About Chemical energetics in A Level Chemistry
Chemical energetics is one of the 18 main topics in our A Level Chemistry (Cambridge (CAIE) 9701) question bank, and about 4% of its real questions come from it. Tayyari Karo has 29 real questions on this topic from 2021 to 2026, from Paper 1, Paper 2, Paper 4; 21 are multiple choice and marked instantly, and the rest are structured questions with a model answer. By difficulty: 16 medium, 10 hard, 3 easy.
Work through the questions below, then open the explanation for each one. Redo the ones you missed a few days later.
Try 6 real questions
These are real past-paper questions. Think of your answer first, then open the explanation.
May/June 2026 · Paper 1 · Chemical energetics
The reaction C(s) + CO2(g) -> 2CO(g) has an enthalpy change of +172 kJ mol-1. Which deduction can be made?
- Carbon can be used as a fuel by reacting it with carbon dioxide.
- The reaction C(s) + CO2(g) -> 2CO(g) absorbs heat from its surroundings.
- The standard enthalpy change of combustion of carbon is +172 kJ mol-1.
- The standard enthalpy change of formation of CO is +86 kJ mol-1.
Show answer and explanation
Answer: B. The reaction C(s) + CO2(g) -> 2CO(g) absorbs heat from its surroundings.
A positive enthalpy change means the reaction is endothermic. The other statements misuse the definitions of combustion and formation.
May/June 2025 · Paper 1 · Chemical energetics
For the reaction 2H2(g) + O2(g) → 2H2O(l), which expression corresponds to the standard enthalpy change of this reaction?
- 2 × ΔHf⦵(H2O)
- ΔHc⦵(H2O) − ΔHc⦵(H2)
- ΔHc⦵(H2)
- 2 × ΔHf⦵(H2O) + 2 × ΔHc⦵(H2)
Show answer and explanation
Answer: A. 2 × ΔHf⦵(H2O)
Since ΔHf of elements in their standard states is zero, ΔHr = 2 × ΔHf⦵(H2O).
October/November 2025 · Paper 1 · Chemical energetics
When 1.0 mol of ethanoic acid, CH3COOH, in aqueous solution is neutralised by excess aqueous sodium hydroxide, 55 kJ mol⁻¹ of energy is released. Which statement about this reaction is correct?
- The reaction is exothermic because only bond breaking takes place.
- The reaction is exothermic because only bond forming takes place.
- The reaction is exothermic because more energy is given out in breaking bonds than is taken in to form bonds.
- The reaction is exothermic because more energy is given out in forming bonds than is taken in to break bonds.
Show answer and explanation
Answer: D. The reaction is exothermic because more energy is given out in forming bonds than is taken in to break bonds.
All chemical reactions involve both bond breaking (endothermic) and bond forming (exothermic); it is exothermic overall because bond forming releases more energy than bond breaking absorbs.
October/November 2024 · Paper 1 · Chemical energetics
Three processes: 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?
- None of the processes have a positive enthalpy change.
- Only process 1 has a positive enthalpy change.
- Only process 2 has a positive enthalpy change.
- Only process 3 has a positive enthalpy change.
Show answer and explanation
Answer: A. None of the processes have a positive enthalpy change.
Neutralisation (1) and combustion (2) are both exothermic, and condensation of a gas to a liquid (3) is also exothermic (releases energy), so none of the processes are endothermic.
October/November 2023 · Paper 1 · Chemical energetics
What is the definition of standard enthalpy change of neutralisation, ΔH°neut?
- ΔH when one mole of an aqueous acid is neutralised by an aqueous alkali
- ΔH when one mole of an aqueous alkali is neutralised by an aqueous acid
- ΔH when one mole of an aqueous acid is neutralised by one mole of an aqueous alkali
- ΔH when an aqueous acid and an aqueous alkali react together to produce one mole of water
Show answer and explanation
Answer: D. ΔH when an aqueous acid and an aqueous alkali react together to produce one mole of water
Standard enthalpy change of neutralisation is defined per mole of water formed from the reaction of an aqueous acid with an aqueous alkali.
May/June 2021 · Paper 1 · Chemical energetics
ΔH°1 is the standard enthalpy of formation of methane. ΔH°2 is the standard enthalpy of combustion of carbon. ΔH°3 is the standard enthalpy of combustion of hydrogen. CH4(g) + 2O2(g) → CO2(g) + 2H2O(l), ΔH°c. Which expression is equivalent to ΔH°c?
- ΔH°1 − ΔH°2 + ΔH°3
- ΔH°1 − 2ΔH°3 − ΔH°2
- ΔH°2 − ΔH°3 + ΔH°1
- ΔH°2 + 2ΔH°3 − ΔH°1
Show answer and explanation
Answer: D. ΔH°2 + 2ΔH°3 − ΔH°1
By Hess's law, ΔH°c(CH4) = ΔH°2 + 2ΔH°3 − ΔH°1, going via elements C(s) and H2(g).
