# Explain why the standard enthalpy of combustion of propane (C₃H₈) is more negative than that of methane (CH₄).

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

> Standard enthalpies of combustion: methane = −890 kJ mol⁻¹; propane = −2220 kJ mol⁻¹.

## Mark scheme (4 marks)

1. Propane has more carbon–hydrogen (and carbon–carbon) bonds than methane / propane has a greater number of covalent bonds.
2. Combustion involves breaking bonds in the fuel and forming bonds in the products (CO₂ and H₂O).
3. More bonds are formed in the products when propane combusts, releasing more energy overall.
4. The enthalpy of combustion is more negative (more exothermic) for propane because the total energy released forming bonds exceeds the total energy absorbed breaking bonds by a greater amount than for methane.

## Key terms

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

## Related

- [Revision notes for IB DP Chemistry Standard 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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