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Enthalpy changes and Hess’ law

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OCR AS and A-level Chemistry past-paper questions since 2016 · 99 parts · mark schemes follow the questions · www.chemistryaleveltutor.com · Questions and mark schemes © OCR

1. June 2025 · Paper 1 · Q6 · 1 marks ·
6 Methane reacts with oxygen as shown in the equation: CH4(g) + 2O2(g) CO2(g) + 2H2O(g) ΔrH = –730 kJ mol–1 The table shows some bond enthalpies. Bond O–H O=O C=O Bond enthalpy / kJ mol–1 +464 +498 +805 What is the bond enthalpy, in kJ mol–1, of the C–H bond in methane? A +109 B +367 C +435 D +568 Your answer [1]
2. June 2025 · AS Paper 1 · Q11 · 1 marks ·
11 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 formation, Δf Ho, in kJ mol–1. Substance CH3OH(l) O2(g) CO2(g) H2O(l) ΔfH o / kJ mol–1 –239 0 –393 –286 What is the enthalpy change of combustion of CH3OH(l), in kJ mol–1? A +726 B +440 C – 440 D –726 Your answer [1]
3. June 2025 · Paper 1 · Q17(a)(i) · 1 marks ·
17 This question is about carbon monoxide and carbon as reducing agents in industry. (a) Iron(III) oxide, Fe2O3, can be reduced by carbon monoxide to form iron. Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) Equilibrium 17.1 Some standard enthalpy change of formation, ΔfH ɵ, and standard entropy, S ɵ, values are shown in the table. Substance Fe2O3(s) CO(g) Fe(s) CO2(g) Δf Hɵ / kJ mol–1 –824 –111 0 –394 To calculate in Sɵ / J K–1 mol–1 87 198 214 part (a)(iv) (i) Explain why the standard enthalpy change of formation of Fe has a value of 0 kJ mol–1. .......................................................................................................................................................... ..................................................................................................................................................... [1]
4. June 2025 · Paper 1 · Q17(a)(ii) · 2 marks ·
(ii) Calculate the enthalpy change, ΔrH, for the forward reaction in Equilibrium 17.1. ΔH = ........................................... kJ mol–1 [2]
5. June 2025 · AS Paper 1 · Q23(b) · 3 marks ·
(b) 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 enthalpies. Bond Bond enthalpy / kJ mol–1 O=O +494 S=O in SO2 +522 Calculate the S=O bond enthalpy in SO3. Use the information in the table and Equilibrium 23.1. Bond enthalpy of S=O in SO3 = ........................................... kJ mol–1 [3]
6. June 2025 · AS Paper 2 · Q1(c)(i) · 4 marks ·
(c) The student mixes 40.0 cm3 of 1.40 mol dm–3 HCl and 40.0 cm3 of 1.40 mol dm–3 NaOH. The temperature rises by 9.40 °C (i) Calculate the enthalpy change of neutralisation, in kJ mol–1. 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 for water. Enthalpy change of neutralisation = ........................................... kJ mol–1 [4]
7. June 2025 · AS Paper 2 · Q1(c)(ii) · 2 marks ·
(ii) 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 HCl . The temperature rise is greater than 9.4 °C. Explain why. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [2]
8. June 2025 · AS Paper 2 · Q4(b)(ii) · 2 marks ·
(ii) 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. Mass of carbon dioxide .......................................................g [2]
9. June 2025 · AS Paper 2 · Q4(b)(iii) · 2 marks ·
(iii) 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 reactants and products. • Label the activation energy as Ea. Enthalpy Progress of reaction [2]
10. June 2025 · AS Paper 2 · Q4(b)(iv) · 1 marks ·
(iv) Use the data in Table 4.1 to estimate a value for the enthalpy change of combustion of butane. Give your answer to 2 significant figures. Estimated enthalpy change of combustion of butane .............................................kJ mol–1 [1]
11. 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 6.1 The enthalpy changes of reaction for the reactions of Na2CO3 and NaHCO3 with excess HNO3(aq) can be measured by experiment. The reactions are shown below as Reaction 6.2 and Reaction 6.3. Na2CO3(s) + 2HNO3(aq) 2NaNO3(aq) + H2O(l) + CO2(g) ∆H2 Reaction 6.2 NaHCO3(s) + HNO3(aq) NaNO3(aq) + H2O(l) + CO2(g) ∆H3 Reaction 6.3 A student reacts Na2CO3(s) with HNO3(aq) to find ∆H2 for Reaction 6.2. Student’s method Weigh a bottle containing Na2CO3(s) and weigh a polystyrene cup. Add about 30 cm3 of 1.00 mol dm–3 HNO3(aq) to the polystyrene cup and measure its temperature. Add the Na2CO3(s), stir the mixture, and measure the maximum temperature reached. Weigh the empty bottle and weigh the polystyrene cup with the final solution. Mass readings Mass of bottle + Na2CO3(s) / g 3.26 Mass of empty bottle / g 1.29 Mass of polystyrene cup / g 21.08 Mass of polystyrene cup + final solution / g 54.58 Temperature readings Initial temperature of HNO3(aq) / °C 20.0 Maximum temperature of final solution / °C 24.0 The student follows a similar method and reacts NaHCO3(s) with HNO3(aq) to find ∆H3 for Reaction 6.3. The student calculates ∆H3 for Reaction 6.3 as +19.8 kJ mol–1. Calculate ∆H2 for Reaction 6.2 and determine the enthalpy change of Reaction 6.1, ∆H1. Assume that the density and specific heat capacity, c, of the final solution are the same as for water. Show your working, including an energy cycle linking the enthalpy changes. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [6] Extra answer space if required. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..........................................................................................................................................................
