Reaction enthalpy from mean bond enthalpies
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.
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.
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.
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 — your first lesson is free.