Chemical energetics — AS & A Level Chemistry (9701) questions by topic
Enthalpy cycles and neutralisation calculations run through this topic: judging whether statements about enthalpy changes of reaction, combustion or formation are correct, calculating temperature rise on mixing acid and alkali, and predicting hydration-enthalpy trends down Group 17.
Calculate, identify, state, explain and express are the leading command words across 53 questions from 2023 to 2025 papers, worth from 1 to 17 marks, where showing working separates full credit from a bare numerical guess. Practising enthalpy-cycle calculations with instant marking catches a doubled or halved mole value, or an unconverted J-to-kJ error, at the exact line it occurs rather than at the final answer.
What examiners look for
- A common mistake was not showing working, which lost credit even when a numerical answer was correct.
- Examiners note candidates suggesting enthalpy change of formation is always negative, which is incorrect.
- Reports flag omitting the word 'one mole' when describing gaseous ions or atoms in a hydration-enthalpy definition.
- A recurring error was 1.55 x 10^-5, from forgetting to double the value for [Ag+].
- Strong answers showed their working as required and correctly identified that O2- has the smallest ionic radius and largest charge density.
May–June 2025, Paper 11
The reaction of hydrogen with oxygen is shown. 2H2(g) + O2(g) → 2H2O(l) Which expression corresponds to the standard enthalpy change of this reaction? A 2 × ΔHf°(H2O) B ΔHc°(H2O)…
Answer this question and get it marked →50 cm^3 of 1.0 mol dm^-3 H2SO4 is added to 100 cm^3 of 1.0 mol dm^-3 NaOH in an insulated vessel. Both solutions are at a temperature of 20 °C before mixing. After mixing, the…
Answer this question and get it marked →May–June 2025, Paper 12
Some standard enthalpy of combustion data are given. substance standard enthalpy change of combustion/kJ mol^-1 C(s) –394 H2(g) –286 CH3OH(l) –726 Using these data, what is the…
Answer this question and get it marked →Under certain conditions, CCl4 and H2O react as shown. The activation energy for the reaction is +62 kJ mol^-1. CCl4 + 2H2O → CO2 + 4HCl ΔH = –61.5 kJ mol^-1 Which enthalpy…
Answer this question and get it marked →May–June 2025, Paper 13
The first stage in the industrial production of nitric acid from ammonia can be represented by the following equation. 4NH3(g) + 5O2(g) ⇌ 4NO(g) + 6H2O(g) Using the following…
Answer this question and get it marked →The equation for the reaction of nitrogen with hydrogen is shown. N2(g) + 3H2(g) ⇌ 2NH3(g) ΔHr° = –92.2 kJ mol^-1 Which statement is correct? A ΔHr° is measured at 298 °C. B ΔHr°…
Answer this question and get it marked →May–June 2025, Paper 14
Solid sulfur consists of molecules made up of eight atoms covalently bonded together. The bonding in sulfur dioxide is O=S=O. enthalpy change of combustion of S8, ΔHc°S8(s) =…
Answer this question and get it marked →Which equation represents the standard enthalpy change of formation, ΔHf°, for ethanol? A 2C(s) + 2½H2(g) + ½O2(g) → C2H5OH(g) B 2C(s) + 2½H2(g) + ½O2(g) → C2H5OH(l) C 2C(s) +…
Answer this question and get it marked →May–June 2025, Paper 41
Table 4.1 gives the enthalpy changes of hydration, ΔHhyd, of three ions, F−, K+ and Ca2+. Table 4.1: ion ΔHhyd / kJ mol−1 F− −506 K+ −322 Ca2+ −1650 (i) Define enthalpy change of…
Answer this question and get it marked →May–June 2025, Paper 42
May–June 2025, Paper 43
Table 4.1 gives the enthalpy changes of hydration, ΔHhyd, of three ions, F⁻, K⁺ and Ca²⁺. Table 4.1: ion ΔHhyd / kJ mol⁻¹ F⁻ −506 K⁺ −322 Ca²⁺ −1650 (i) Define enthalpy change of…
Answer this question and get it marked →May–June 2025, Paper 44
Nitrogen triiodide, NI3, decomposes explosively. 2NI3(s) → N2(g) + 3I2(g) (i) Describe the observation when NI3 is heated. (ii) Define the term entropy. (iii) Explain why the…
Answer this question and get it marked →(i) Define the term lattice energy. (ii) Define the term standard enthalpy change of solution, ΔHsol⦵.