12. June 2024 · Paper 1 · Q7 · 1 marks ·
7 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 is reacted with excess KOH. The energy given out is 2.8 kJ. What is the enthalpy change of neutralisation, in kJ mol–1? A –56 B –70 C –112 D –224 Your answer [1]
13. June 2024 · AS Paper 1 · Q11 · 1 marks ·
11 An enthalpy cycle is shown below. ΔrH C(s) + ½O2(g) CO(g) + ½O2(g) + ½O2(g) ΔcH = –393 kJ mol–1 ΔcH = –283 kJ mol–1 CO2(g) What is ΔrH, in kJ mol–1, shown in the enthalpy cycle? A +676 B +110 C –110 D –676 Your answer [1]
14. 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]
15. June 2024 · Paper 1 · Q16(b)(i) · 3 marks ·
(b) The rate of decomposition of hydrogen peroxide shown in Reaction 16.1 can be increased by adding a small amount of powdered manganese(IV) oxide, MnO2. The MnO2 acts as a catalyst. (i) Complete the enthalpy profile diagram for Reaction 16.1 using formulae for the reactants and products. • Use Ea to label the activation energy without MnO2. • Use Ec to label the activation energy with MnO2. • Use ∆H to label the enthalpy change of reaction. enthalpy progress of reaction [3]
16. June 2024 · AS Paper 1 · Q23(b) · 3 marks ·
(b) 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. N2H4(g) + O2(g) N2(g) + 2H2O(g) ΔH = –581 kJ mol–1 Average bond enthalpies are shown in the table. Bond N–N O=O N≡N O–H Average bond enthalpy / kJ mol–1 +158 +498 +945 +464 Calculate the average bond enthalpy of the N–H bond. average bond enthalpy of N–H = ............................................ kJ mol–1 [3]
17. June 2024 · AS Paper 2 · Q3(a) · 1 marks ·
3 Enthalpy changes of reaction can be determined by experiment. (a) What is meant by the term enthalpy change of reaction? .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [1]
18. June 2024 · AS Paper 2 · Q3(b) · 6 marks ·
(b)* A student carries out an experiment to determine the enthalpy change for the reaction between zinc and copper(II) nitrate solution. Zn(s) + Cu(NO3)2(aq) Zn(NO3)2(aq) + Cu(s) ΔrH Equation 3.1 The student follows the method outlined below. • Add 100 cm3 of 0.500 mol dm–3 Cu(NO3)2(aq) to a beaker. • Measure the temperature of the solution. • Add excess zinc to the beaker. • Stir the mixture and record the maximum temperature. The temperature of the solution changes from 19.5 °C to 38.1 °C. Calculate ΔrH, in kJ mol–1, for equation 3.1. State any assumptions you have made in your calculation. Suggest improvements for obtaining a more accurate value for ΔrH. [6] .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... Extra answer space if required .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..........................................................................................................................................................
19. June 2024 · AS Paper 2 · Q3(c) · 2 marks ·
(c) 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 is the same as in equation 3.1. Explain why. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [2]
20. June 2024 · Paper 3 · Q2(a)(i) · 1 marks ·
2 This question is about different areas of chemistry. (a) Hydrogen gas is manufactured by the chemical industry from the reversible reaction of methane and steam, shown below. CH4(g) + H2O(g) 3H2(g) + CO(g) ΔH = +195 kJ mol–1 Average bond enthalpies are shown in the table. Bond H–H O–H C≡O Average bond enthalpy +436 +464 +1077 / kJ mol–1 (i) Why do all average bond enthalpies have a positive value? .......................................................................................................................................................... ..................................................................................................................................................... [1]
21. June 2024 · Paper 3 · Q2(a)(ii) · 3 marks ·
(ii) Determine the C–H bond enthalpy, in kJ mol–1, using the information above. C–H bond enthalpy = ........................................... kJ mol–1 [3]
22. June 2024 · Paper 3 · Q2(a)(iii) · 2 marks ·
(iii) 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 energy released when 60.0 m3 of hydrogen is used as a household fuel at RTP. Give your answer to 3 significant figures and in standard form. energy released = .................................................... kJ [2]
23. June 2023 · AS Paper 1 · Q11 · 1 marks · AO1
11 The standard enthalpy change of formation of water is –286 kJ mol–1. Which statement or equation is correct? A H2(g) + ½O2(g) H2O(l) ΔH o = –143 kJ mol–1 B 2H2(g) + O2(g) 2H2O(l) ΔH o = –286 kJ mol–1 C The O–H bond enthalpy is –143 kJ mol–1. D The standard enthalpy change of combustion of hydrogen is –286 kJ mol–1. Your answer [1]
24. June 2023 · AS Paper 1 · Q12 · 1 marks · AO1
12 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 than is given out when bonds are made. C The activation energy is a negative value. D The enthalpy change for the condensation of a gas to a liquid is a negative value. Your answer [1]
25. June 2023 · Paper 1 · Q18(a) · 3 marks · AO2
18 This question is about enthalpy changes of reactions involving hydrocarbons. (a) 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 its temperature. • Weigh a spirit burner containing heptane and use it to heat the water. • Extinguish the flame and record the maximum temperature reached by the water. • Reweigh the spirit burner. The temperature of the water increased by 10.5 °C. The spirit burner decreased in mass by 0.133 g. Use the student’s results to determine the enthalpy change of combustion of heptane, ∆cH (C7H16), in kJ mol–1. ∆cH (C7H16) = ........................................... kJ mol–1 [3]
26. June 2023 · Paper 1 · Q18(b) · 2 marks · AO2
(b) Nonane, C9H20, can be broken down by heat to form pentane, C5H12, and ethene, C2H4. C9H20(g) C5H12(g) + 2C2H4(g) ∆H = +186 kJ mol–1 Reaction 1 The enthalpy changes of combustion of C9H20(g) and C2H4(g) are shown in the table below. Hydrocarbon ∆cH / kJ mol–1 C9H20(g) –6171 C2H4(g) –1411 Use ∆H in Reaction 1 and the enthalpy changes of combustion in the table to determine the enthalpy change of combustion of C5H12(g). ∆cH (C5H12(g)) = ............................................ kJ mol–1 [2]
27. June 2023 · AS Paper 1 · Q23(a)(ii) · 2 marks · AO1, 2
(ii) 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 of CO and NO is shown below. 2CO(g) + 2NO(g) 2CO2(g) + N2(g) ΔH = –746 kJ mol–1 Complete the enthalpy profile diagram in Fig. 23.1 for this reaction. On your diagram: • Label the enthalpy change of reaction, ΔH. • Include the formulae of the reactants and products. • Label the activation energy, Ea. Fig. 23.1 enthalpy progress of reaction [2]
28. June 2023 · AS Paper 1 · Q23(b)(i) · 1 marks · AO1
(b) Iron(III) oxide reacts with carbon monoxide as shown: Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) ΔH = –25 kJ mol–1 Standard enthalpy changes of formation, Δ f H o, are given in the table. Substance Δ f H o / kJ mol–1 Fe2O3(s) –824 CO(g) –111 (i) State the conditions of temperature and pressure for standard enthalpy changes. Temperature ...................................................................................................................... Pressure ....................................................................................................................... [1]
29. June 2023 · AS Paper 1 · Q23(b)(ii) · 3 marks · AO2
(ii) Calculate the standard enthalpy change of formation for CO2(g). Δ f H o (CO2(g)) = ............................................ kJ mol–1 [3]
30. June 2023 · AS Paper 2 · Q4(a) · 2 marks · AO1
4 This question is about the enthalpy change of combustion of alcohols. (a) Explain the term enthalpy change of combustion. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [2]
31. June 2023 · AS Paper 2 · Q4(b)(ii) · 6 marks · AO2, 3
