# Explain why the standard enthalpy of combustion of methane is more exothermic per mole than that of methanol, despite both molecules containing one carbon atom.

> IB DP Chemistry Higher Level (2023 syllabus) — R1.3 Energy from fuels · Explain · 4 marks

> Methane (CH₄) has a standard enthalpy of combustion of −890 kJ mol⁻¹, whilst methanol (CH₃OH) has a standard enthalpy of combustion of −726 kJ mol⁻¹.

## Mark scheme (4 marks)

1. Methanol already contains an oxygen atom (or a C–O bond / O–H bond), meaning it is partially oxidised compared to methane.
2. Combustion is an oxidation process; since methanol is already partially oxidised, less energy is released when it is fully oxidised to CO₂ and H₂O.
3. Methane has more C–H bonds (4) than methanol (3), and C–H bonds release more energy per bond when oxidised than C–O or O–H bonds that replace them in methanol.
4. The overall enthalpy of combustion depends on the difference in bond energies broken and formed; for methane, the net energy released is greater because the reactant bonds are all at a lower oxidation state, giving a larger negative ΔH.

## Key terms

- [standard enthalpy of combustion](https://www.gradenine.co.uk/glossary/standard-enthalpy-of-combustion)
- [exothermic](https://www.gradenine.co.uk/glossary/exothermic)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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