH4(g) + H2O(g) Average bond enthalpies are shown in the table.
- November 2021, AS Paper 2, Q4(a) (6 marks): A student carries out an investigation to find the enthalpy change for the decomposition of magnesium carbonate, ∆H1 (Reaction 1). Reaction 1 MgCO3(s) MgO(s) +
- 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, Paper 3, Q3(a)(i) (3 marks): Determine the enthalpy change of combustion of propane, in kJ mol−1. Give your answer to 3 significant figures. ∆cH (C3H8) = kJ mol−1 [3]
- November 2020, Paper 3, Q3(a)(ii) (1 mark): The student finds that the experimental enthalpy change ∆cH (C3H8) is much less exothermic than the accurate standard enthalpy change ∆cH (C3H8) in databases. O
- November 2020, Paper 3, Q3(b) (6 marks): Propane, C3H8, (boiling point −42 °C) is used as ‘camping gas’. A student plans to determine the enthalpy change of combustion of propane, ∆cH (C3H8), by two me
- November 2020, AS Paper 1, Q11 (1 mark): Hydrogen and oxygen react as shown below. 2H2(g) + O2(g) → 2H2O(g) ∆rH = −486 kJ mol−1 Bond enthalpies are shown in the table. Bond H−H O=O Bond enthalpy +436 +
- 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
- November 2020, AS Paper 2, Q2(a) (2 marks): Explain the term enthalpy change of combustion, ΔcH. [2]
- November 2020, AS Paper 2, Q2(b) (4 marks): A student carries out an experiment to determine the enthalpy change of combustion of cyclohexane, C6H12, using the apparatus shown in the diagram. thermometer
- November 2020, AS Paper 2, Q2(c)(i) (2 marks): The uncertainty in each thermometer reading is ±0.5 °C and the uncertainty in the measured volume of water is ±2 cm3. Determine whether the temperature change o
- November 2020, AS Paper 2, Q2(c)(ii) (2 marks): Suggest two reasons, apart from measurement uncertainties, why the experimental value for ΔcH is less exothermic than the data book value. Reason 1 Reason 2 [2]
- November 2020, AS Paper 2, Q2(c)(iii) (2 marks): In the experiment the water in the beaker was heated for 5 minutes. The student thought that the experiment could be improved by heating the water for 10 minute
- June 2019, Paper 1, Q10 (1 mark): Radical reactions are responsible for the catalysed breakdown of the ozone layer. The overall equation is shown below. 3O2 ∆rH = −284 kJ mol−1 2O3 The molar gas
- June 2019, Paper 1, Q19(a)(i) (1 mark): Explain why the enthalpy change for Reaction 1 has a negative value. Use ideas about enthalpy changes associated with bond breaking and bond making. [1]
- 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, Paper 3, Q5(a)(i) (6 marks): The enthalpy change of reaction, ∆rH, for converting anhydrous copper(II) sulfate to hydrated copper(II) sulfate is difficult to measure directly by experiment.
- June 2019, Paper 3, Q5(a)(ii) (1 mark): The thermometer had an uncertainty in each temperature reading of ± 0.1 °C. The student calculates a 20% uncertainty in the temperature change in Experiment 2.