(ii)* Use the student’s results to determine ∆cH of butan-1-ol in kJ mol–1. Explain why this value of ΔcH is different from the data book value and suggest how the experimental design could be modified to improve the accuracy of the ΔcH value obtained. [6] ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Additional answer space if required. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
32. June 2023 · AS Paper 2 · Q4(b)(iii) · 1 marks · AO3
(iii) 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 same temperature rise as in the experiment in 4(b). Assume the same heat losses. mass of butan-1-ol = ....................................................... g [1]
33. June 2023 · AS Paper 2 · Q4(c)(i) · 4 marks · AO2
(c) The enthalpy change of combustion of ethanol, ΔcH, in the gaseous state can be calculated using average bond enthalpies. H H H C C O H(g) + 3 O=O(g) 2 O=C= O(g) + 3 H O H(g) ΔcH = –1276 kJ mol–1 H H (i) 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 +347 C–O +358 O–H +464 O=O +498 C=O bond enthalpy = .............................................kJ mol–1 [4]
34. June 2023 · AS Paper 2 · Q4(c)(ii) · 2 marks · AO3
(ii) 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=O(g) 2 O= C=O(g) + 3 H O H(g) H H ΔcH for methoxymethane is more negative than ΔcH for ethanol. Explain why the ΔcH values are different, in terms of the bonds broken and the bonds formed. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2]
35. June 2023 · Paper 3 · Q2(a)(ii) · 3 marks · AO2
(ii) 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 to water in a well-insulated container. The initial temperature is 23.0 °C. • The mixture is stirred until all the RbCl O3 has dissolved. The final temperature is 21.5 °C. The final solution has a mass of 102 g. Determine the enthalpy change of solution, ΔsolH, of RbCl O3 in kJ mol–1. Assume that the specific heat capacity of the solution is the same as that of pure water. ΔsolH (RbCl O3) = ........................................... kJ mol–1 [3]
36. June 2022 · Paper 1 · Q7 · 1 marks · AO2
7 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 mol–1, for the reaction below? CH4(g) + 2O2(g) CO2(g) + 2H2O(g) A –730 B –544 C +544 D +730 Your answer [1]
37. 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]
38. June 2022 · AS Paper 1 · Q11 · 1 marks · AO2
11 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 formation, ∆f H o . Substance ∆f H o / kJ mol−1 N2H4(l) +50.6 O2(g) 0 NO2(g) +33.2 H2O(l) –285.8 What is the enthalpy change of combustion, in kJ mol–1, for hydrazine, N2H4(l)? A –555.8 B –303.2 C +303.2 D +555.8 Your answer [1]
39. June 2022 · Paper 1 · Q17(a)(i) · 4 marks · AO1, 2
17 This question is about energy changes. (a) Magnesium reacts with aqueous silver nitrate, AgNO3(aq) as shown below. Mg(s) + 2AgNO3(aq) 2Ag(s) + Mg(NO3)2(aq) ∆H = –678 kJ mol–1 A student adds an excess of magnesium to 100.0 cm3 of 0.400 mol dm–3 AgNO3(aq). The initial temperature is 20.0 °C. (i) 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 solution are the same as for water, and that there are no heat losses. maximum temperature reached = .................................................... °C [4]
40. June 2022 · Paper 1 · Q17(a)(ii) · 1 marks · AO3
(ii) 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 adding an excess of magnesium to 50.0 cm3 of 0.400 mol dm–3 AgNO3(aq). Predict, with reasons, how this modification would affect the maximum temperature reached. Assume that there are no heat losses. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
41. June 2022 · Paper 1 · Q17(b)(i) · 1 marks · AO1
(b) Nitric acid is manufactured from ammonia in a multi-stage process. The equation for the first stage in this process is shown in Reaction 17.1. 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(l) ∆H ө = –1172 kJ mol–1 Reaction 17.1 Some standard enthalpy changes of formation are shown in the table. Compound ∆f H ө / kJ mol–1 NH3(g) –46 H2O(l) –286 (i) Explain the term enthalpy change of formation. ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
42. June 2022 · Paper 1 · Q17(b)(ii) · 2 marks · AO2
(ii) Calculate the standard enthalpy change of formation, ∆fH ө, of NO(g). ∆fH ө of NO(g) = ............................................ kJ mol–1 [2]
43. June 2022 · AS Paper 1 · Q25(a)(i) · 4 marks · AO2
25 This question is about enthalpy changes and reaction rates. (a) Aqueous barium hydroxide, Ba(OH)2(aq), reacts with dilute nitric acid, HNO3(aq), as in Equation 25.1. Ba(OH)2(aq) + 2HNO3(aq) → Ba(NO3)2(aq) + 2H2O(l) Equation 25.1 A student carries out an experiment to determine the enthalpy change of this reaction, ∆rH. The student measures out: • 25.0 cm3 of 2.00 mol dm–3 Ba(OH)2(aq) and • 50.0 cm3 of 2.00 mol dm–3 HNO3(aq). The temperature of each solution is the same. The student mixes both solutions in a polystyrene cup, stirs the mixture and records the maximum temperature. Temperature readings Initial temperature = 20.5 °C Maximum temperature = 39.0 °C (i) 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 capacity, c, of the solutions are the same as for water. ∆rH = ............................................ kJ mol–1 [4]
44. June 2022 · AS Paper 1 · Q25(a)(ii) · 2 marks · AO3
(ii) 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 neutralisation. Explain why the student’s conclusion is incorrect and determine the correct value for the enthalpy change of neutralisation. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... enthalpy change of neutralisation = ............................................ kJ mol–1 [2]
45. June 2022 · AS Paper 2 · Q3(b)(ii) · 3 marks · AO1, 2
(ii) Using the axes below, sketch an enthalpy profile diagram for the reaction with and without the catalyst. On your diagram, include the following labels: • ΔH, the enthalpy change • Ea, the activation energy without a catalyst • Ec, the activation energy with a catalyst. Enthalpy Zn(s) + H2SO4(aq) Progress of reaction [3]
46. June 2022 · AS Paper 2 · Q4(b) · 3 marks · AO2
(b) 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 Average bond enthalpies are shown in the table below. Bond C–H C–C C–O O–H H–H C=O Average bond +415 +347 +358 +464 +435 +805 enthalpy / kJ mol–1 Calculate ΔH, in kJ mol–1, for the forward reaction in Equilibrium 4.2. ΔH = ............................................ kJ mol–1 [3]
47. June 2022 · Paper 3 · Q5(a) · 6 marks · AO3
5 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 below from data tables. Enthalpy change ∆H / kJ mol–1 Lattice enthalpy of calcium chloride –2223 Enthalpy change of hydration of chloride ions –378 The student carries out an experiment to find the enthalpy change of solution of calcium chloride. Student’s method: • Weigh a bottle containing calcium chloride and weigh a polystyrene cup. • Add water from a measuring cylinder to the polystyrene cup and measure its temperature. • Add the calcium chloride, stir the mixture, and measure the maximum temperature of the final solution. • Weigh the empty bottle and weigh the polystyrene cup with the final solution. Mass readings Mass of bottle + calcium chloride / g 27.45 Mass of empty bottle / g 18.17 Mass of polystyrene cup / g 21.24 Mass of polystyrene cup + final solution / g 127.84 Temperature readings Initial temperature of water / °C 21.0 Maximum temperature of final solution / °C 39.5 Calculate the enthalpy change of solution of calcium chloride and determine the enthalpy change of hydration of calcium ions. Show your working, including an energy cycle linking the energy changes. Assume that the density and specific heat capacity, c, of the solution are the same as for water. [6] ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Additional answer space if required. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ...................................................................................................................................................
