H2 + Cl2 → 2HCl. Bond energies in kJ/mol are H-H 436, Cl-Cl 243 and H-Cl 431. Calculate the energy change.
[3 marks]
Total for this question: 3
All calculations
Energy change from bond energies
Energy changes
AQA 8462
ORIGINAL
Higher tier
energy change = bonds broken - bonds formed
Breaking takes energy; making releases it. Keep that order in the subtraction.
Board and spec code confirmed against AQA 8462 · registry checked 2026-07-17How this checking works
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
Q1
Do the calculation on paper first — then mark it.
Where the marks get lost
- Subtracting broken bonds from formed bonds.
- Forgetting the coefficient gives two H-Cl bonds.
Exam tip: List every bond broken and formed before inserting values; coefficients multiply bond counts.
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