CH4(g) + 2O2(g) → CO2(g) + 2H2O(g). Mean bond enthalpies: C-H 413, O=O 498, C=O in CO2 805, O-H 464 kJ/mol. Calculate dH.
[3 marks]
Total for this question: 3
All calculations
Reaction enthalpy from mean bond enthalpies
Energetics
AQA 7405
ORIGINAL
dH = sum(bonds broken) - sum(bonds formed)
Count bonds in the balanced gaseous equation, then subtract energy released making bonds from energy absorbed breaking them.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How 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
- Counting two O-H bonds instead of four in two water molecules.
- Using liquid-water formation data with gaseous mean bond enthalpies.
Exam tip: Mean bond enthalpies apply to gaseous molecules and therefore give an approximate reaction enthalpy.
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. Free intro call, then a free first lesson.
Stuck on mean bond enthalpy?
Everything here stays free. If mean bond enthalpy is where the marks keep going, I teach it one to one from the mark schemes. Free intro call, then a free first lesson.