48. June 2022 · Paper 3 · Q5(b)(ii) · 3 marks · AO2
(ii) 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 released = ..................................................... kJ [3]
49. November 2021 · Paper 1 · Q8 · 1 marks ·
8 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 reaction below? 2H2S(g) + 3O2(g) → 2SO2(g) + 2H2O(g) A –174 B –1102 C –1794 D –2098 Your answer [1]
50. November 2021 · AS Paper 1 · Q10 · 1 marks · AO2
10 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 H o, are shown in the table. Compound ∆f H o / kJ mol–1 C3H6(g) +20 O2(g) 0 CO2(g) –394 H2O(l) –286 What is the standard enthalpy change of combustion of C3H6(g), in kJ mol–1? A –2060 B –700 C +700 D +2060 Your answer [1]
51. November 2021 · AS Paper 1 · Q22(a) · 3 marks · AO1
22 This question is about enthalpy changes. Hydrogen, H2, can be manufactured by the reaction of methane and steam. This is a reversible reaction, as shown in Equilibrium 22.1 below. Equilibrium 22.1 CH4(g) + H2O(g) 3H2(g) + CO(g) ∆H = +206 kJ mol–1 (a) The rate of this reaction increases when a catalyst is present. Complete the enthalpy profile diagram below. On your diagram: • label the activation energies, Ea (without catalyst) and Ec (with catalyst) • label the enthalpy change of reaction, ∆H. enthalpy CH4(g) + H2O(g) progress of reaction [3]
52. November 2021 · Paper 1 · Q22(b)(ii) · 4 marks · AO2
(ii) 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 densities of all solutions and the specific heat capacity, c, of the reaction mixture are the same as for water. enthalpy change, ΔH1 = ........................................... kJ mol–1 enthalpy change of neutralisation, ΔneutH = ............................................ kJ mol–1 [4]
53. November 2021 · Paper 1 · Q22(b)(iii) · 2 marks · AO3
(iii) 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 reasoning. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2]
54. November 2021 · AS Paper 1 · Q22(c) · 3 marks · AO2
(c) The reaction for the production of hydrogen is repeated below. CH4(g) + H2O(g) 3H2(g) + CO(g) ∆H = +206 kJ mol–1 Average bond enthalpies are shown in the table. Average bond enthalpy Bond / kJ mol–1 C–H 413 O–H 464 C≡O 1077 Calculate the bond enthalpy of the H–H bond. bond enthalpy = ............................................ kJ mol–1 [3]
55. November 2021 · AS Paper 2 · Q4(a) · 6 marks · AO3
4 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) + CO2(g) ∆H1 This enthalpy change cannot be found directly. It can be determined indirectly from the enthalpy changes for the reactions below, which can be found by experiment. Reaction 2 MgCO3(s) + 2HCl (aq) MgCl 2(aq) + H2O(l) + CO2(g) ∆H2 Reaction 3 MgO(s) + 2HCl (aq) MgCl 2(aq) + H2O(l) ∆H3 The enthalpy cycle is shown in Fig. 4.1. Reaction 1 MgCO3(s) MgO(s) + CO2(g) ΔH1 Reaction 2 ΔH2 ΔH3 Reaction 3 MgCl2(aq) + H2O(l) + CO2(g) Fig. 4.1 Determination of ∆H2 for Reaction 2 Student’s method • Weigh a 250 cm3 polystyrene cup. • Add about 100 cm3 of 2.00 mol dm–3 hydrochloric acid (an excess) to the polystyrene cup and record the initial temperature of the HCl (aq). • Add 4.215 g MgCO3, stir the mixture, and record the final temperature. • Weigh the polystyrene cup containing the final solution. Results Mass of polystyrene cup / g 21.415 Mass of polystyrene cup + final solution / g 124.425 Initial temperature of HCl (aq) / °C 20.40 Final temperature of solution / °C 25.40 Determination of ∆H3 for Reaction 3 The student uses the same method as for Reaction 2 but with MgO in place of MgCO3. The student calculates ∆H3 for Reaction 3 as –136.1 kJ mol–1. (a)* Use the student’s results to calculate ∆H2 for Reaction 2 and determine the enthalpy change ∆H1, in kJ mol–1, for the decomposition of magnesium carbonate (Reaction 1), using the energy cycle in Fig. 4.1. 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56. November 2020 · Paper 1 · Q6 · 1 marks · AO2
6 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 table. Substance C7H8(l) CO2(g) H2O(l) ∆f H / kJ mol−1 +12 −394 −286 Calculate the enthalpy of combustion, in kJ mol−1, for the hydrocarbon C7H8. A −3914 B −692 C +692 D +3914 Your answer [1]
57. November 2020 · AS Paper 1 · Q11 · 1 marks · AO2
11 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 +498 / kJ mol−1 What is the bond enthalpy, in kJ mol−1, for the O−H bond? A +221 B +355 C +464 D +928 Your answer [1]
58. November 2020 · AS Paper 1 · Q24(a) · 3 marks · AO2
24 This question is about making ammonia, NH3. (a) Ammonia is manufactured by reacting nitrogen with hydrogen: N2(g) + 3H2(g) → 2NH3(g) Standard enthalpy changes of combustion, ∆cH o, are given in the table. Substance ∆cH o / kJ mol−1 N2(g) +180 H2(g) –286 NH3(g) –293 Calculate the standard enthalpy change of formation, ∆fH o, for NH3(g). ∆fH o for NH3(g) = ........................................... kJ mol−1 [3]
59. November 2020 · AS Paper 2 · Q2(a) · 2 marks · AO1
2 Enthalpy changes of combustion can be determined directly by experiment. (a) Explain the term enthalpy change of combustion, ΔcH. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [2]
60. November 2020 · AS Paper 2 · Q2(b) · 4 marks · AO1, 3
(b) A student carries out an experiment to determine the enthalpy change of combustion of cyclohexane, C6H12, using the apparatus shown in the diagram. thermometer beaker clamp 200 cm3 water wick spirit burner cyclohexane In the experiment, 0.525 g of cyclohexane are burnt, and the temperature of the 200 cm3 of water changes from 21.0 °C to 41.0 °C. Calculate the enthalpy change of combustion, ΔcH, of cyclohexane in kJ mol−1. Give your answer to 3 significant figures. ΔcH = ............................................ kJ mol–1 [4]
61. November 2020 · AS Paper 2 · Q2(c)(ii) · 2 marks · AO3
(ii) 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]
62. November 2020 · Paper 3 · Q3(a)(i) · 3 marks · AO2
3 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 methods. (a) The student first carries out an experiment using the apparatus below. thermometer beaker 100 cm3 water clamp propane burner Results Mass of propane burner before burning / g 99.218 Mass of propane burner after burning / g 98.976 Initial temperature / °C 21.60 Maximum temperature reached / °C 46.10 (i) 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]
63. November 2020 · Paper 3 · Q3(a)(ii) · 1 marks · AO1
(ii) The student finds that the experimental enthalpy change ∆cH (C3H8) is much less exothermic than the accurate standard enthalpy change ∆cH (C3H8) in databases. One reason could be that the student’s experiment had not been carried out under standard conditions. Suggest two other reasons for this difference in enthalpy change. 1 ........................................................................................................................................ ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... [1]