Answer this question and get it marked →May–June 2024, Paper 11
An energy cycle is shown. At the top is CH4 + 2O2; an arrow X leads down to C + 2H2 + 2O2, and an arrow Y leads to CO2 + 2H2O. An arrow Z links C + 2H2 + 2O2 to CO2 + 2H2O. The…
Answer this question and get it marked →For a certain endothermic reaction, the activation energy is numerically equal to twice the enthalpy change of reaction. Which reaction pathway diagram is correct for this…
Answer this question and get it marked →May–June 2024, Paper 12
Ethane can react with fluorine to produce 1,2-difluoroethane, C2H4F2. C2H6 + 2F2 - C2H4F2 + 2HF deltaH = -960 kJ mol^-1 Bond energies / kJ mol^-1: C-H = 410; C-C = 350; F-F = 158;…
Answer this question and get it marked →Which equation has an enthalpy change equal to the standard enthalpy of formation of sodium oxide? A Na(s) + 1/4 O2(g) - 1/2 Na2O(s) B Na(s) + O2(g) - Na2O(s) C 2Na(s) + 1/2 O2(g)…
Answer this question and get it marked →May–June 2024, Paper 13
The apparatus used to determine a value for the enthalpy of combustion of butan-1-ol is shown: a copper can holding 175 cm^3 of water with a thermometer, heated by a burner…
Answer this question and get it marked →In the high temperatures of car engines, nitrogen reacts with oxygen to produce nitrogen monoxide. 1/2 N2(g) + 1/2 O2(g) - NO(g) deltaH(standard) = +90 kJ mol^-1 This reaction has…
Answer this question and get it marked →May–June 2024, Paper 21
May–June 2024, Paper 22
Separate samples of Na2CO3 and NaHCO3 react with HCl(aq) to produce the same products, as shown in Table 2.1. Table 2.1 reaction equation ΔH / kJ mol⁻¹ --- --- --- 1 Na2CO3 + 2HCl…
Answer this question and get it marked →May–June 2024, Paper 23
Write an equation to show the reaction for the standard enthalpy change of formation of H2O. Include state symbols.
Answer this question and get it marked →May–June 2024, Paper 42
Carbon disulfide, CS2, is flammable and reacts readily with oxygen, as shown in reaction 1: CS2(g) + 3O2(g) - CO2(g) + 2SO2(g). Table 3.1 shows the standard enthalpy of formation,…
Answer this question and get it marked →October–November 2024, Paper 11
The standard enthalpy change of combustion of carbon is −394 kJ mol⁻¹. The standard enthalpy change of combustion of hydrogen is −286 kJ mol⁻¹. The standard enthalpy change of…
Answer this question and get it marked →Three processes are described. 1 H⁺(aq) + OH⁻(aq) → H₂O(l) 2 CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) 3 NH₃(g) → NH₃(l) Which statement is correct? A None of the processes have a…
Answer this question and get it marked →October–November 2024, Paper 12
For which reaction is the enthalpy change an enthalpy change of formation? A C(g) + 2H₂(g) → CH₄(g) B ½N₂(g) + ½O₂(g) → NO(g) C Na₂O(s) + SO₃(g) → Na₂SO₄(s) D PCl₃(g) + Cl₂(g) →…
Answer this question and get it marked →Two standard enthalpy change of formation values are given. ΔH°f [VCl₂] = −452 kJ mol⁻¹ ΔH°f [VCl₃] = −573 kJ mol⁻¹ What is the enthalpy change for the reaction 3VCl₂ → 2VCl₃ + V…
Answer this question and get it marked →October–November 2024, Paper 13
The standard enthalpy change of combustion of carbon is −394 kJ mol⁻¹. The standard enthalpy change of combustion of hydrogen is −286 kJ mol⁻¹. The standard enthalpy change of…
Answer this question and get it marked →Three processes are described. 1 H⁺(aq) + OH⁻(aq) → H₂O(l) 2 CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) 3 NH₃(g) → NH₃(l) Which statement is correct? A None of the processes have a…
Answer this question and get it marked →October–November 2024, Paper 41
(a) Predict and explain the variation in enthalpy change of hydration for the ions F-, Cl-, Br- and I-.
Answer this question and get it marked →October–November 2024, Paper 42
(a) Predict and explain the variation in enthalpy change of hydration for the ions Na+, Mg2+ and Al3+.
Answer this question and get it marked →October–November 2024, Paper 43
(a) Predict and explain the variation in enthalpy change of hydration for the ions F-, Cl-, Br- and I-.
