Energetics
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
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
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)
Explain what is meant by 'mean bond enthalpy' and give two reasons why an enthalpy change calculated from mean bond enthalpies is only approximate.
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.
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)
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
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Sign errors and missing working throw away easy energetics marks — I teach the layout that keeps every consequential mark alive, and the first lesson is free.