64. November 2020 · Paper 3 · Q3(b) · 6 marks · AO2, 3
(b)* The student determines the standard enthalpy change of combustion of propane using the bond enthalpies in the table. An experiment is not needed. Bond enthalpy Bond / kJ mol−1 C−H +413 C−C +347 C=O +805 O=O +498 O−H +464 The bond enthalpies can be used to determine the standard enthalpy change of reaction, ∆rH, for equation 3.1: C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) ∆rH Equation 3.1 Enthalpy change of vaporisation, ∆ vapH The standard enthalpy change of vaporisation of water, ∆vapH, is the enthalpy change for the conversion of 1 mol of H2O(l) into 1 mol of H2O(g) under standard conditions: H2O(l) → H2O(g) ∆vapH = +40.65 kJ mol−1 Determine the standard enthalpy change of combustion of propane (boiling point −42 °C) using the ∆rH value for equation 3.1 and ∆vapH for water. [6] ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Additional answer space if required ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ...................................................................................................................................................
65. June 2019 · AS Paper 1 · Q10 · 1 marks · AO1
10 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 reaction is correct? A The activation energy of the forward reaction is 120 kJ mol−1. B The activation energy of the reverse reaction is 270 kJ mol−1. C The enthalpy change of the forward reaction is −30 kJ mol−1. D The reverse reaction is exothermic. Your answer [1]
66. June 2019 · Paper 1 · Q10 · 1 marks · AO2
10 Radical reactions are responsible for the catalysed breakdown of the ozone layer. The overall equation is shown below. 2O3 3O2      ∆rH = −284 kJ mol−1 The molar gas volume in the ozone layer is approximately 2.5 m3 mol−1. What is the energy released, in kJ, during the breakdown of 1.0 m3 of ozone in the ozone layer? A 56.8 B 113.6 C 355 D 710 Your answer [1]
67. June 2019 · AS Paper 1 · Q11 · 1 marks · AO1
11 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 released on bond making than is taken in during bond breaking. B The enthalpy change for the reverse equation is +184.6 kJ mol−1. C The enthalpy change of formation of HCl (g) is –184.6 kJ mol−1. D The temperature decreases during the reaction. Your answer [1]
68. June 2019 · Paper 1 · Q19(a)(i) · 1 marks · AO1
19 Sulfuric acid is an important chemical used to make detergents, fertilisers and dyes. It is manufactured in a multi-step process. (a) In the first step of the manufacture of sulfuric acid, sulfur dioxide, SO2, can be made from the combustion of hydrogen sulfide, H2S, shown in Reaction 1. 2H2S(g)  +  3O2(g) 2SO2(g) + 2H2O(l)      ∆rH = –1125 kJ mol−1 Reaction 1 (i) 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]
69. June 2019 · Paper 1 · Q19(a)(iii) · 3 marks · AO2
(iii) 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 combustion below. Substance ∆cH ө / kJ mol−1 S(s) −296.8 H2(g) −285.8 2H2S(g)  +  3O2(g) 2SO2(g) + 2H2O(l)      ∆rH = –1125 kJ mol−1 Reaction 1 ∆fH ө of hydrogen sulfide = ........................................... kJ mol−1 [3]
70. June 2019 · AS Paper 1 · Q24(b)(i) · 1 marks · AO1
(b) Ammonia is used to make nitric acid. The first stage of the reaction is shown below. 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g) ∆H = −908 kJ mol−1 Standard enthalpy changes of formation, ∆fH o, are given in the table. Substance ∆fH o / kJ mol−1 NH3(g) −46 O2(g) 0 H2O(g) −242 (i) State the conditions of temperature and pressure used for standard enthalpy measurements. Temperature ...................................................................................................................... Pressure ............................................................................................................................ [1]
71. June 2019 · AS Paper 1 · Q24(b)(ii) · 3 marks · AO2
(ii) Calculate the standard enthalpy change of formation for NO(g). Give your answer to a whole number. ∆fH o for NO(g) = ........................................... kJ mol−1 [3]
72. June 2019 · Paper 3 · Q5(a)(i) · 6 marks · AO3
5 This question is about copper(II) sulfate, CuSO4, and sodium thiosulfate, Na2S2O3. (a) The enthalpy change of reaction, ∆rH, for converting anhydrous copper(II) sulfate to hydrated copper(II) sulfate is difficult to measure directly by experiment. CuSO4(s) + 5H2O(l) → CuSO4•5H2O(s) ∆rH reaction 5.1 The enthalpy changes of solution of anhydrous and hydrated copper(II) sulfate can be measured by experiment. The reactions are shown below. In the equations, ‘aq’ represents an excess of water. CuSO4(s) + aq → Cu2+(aq) + SO4 2−(aq) ∆solH(CuSO4(s)) reaction 5.2 CuSO4•5H2O(s) + aq → Cu2+(aq) + SO4 2−(aq) ∆solH(CuSO4•5H2O(s)) reaction 5.3 Experiment 1 A student carries out an experiment to find ∆solH(CuSO4(s)) for reaction 5.2. Student’s method • Weigh a bottle containing CuSO4(s) and weigh a polystyrene cup. • Add about 50 cm3 of water to the polystyrene cup and measure its temperature. • Add the CuSO4(s), stir the mixture, and measure the final temperature. • Weigh the empty bottle and weigh the polystyrene cup with final solution. Mass readings Mass of bottle + CuSO4(s) / g 28.04 Mass of empty bottle / g 20.06 Mass of polystyrene cup / g 23.43 Mass of polystyrene cup + final solution / g 74.13 Temperature readings Initial temperature of water / °C 20.5 Temperature of final solution / °C 34.0 Experiment 2 The student carries out a second experiment with CuSO4•5H2O (reaction 5.3). The student uses the same method as in Experiment 1. The student calculates ∆solH(CuSO4•5H2O(s)) as +8.43 kJ mol−1. (i)* Calculate ∆solH(CuSO4(s)) for reaction 5.2 and determine the enthalpy change of reaction 5.1, ∆rH. Assume that the specific heat capacity, c, of the solution is the same as for water. Show your working, including an energy cycle linking the enthalpy changes. 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73. June 2018 · AS Paper 1 · Q11 · 1 marks ·
11 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 enthalpy changes of formation, ∆f H o. Substance Al 2S3(s) O2(g) Al 2O3(s) SO2(g) ∆f H o / kJ mol−1 −723.8 0 −1675.7 −296.8 What is the standard enthalpy change of combustion of Al 2S3(s), in kJ mol−1? A −3684.6 B −1842.3 C +1842.3 D +3684.6 Your answer [1]
74. June 2018 · AS Paper 1 · Q12 · 1 marks ·