Answer this question and get it marked →February–March 2023, Paper 12
The equation for a chemical reaction is shown. All substances are in their standard states. XeF6 + 3H2O → XeO3 + 6HF Which statement describes the standard enthalpy change of…
Answer this question and get it marked →The table shows bond energies for some diatomic molecules. Deuterium, D, is an isotope of hydrogen. bond energy / kJ mol⁻¹ --- --- F–F 158 Cl–Cl 242 Br–Br 193 I–I 151 H–H 436 D–D…
Answer this question and get it marked →May–June 2023, Paper 11
Calcium carbide, CaC2, reacts with water, as shown. The data below the equation show, in kJ mol⁻¹, the standard enthalpies of formation of the compounds involved. CaC2(s) + H2O(l)…
Answer this question and get it marked →May–June 2023, Paper 12
A student mixed 25.0 cm3 of 4.00 mol dm^-3 hydrochloric acid with an equal volume of 4.00 mol dm^-3 sodium hydroxide. The initial temperature of both solutions was 15.0 degC. The…
Answer this question and get it marked →Nitrogen monoxide is rapidly oxidised to nitrogen dioxide. 2NO(g) + O2(g) - 2NO2(g) Nitrogen dioxide can then dimerise to form dinitrogen tetroxide. 2NO2(g) - N2O4(g) delta H =…
Answer this question and get it marked →May–June 2023, Paper 13
Which equation represents the reaction whose standard enthalpy change is the standard enthalpy change of formation of water? A 2H2(g) + O2(g) - 2H2O(l) B H2(g) + 1/2 O2(g) -…
Answer this question and get it marked →The enthalpy change for neutralisation of HNO3(aq) with NaOH(aq) is -57.0 kJ mol^-1. In an experiment, 20.0 cm3 of 4.00 mol dm^-3 HNO3 is mixed with 30.0 cm3 of 2.00 mol dm^-3…
Answer this question and get it marked →Some bond energies are listed. bond bond energy / kJ mol^-1 --- --- H-H 436 O-H 463 O-O 146 O=O 496 One mole of hydrogen reacts with oxygen to give water vapour. Using these bond…
Answer this question and get it marked →May–June 2023, Paper 21
A neutralisation reaction occurs when NaOH(aq) is added to H2SO4(aq). equation 1: 2NaOH(aq) + H2SO4(aq) - Na2SO4(aq) + 2H2O(l) Define enthalpy change of neutralisation, delta H…
Answer this question and get it marked →May–June 2023, Paper 22
In industry, ethanol is made by reacting ethene with steam in the presence of H3PO4. reaction 1: C2H4(g) + H2O(g) <= C2H5OH(g) Use the bond energy values in Table 4.1 to calculate…
Answer this question and get it marked →May–June 2023, Paper 23
A neutralisation reaction occurs when NaOH(aq) is added to H2SO4(aq). equation 1: 2NaOH(aq) + H2SO4(aq) - Na2SO4(aq) + 2H2O(l) Define enthalpy change of neutralisation, delta H…
Answer this question and get it marked →May–June 2023, Paper 41
Complete Table 8.1 by placing one tick in each row to indicate the sign of each type of energy change under standard conditions: lattice energy, enthalpy change of hydration, and…
Answer this question and get it marked →May–June 2023, Paper 42
Complete Table 3.1 by placing one tick in each row to indicate the sign of each type of energy change under standard conditions: bond energy, enthalpy change of atomisation, and…
Answer this question and get it marked →May–June 2023, Paper 43
Complete Table 8.1 by placing one tick in each row to indicate the sign of each type of energy change under standard conditions: lattice energy, enthalpy change of hydration, and…
Answer this question and get it marked →October–November 2023, Paper 11
Which statement about enthalpy changes is correct? A: Enthalpy changes of reaction are always negative. B: Enthalpy changes of combustion are always positive. C: Enthalpy changes…
Answer this question and get it marked →What is the definition of standard enthalpy change of neutralisation, delta-H neut? A: delta-H when one mole of an aqueous acid is neutralised by an aqueous alkali. B: delta-H…
Answer this question and get it marked →October–November 2023, Paper 12
Nitric acid is made industrially by the oxidation of ammonia; the overall equation 1 is NH3 + 2O2 - HNO3 + H2O. The process happens in three stages: stage 1: 4NH3 + 5O2 - 4NO +…
Answer this question and get it marked →October–November 2023, Paper 13
Which statement about enthalpy changes is correct? A Enthalpy changes of reaction are always negative. B Enthalpy changes of combustion are always positive. C Enthalpy changes of…
Answer this question and get it marked →What is the definition of standard enthalpy change of neutralisation, delta H neut standard? A delta Hr standard when one mole of an aqueous acid is neutralised by an aqueous…
Answer this question and get it marked →October–November 2023, Paper 41
Hydrogen peroxide is a liquid at 298 K. It is moderately stable under room conditions but will decompose quickly if a catalyst is added (reaction 1: 2H₂O₂(l) → 2H₂O(l) + O₂(g)).…
Answer this question and get it marked →October–November 2023, Paper 43
Hydrogen peroxide is a liquid at 298 K. It is moderately stable under room conditions but will decompose quickly if a catalyst is added (reaction 1: 2H₂O₂(l) → 2H₂O(l) + O₂(g)).…
Answer this question and get it marked →Other AS & A Level Chemistry topics
- Practical skills (106)
- Hydrocarbons (65)
- Chemical bonding (63)
- The Periodic Table: chemical periodicity (59)
- Equilibria (53)
- Introduction to organic chemistry (53)
- Reaction kinetics (52)
- Carboxylic acids and derivatives (52)
- Group 2 (51)
- Carbonyl compounds (45)
- Atomic structure (44)
- Halogen compounds (37)