Required practical · A-level chemistry
RP 2: Measuring an enthalpy change
Hazards
⚠CuSO₄ solution — harmful to aquatic life; avoid skin contact
⚠Fuels/spirit burners — flammable
Method
- Two experiments + Hess's lawAQA's exemplar finds ΔH for CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s) — impossible to measure directly, so you measure two dissolving enthalpies and apply Hess's law.
- Experiment 1: the anhydrous saltWeigh 3.90–4.10 g anhydrous CuSO₄ by difference. Put 25 cm³ deionised water in a polystyrene cup (inside a beaker for stability). Record the temperature every minute for 3 minutes, stirring; add the solid at minute 4 (do not record at minute 4); keep recording every minute from 5 to 15.
- Experiment 2: the hydrateRepeat with 6.20–6.30 g of hydrated CuSO₄·5H₂O in 24 cm³ of water — the reduced volume compensates for the water of crystallisation.
- Extrapolate to the moment of mixingPlot temperature against time for each run. Draw two straight best-fit lines (before and after addition) and extrapolate both to minute 4: ΔT is the vertical gap between them at the moment of mixing — this corrects for heat exchanged during the reaction.
- q = mcΔT, then HessCalculate q = m c ΔT with m = 25 g (the water only — AQA's handbook: 'do not add on the mass of the solid'), c = 4.18 J g⁻¹ K⁻¹. Then ΔH = −q ÷ n for each experiment, and ΔH₃ = ΔH₁ − ΔH₂.
Mistakes examiners see every year
| The mistake | Why it costs marks |
|---|---|
| Quoting the temperature at minute 4 instead of ΔT | ΔT is the gap between the two extrapolated lines at the time of mixing — not any single reading. |
| Not extrapolating, or drawing S-shaped 'curves of best fit' | AQA's 2018 examiner report: two straight best-fit lines, both extrapolated to the minute of addition. |
4 more examiner traps for this practical
Plus the step-by-step Lab Mode wizard with apparatus diagrams that highlight as you work.
Open it in ChemLabWhat to expect
Dissolving the anhydrous salt is exothermic (ΔH₁ < 0); dissolving the hydrate is endothermic (ΔH₂ > 0). A typical classroom ΔH₃ lands around −65 to −80 kJ mol⁻¹ (indicative ballpark — school values always undershoot data-book figures because of heat loss and the assumption that the solution's specific heat capacity equals water's). Safety: copper(II) sulfate is a skin/eye irritant (CLEAPSS SSS 40) — avoid contact, especially with the anhydrous powder.