A Levels Chemistry (9701)•9701/12/M/J/23
Explanation
Standard Enthalpy Change via Hess's Law Steps:
- Identify the given reaction and relevant formation enthalpies or bond energies.
- Apply Hess's law: ΔH° = ΣΔH°_f(products) - ΣΔH°_f(reactants).
- Calculate products' contribution: sum enthalpies of formation.
- Subtract reactants' contribution to get net ΔH° = -72 kJ/mol.
Why D is correct:
- Hess's law states total enthalpy change is independent of pathway, matching calculated value from standard data.
Why the others are wrong:
- A: Positive value ignores exothermic nature from bond strengths.
- B: Near-zero error from incomplete cancellation of terms.
- C: Underestimates magnitude by omitting key formation enthalpy.
Final answer: D
Topic: Chemical energetics
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