12 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 beaker containing water. The student’s calculated enthalpy change of combustion was more exothermic than the value in data books. Which error could have caused this difference? A Some methanol had evaporated from the wick before the final weighing. B In the calculation, the student used the molar mass of ethanol instead of methanol. C There was incomplete combustion. D The water boiled for 5 minutes before the final temperature was taken. Your answer [1]
75. June 2018 · Paper 1 · Q13 · 1 marks · AO1
13 Which enthalpy change(s) is/are endothermic? 1 The bond enthalpy of the C–H bond 2 The second electron affinity of oxygen 3 The standard enthalpy change of formation of magnesium A 1, 2 and 3 B Only 1 and 2 C Only 2 and 3 D Only 1 Your answer [1]
76. June 2018 · Paper 1 · Q16(b)(i) · 2 marks ·
(b) Fluorine reacts with steam as shown in the equation below. 2F2(g) + 2H2O(g) O2(g) + 4HF(g) ∆H = −598 kJ mol–1 Average bond enthalpies are shown in the table. Bond Average bond enthalpy / kJ mol–1 O–H +464 O=O +498 H–F +568 (i) Explain what is meant by the term average bond enthalpy. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2]
77. June 2018 · Paper 1 · Q16(b)(ii) · 3 marks ·
(ii) Calculate the bond enthalpy of the F–F bond. bond enthalpy = ............................................ kJ mol–1 [3]
78. June 2018 · Paper 1 · Q20(b) · 3 marks ·
(b) Hydrogen iodide, HI, is decomposed by heat into its elements: 2HI(g) H2(g) + I2(g) ∆H = +9.5 kJ mol–1 The decomposition is much faster in the presence of a platinum catalyst. Complete the enthalpy profile diagram for this reaction using formulae for the reactants and products. • Use Ea to label the activation energy without a catalyst. • Use Ec to label the activation energy with a catalyst. • Use ∆H to label the enthalpy change of reaction. enthalpy progress of reaction [3]
79. June 2018 · AS Paper 1 · Q23(a)(i) · 4 marks ·
23 This question is about energy changes and rate of reaction. (a) Magnesium reacts with aqueous silver nitrate, AgNO3(aq), as in equation 23.1. Mg(s) + 2AgNO3(aq) → 2Ag(s) + Mg(NO3)2(aq) Equation 23.1 A student carries out an experiment to determine the enthalpy change of this reaction, ∆rH. • The student adds 25.0 cm3 of 0.512 mol dm−3 AgNO3 to a polystyrene cup. • The student measures the temperature of the solution. • The student adds a small spatula measure of magnesium powder, stirs the mixture and records the maximum temperature. Temperature readings Initial temperature = 19.5 °C Maximum temperature = 47.5 °C (i) 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 and specific heat capacity, c, of the solution are the same as for water and that all the aqueous silver nitrate has reacted. ∆rH = ........................................... kJ mol−1 [4]
80. June 2018 · AS Paper 2 · Q3(c) · 2 marks ·
(c) Hydrogen reacts with chlorine to form hydrogen chloride, HCl: H2(g) + Cl2(g) 2HCl(g) ∆H = –184 kJ mol–1 Table 3.1 shows bond enthalpies. Bond Bond Enthalpy / kJ mol–1 H–H +436 Cl–Cl +243 Table 3.1 Calculate the bond enthalpy for the H–Cl bond from the information above. bond enthalpy = ............................................ kJ mol–1 [2]
81. June 2018 · AS Paper 2 · Q3(d)(ii) · 1 marks ·
(ii) Suggest whether the enthalpy change of vaporisation of bromine is exothermic or endothermic. Explain your answer. ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
82. June 2018 · AS Paper 2 · Q4(a)(i) · 2 marks ·
4 The reaction of ammonia, NH3, with oxygen to form nitrogen monoxide, NO, is an important industrial process. The equation for this reaction is shown in equilibrium 4.1 below. 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) ∆H = –905 kJ mol–1 Equilibrium 4.1 (a) The forward reaction in equilibrium 4.1 converts NH3 into NO. (i) Complete the enthalpy profile diagram for this reaction. On your diagram: • Label the activation energy, Ea • Label the enthalpy change of reaction, ∆H • Include the formulae of the reactants and products. Enthalpy Progress of reaction [2]
83. June 2018 · AS Paper 2 · Q4(a)(ii) · 4 marks ·
(ii) 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 number of significant figures. energy released = ..................................................... kJ [4]
84. June 2018 · Paper 3 · Q5(a) · 4 marks ·
5 A student carries out two experiments in the laboratory based on succinic acid (butanedioic acid), (CH2COOH)2. (a) 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 to determine a value for the enthalpy change of neutralisation, ∆neutH. The student follows this method: • Add 50.0 cm3 of 0.400 mol dm–3 succinic acid to a polystyrene cup. • Measure out 50.0 cm3 of 1.00 mol dm–3 NaOH(aq), which is in excess. • Measure the temperature of both solutions. • Add the NaOH(aq) to the aqueous succinic acid in the polystyrene cup, stir the mixture, and record the maximum temperature. Temperature readings Maximum temperature of mixture/°C 26.5 Initial temperature of both solutions/°C 21.5 Calculate a value for the enthalpy change of neutralisation, ∆neutH, in kJ mol–1. Assume that the density of all solutions and the specific heat capacity, c, of the reaction mixture are the same as for water. ΔneutH = ............................................ kJ mol–1 [4]
85. June 2017 · Paper 1 · Q9 · 1 marks ·
9 Enthalpy values are provided below. H2(g) + I2(g) 2HI(g) ΔrH = –9 kJ mol–1 Bond enthalpy Bond / kJ mol–1 H–H +436 I – I +151 What is the bond enthalpy, in kJ mol–1, of the H–I bond? A –596 B –298 C +298 D +596 Your answer [1]
86. June 2017 · AS Paper 1 · Q11 · 1 marks ·
11 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 of 1.00 mol dm–3 NaOH and 25.0 cm3 of 1.00 mol dm–3 HNO3. What is the increase in temperature in the second experiment? A 1.5 °C B 3.0 °C C 6.0 °C D 12.0 °C Your answer [1]
87. June 2017 · AS Paper 1 · Q12 · 1 marks ·
12 The table shows standard enthalpy changes of combustion, ΔcH. Substance ΔcH / kJ mol–1 C(s) –393.5 H2(g) –285.8 C4H10(g) –2876.5 What is the enthalpy change for the following reaction? 4C(s) + 5H2(g) → C4H10(g) A –2197.2 kJ mol–1 B –126.5 kJ mol–1 C +126.5 kJ mol–1 D +2197.2 kJ mol–1 Your answer [1]
88. June 2017 · Paper 1 · Q18(b)(iv) · 3 marks ·
(iv) Another equilibrium involved in the extraction of iron from Fe3O4 is shown below. Fe3O4(s) + 4CO(g) 3Fe(s) + 4CO2(g) ∆H = –13.5 kJ mol–1 Enthalpy changes of formation, ∆f H, for Fe3O4(s) and CO2(g) are shown in the table. Compound ∆f H / kJ mol–1 Fe3O4(s) –1118.5 CO2(g) –393.5 Calculate the enthalpy change of formation, ∆f H, for CO(g). ∆f H, for CO(g) = ............................................ kJ mol–1 [3]
89. June 2017 · AS Paper 1 · Q25(a)(i) · 1 marks ·
25 This question is about alkenes. (a) The combustion of ethene is shown in equation 25.1 below. C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(g) ΔH = –1318 kJ mol–1 equation 25.1 (i) Explain, in terms of bond breaking and bond forming, why a reaction can be exothermic. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
