All question types · Thermodynamics
Hess's law from enthalpies of formation
A-Level (AQA) · C3L6 · UKChO Round 1 · IChO · unreviewed
Reaction 2A + B → 3C + D with ΔfH known for every species.
Worked example
For 2A + B → 3C + D: ΔfH(A) = −85.9 kJ mol⁻¹, ΔfH(B) = −55.0 kJ mol⁻¹, ΔfH(C) = −274 kJ mol⁻¹, ΔfH(D) = −15.2 kJ mol⁻¹. Find ΔrH.
1. Weighted sum of 3·ΔfH(C) + 1·ΔfH(D) − 2·ΔfH(A) − 1·ΔfH(B) → ΔrH
= −610.4 kJ mol⁻¹
Answer: ΔrH = −610 kJ mol⁻¹
Method
ΔrH = Σν ΔfH(products) − Σν ΔfH(reactants) = 3ΔfH(C) + ΔfH(D) − 2ΔfH(A) − ΔfH(B).
Common mistakes
- Each ΔfH must be multiplied by its stoichiometric coefficient (3 for C, 2 for A).
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