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.
Verified against AQA 8462 (2026 spec)
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
3 marks
H2 + Cl2 → 2HCl. Bond energies in kJ/mol are H-H 436, Cl-Cl 243 and H-Cl 431. Calculate the energy change.
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.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.