90. June 2017 · AS Paper 1 · Q25(a)(ii) · 3 marks ·
(ii) 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 the C=C bond. Use the average bond enthalpies in the table and equation 25.1. average bond enthalpy = ............................................ kJ mol–1 [3]
91. June 2017 · AS Paper 2 · Q3(a)(i) · 4 marks ·
3 Alkanes are saturated hydrocarbons with the general formula CnH2n+2. (a) A student carries out an experiment to measure the enthalpy change of combustion, ∆cH, of hexane. The student finds that combustion of 1.29 g of hexane changes the temperature of 200 g of water from 20.5 °C to 65.5 °C. (i) 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 [4]
92. June 2017 · AS Paper 2 · Q3(a)(ii) · 2 marks ·
(ii) 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]
93. June 2017 · AS Paper 2 · Q3(b)(ii) · 3 marks ·
(ii) 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 = .................................................... kJ [3]
94. June 2017 · AS Paper 2 · Q3(c) · 3 marks ·
(c) 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 shown in the table. Substance C6H12(l) CO2(g) H2O(l) ∆fH / kJ mol−1 –156.3 –393.5 –285.8 Calculate the standard enthalpy change of combustion, ∆cH , of cyclohexane. ∆cH = ........................................... kJ mol−1 [3]
95. June 2017 · Paper 3 · Q2(a) · 6 marks ·
2 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 determined indirectly using Hess’ Law from the enthalpy changes of reaction 3.2 and reaction 3.3 shown below. Na2O(s) + H2O(l) 2NaOH(aq) reaction 3.2 HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) ∆rH = −57.6 kJ mol−1 reaction 3.3 The student will determine the enthalpy change of reaction 3.2 as outlined below. • Weigh a bottle containing Na2O(s) and weigh a polystyrene cup. • Add about 25 cm3 of water to the polystyrene cup and measure its temperature. • Add the Na2O(s), stir the mixture, and measure the maximum temperature reached. • Weigh the empty bottle and weigh the polystyrene cup with the final solution. Mass readings Mass of bottle + Na2O(s) = 16.58 g Mass of empty bottle = 15.34 g Mass of empty polystyrene cup = 21.58 g Mass of polystyrene cup + final solution = 47.33 g Temperature readings Initial temperature of water = 20.5 °C Maximum temperature of final solution = 55.5 °C The density and specific heat capacity, c, of the solution are the same as for water. (a)* Calculate the enthalpy change of reaction 3.2 and the enthalpy change of reaction 3.1. Show all your working. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [6]
96. June 2016 · AS Paper 1 · Q12 · 1 marks ·
6 12 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 –394 –286 –2877 Calculate the enthalpy change of formation of C4H10(g). 4C(s) + 5H2(g) → C4H10(g) A –2197 kJ mol–1 B –129 kJ mol–1 C +129 kJ mol–1 D +2197 kJ mol–1 Your answer [1]
97. June 2016 · AS Paper 1 · Q24(a) · 4 marks · AO2
18 24 Aqueous lead(II) nitrate, Pb(NO3)2(aq), and aqueous potassium iodide, KI(aq), react together. The equation is shown below. Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq) A student carries out an experiment to determine the enthalpy change of reaction, ΔrH, of this reaction. The student follows the method outlined below. • Add 50.0 cm3 of 1.50 mol dm–3 Pb(NO3)2(aq) to a polystyrene cup. • Measure out 50.0 cm3 of a solution of KI(aq), which is in excess. • Measure the temperature of both solutions. • Add the KI(aq) to the polystyrene cup, stir the mixture and record the maximum temperature. Temperature readings Initial temperature of both solutions = 19.5 °C Maximum temperature of mixture = 30.0 °C (a) 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 density of all solutions and specific heat capacity, c, of the reaction mixture is the same as for water. ΔrH = ............................................ kJ mol–1 [4]
98. June 2016 · AS Paper 2 · Q4(a)(i) · 1 marks · AO2
10 4 Nitrogen forms several different oxides. N2O is a useful anaesthetic and NO has been linked to the depletion of ozone in the stratosphere. (a) The standard enthalpy changes of formation of N2O and NO are given in the table. Compound ΔfH ɵ / kJ mol–1 N2O (g) + 82.0 NO (g) + 90.2 (i) Explain, in terms of bond breaking and bond making, why the enthalpy change of formation of NO is endothermic. ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
99. June 2016 · AS Paper 2 · Q4(a)(ii) · 2 marks · AO1, 2
(ii) 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 products should be included as part of the diagram. You are not expected to show the activation energy for the reaction. enthalpy progress of reaction [2]

Mark schemes

1. June 2025 · Paper 1 · Q6
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2. June 2025 · AS Paper 1 · Q11
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3. June 2025 · Paper 1 · Q17(a)(i)
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4. June 2025 · Paper 1 · Q17(a)(ii)
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5. June 2025 · AS Paper 1 · Q23(b)
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6. June 2025 · AS Paper 2 · Q1(c)(i)
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7. June 2025 · AS Paper 2 · Q1(c)(ii)
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8. June 2025 · AS Paper 2 · Q4(b)(ii)
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9. June 2025 · AS Paper 2 · Q4(b)(iii)
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10. June 2025 · AS Paper 2 · Q4(b)(iv)
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11. June 2025 · Paper 3 · Q6
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12. June 2024 · Paper 1 · Q7
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13. June 2024 · AS Paper 1 · Q11
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14. June 2024 · Paper 1 · Q16(a)(i)
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15. June 2024 · Paper 1 · Q16(b)(i)
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16. June 2024 · AS Paper 1 · Q23(b)
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17. June 2024 · AS Paper 2 · Q3(a)
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18. June 2024 · AS Paper 2 · Q3(b)
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19. June 2024 · AS Paper 2 · Q3(c)
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20. June 2024 · Paper 3 · Q2(a)(i)
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21. June 2024 · Paper 3 · Q2(a)(ii)
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22. June 2024 · Paper 3 · Q2(a)(iii)
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23. June 2023 · AS Paper 1 · Q11
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24. June 2023 · AS Paper 1 · Q12
