Burning 0.500 g of methanol (Mr = 32.0) raised the temperature of 150 g of water by 22.0 K. Calculate the enthalpy of combustion of methanol in kJ mol−1. (c = 4.18 J g−1 K−1)
[3 marks]
Total for this question: 3
Chemistry
Energetics
AQA 3.1.4
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works
The topic on one screen
- Exothermic reactions release heat: ΔH is negative. Endothermic: ΔH is positive. Always give the sign.
- Calorimetry: q = mcΔT. Use the mass of WATER (or solution), not the fuel; then ΔH = -q / n (per mole).
- Hess's law: the enthalpy change is independent of route. ΔH(reaction) = sum ΔHf(products) - sum ΔHf(reactants).
- Mean bond enthalpy is the energy to break one mole of a bond in the gaseous state, averaged over many different compounds.
- Bond-enthalpy method: ΔH = (sum of bonds broken) - (sum of bonds formed). It is only approximate — the mean differs from the real bond, and it ignores state changes.
- Show working: write the sums of bonds broken and bonds formed, or the Hess cycle, so error-carried-forward marks are available.
Where students actually lose marks
A significant number lost the final calorimetry mark by giving a positive answer for an exothermic change. If it releases heat, the sign must be negative — check it every time.
June 2024 Paper 2 examiner report (Q11.1)
Very few recognised that a value from mean bond enthalpies does not account for the enthalpy changes when substances go from liquid to gas (and back). That, plus 'averaged over a range of compounds', is why it is approximate.
June 2024 Paper 2 examiner report (Q11.2)
The commonest bond-enthalpy error is too little working: show the totals for bonds broken and bonds formed so that a slip still earns consequential marks.
June 2023 Paper 2 examiner report (Q08.6)
Try it — exam-style
Hard
ORIGINAL · after June 2024 Paper 2 Q11.1
Medium
ORIGINAL · after June 2024 Paper 2 Q11.2
Explain what is meant by 'mean bond enthalpy' and give two reasons why an enthalpy change calculated from mean bond enthalpies is only approximate.
[3 marks]
Total for this question: 3
Medium
ORIGINAL · after June 2023 Paper 2 Q08.6
Use mean bond enthalpies to calculate ΔH for H2(g) + Cl2(g) → 2HCl(g). Bond enthalpies / kJ mol−1: H-H = 436, Cl-Cl = 242, H-Cl = 431.
[2 marks]
Total for this question: 2
Hard
ORIGINAL · after June 2023 Paper 2 Q08.7
Use enthalpies of formation to calculate ΔH for CH4(g) + 2O2(g) → CO2(g) + 2H2O(l). ΔHf / kJ mol−1: CH4 = -75, CO2 = -394, H2O = -286. (ΔHf of O2 = 0)
[2 marks]
Total for this question: 2
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
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