- June 2019, AS Paper 1, Q10 (1 mark): A reversible reaction has the enthalpy profile diagram shown below. 150 energy reactants /kJ mol–1 120 products progress of reaction Which statement about this
- June 2019, AS Paper 1, Q11 (1 mark): Hydrogen and chlorine react as shown below. H2(g) + Cl 2(g) → 2HCl (g) ∆H o = −184.6 kJ mol−1 Which statement about this reaction is correct? A Less energy is r
- June 2019, AS Paper 1, Q24(b)(i) (1 mark): State the conditions of temperature and pressure used for standard enthalpy measurements. Temperature Pressure [1]
- 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, Paper 1, Q16(b)(i) (2 marks): Explain what is meant by the term average bond enthalpy. [2]
- June 2018, Paper 1, Q16(b)(ii) (3 marks): Calculate the bond enthalpy of the F–F bond. bond enthalpy = kJ mol–1 [3]
- June 2018, Paper 3, Q5(a) (4 marks): Aqueous succinic acid can be neutralised by aqueous sodium hydroxide, NaOH(aq): (CH2COOH)2(aq) + 2NaOH(aq) → (CH2COONa)2(aq) + 2H2O(l) This reaction can be used
- 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 2018, AS Paper 1, Q12 (1 mark): A student carried out an experiment to measure the enthalpy change of combustion of methanol. The energy from the combustion of methanol was used to heat a beak
- June 2018, AS Paper 1, Q23(a)(i) (4 marks): Calculate ∆rH, in kJ mol−1, for the reaction shown in equation 23.1. Give your answer to an appropriate number of significant figures. Assume that the density a
- June 2018, AS Paper 2, Q3(c) (2 marks): Hydrogen reacts with chlorine to form hydrogen chloride, HCl: 2HCl(g) ∆H = –184 kJ mol–1 H2(g) + Cl2(g) Table 3.1 shows bond enthalpies. Bond Enthalpy/kJ mol–1
- June 2018, AS Paper 2, Q3(d)(i) (1 mark): Write an equation, including state symbols, to represent the enthalpy change of vaporisation of bromine. [1]
- June 2018, AS Paper 2, Q3(d)(ii) (1 mark): Suggest whether the enthalpy change of vaporisation of bromine is exothermic or endothermic. Explain your answer. [1]
- June 2018, AS Paper 2, Q4(a)(i) (2 marks): Complete the enthalpy profile diagram for this reaction. On your diagram: • Label the activation energy, Ea • Label the enthalpy change of reaction, ∆H • Includ
- June 2018, AS Paper 2, Q4(a)(ii) (4 marks): 5.10 tonnes of NH3 are converted into NO. Calculate the energy released, in kJ, for this conversion. Give your answer in standard form and to an appropriate num
- June 2017, Paper 1, Q9 (1 mark): Enthalpy values are provided below. 2HI(g) ΔrH = –9 kJ mol–1 H2(g) + I2(g) Bond enthalpy Bond / kJ mol–1 H–H +436 I – I +151 What is the bond enthalpy, in kJ mo
- 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, Paper 3, Q2(b) (2 marks): The uncertainty in each temperature reading is ±0.1 °C. The uncertainty in each mass reading is ±0.005 g. Determine whether the mass of Na2O or the temperature
- June 2017, Paper 3, Q2(c) (2 marks): Suggest a modification to this experiment, using the same apparatus, which would reduce the percentage errors in the measurements. Explain your reasoning. [2]
- June 2017, AS Paper 1, Q11 (1 mark): 50.0 cm3 of 1.00 mol dm–3 NaOH is neutralised by 50.0 cm3 of 1.00 mol dm–3 HNO3. The temperature increases by 6.0 °C. The experiment is repeated using: 25.0 cm3
- 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 1, Q25(a)(i) (1 mark): Explain, in terms of bond breaking and bond forming, why a reaction can be exothermic. [1]
- June 2017, AS Paper 1, Q25(a)(ii) (3 marks): Average bond enthalpies are shown in the table. Average bond enthalpy Bond /kJ mol–1 O–H +464 O=O +498 C–H +413 C=O +805 Calculate the average bond enthalpy of
- June 2017, AS Paper 2, Q3(a)(i) (4 marks): Calculate the enthalpy change of combustion, ∆cH, of hexane, in kJ mol−1. Give your final answer to an appropriate number of significant figures. ∆cH kJ mol−1 [
- June 2017, AS Paper 2, Q3(a)(ii) (2 marks): The calculated value of ∆cH for hexane from this experiment is different from the data book value. Suggest two reasons for this difference. 1 2 [2]
- June 2017, AS Paper 2, Q3(b)(i) (1 mark): The values for the first three alkanes are plotted on the graph below. Plot the value for butane on the graph. relative molecular mass 0 20 40 60 80 100 –1000 –
- June 2017, AS Paper 2, Q3(b)(ii) (3 marks): Use the graph to estimate the energy released during complete combustion of 1.80 g of pentane. Show relevant working below and on the graph. energy released = k
- 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
- June 2016, AS Paper 1, Q24(a) (4 marks): Calculate ΔrH, in kJ mol–1, for the reaction shown in the equation above. Give your answer to an appropriate number of significant figures. Assume that the dens
- June 2016, AS Paper 2, Q4(a)(i) (1 mark): Explain, in terms of bond breaking and bond making, why the enthalpy change of formation of NO is endothermic. [1]
- June 2016, AS Paper 2, Q4(a)(ii) (2 marks): Draw a fully labelled enthalpy profile diagram to represent the enthalpy change of formation of N2O. The formulae, with state symbols, of the reactants and prod
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