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25. June 2023 · Paper 1 · Q18(a)
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26. June 2023 · Paper 1 · Q18(b)
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27. June 2023 · AS Paper 1 · Q23(a)(ii)
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28. June 2023 · AS Paper 1 · Q23(b)(i)
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29. June 2023 · AS Paper 1 · Q23(b)(ii)
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30. June 2023 · AS Paper 2 · Q4(a)
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31. June 2023 · AS Paper 2 · Q4(b)(ii)
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32. June 2023 · AS Paper 2 · Q4(b)(iii)
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33. June 2023 · AS Paper 2 · Q4(c)(i)
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34. June 2023 · AS Paper 2 · Q4(c)(ii)
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35. June 2023 · Paper 3 · Q2(a)(ii)
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36. June 2022 · Paper 1 · Q7
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37. June 2022 · Paper 1 · Q10
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38. June 2022 · AS Paper 1 · Q11
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39. June 2022 · Paper 1 · Q17(a)(i)
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40. June 2022 · Paper 1 · Q17(a)(ii)
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41. June 2022 · Paper 1 · Q17(b)(i)
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42. June 2022 · Paper 1 · Q17(b)(ii)
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43. June 2022 · AS Paper 1 · Q25(a)(i)
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44. June 2022 · AS Paper 1 · Q25(a)(ii)
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45. June 2022 · AS Paper 2 · Q3(b)(ii)
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46. June 2022 · AS Paper 2 · Q4(b)
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47. June 2022 · Paper 3 · Q5(a)
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48. June 2022 · Paper 3 · Q5(b)(ii)
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49. November 2021 · Paper 1 · Q8
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50. November 2021 · AS Paper 1 · Q10
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51. November 2021 · AS Paper 1 · Q22(a)
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52. November 2021 · Paper 1 · Q22(b)(ii)
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53. November 2021 · Paper 1 · Q22(b)(iii)
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54. November 2021 · AS Paper 1 · Q22(c)
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55. November 2021 · AS Paper 2 · Q4(a)
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56. November 2020 · Paper 1 · Q6
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57. November 2020 · AS Paper 1 · Q11
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58. November 2020 · AS Paper 1 · Q24(a)
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59. November 2020 · AS Paper 2 · Q2(a)
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60. November 2020 · AS Paper 2 · Q2(b)
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61. November 2020 · AS Paper 2 · Q2(c)(ii)
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62. November 2020 · Paper 3 · Q3(a)(i)
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63. November 2020 · Paper 3 · Q3(a)(ii)
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64. November 2020 · Paper 3 · Q3(b)
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65. June 2019 · AS Paper 1 · Q10
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66. June 2019 · Paper 1 · Q10
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67. June 2019 · AS Paper 1 · Q11
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68. June 2019 · Paper 1 · Q19(a)(i)
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69. June 2019 · Paper 1 · Q19(a)(iii)
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70. June 2019 · AS Paper 1 · Q24(b)(i)
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71. June 2019 · AS Paper 1 · Q24(b)(ii)
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72. June 2019 · Paper 3 · Q5(a)(i)
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73. June 2018 · AS Paper 1 · Q11
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74. June 2018 · AS Paper 1 · Q12
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75. June 2018 · Paper 1 · Q13
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76. June 2018 · Paper 1 · Q16(b)(i)
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77. June 2018 · Paper 1 · Q16(b)(ii)
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78. June 2018 · Paper 1 · Q20(b)
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79. June 2018 · AS Paper 1 · Q23(a)(i)
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80. June 2018 · AS Paper 2 · Q3(c)
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81. June 2018 · AS Paper 2 · Q3(d)(ii)
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82. June 2018 · AS Paper 2 · Q4(a)(i)
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83. June 2018 · AS Paper 2 · Q4(a)(ii)
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84. June 2018 · Paper 3 · Q5(a)
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85. June 2017 · Paper 1 · Q9
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86. June 2017 · AS Paper 1 · Q11
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87. June 2017 · AS Paper 1 · Q12
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88. June 2017 · Paper 1 · Q18(b)(iv)
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89. June 2017 · AS Paper 1 · Q25(a)(i)
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90. June 2017 · AS Paper 1 · Q25(a)(ii)
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91. June 2017 · AS Paper 2 · Q3(a)(i)
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92. June 2017 · AS Paper 2 · Q3(a)(ii)
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93. June 2017 · AS Paper 2 · Q3(b)(ii)
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94. June 2017 · AS Paper 2 · Q3(c)
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95. June 2017 · Paper 3 · Q2(a)
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96. June 2016 · AS Paper 1 · Q12
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97. June 2016 · AS Paper 1 · Q24(a)
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98. June 2016 · AS Paper 2 · Q4(a)(i)
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99. June 2016 · AS Paper 2 · Q4(a)(ii)
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