# Explain why the complete combustion of ethanol releases more energy than the complete combustion of methanol, in terms of the bonds broken and formed during each reaction.

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

> Ethanol (C₂H₅OH) and methanol (CH₃OH) are both alcohols used as fuels. The complete combustion of each produces carbon dioxide and water.

## Mark scheme (4 marks)

1. Combustion is exothermic because energy released in bond formation (in products) exceeds energy required for bond breaking (in reactants).
2. Ethanol has more bonds (more C–H, C–C, and C–O bonds) than methanol, so more energy is required to break the bonds in ethanol.
3. Complete combustion of ethanol produces more moles of CO₂ and H₂O per mole of fuel than methanol, so more bonds are formed in the products.
4. The net energy released (enthalpy of combustion) is greater for ethanol because the overall difference between energy released in bond formation and energy absorbed in bond breaking is larger.

## Key terms

- [complete combustion](https://www.gradenine.co.uk/glossary/complete-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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-complete-combustion-of-e44d91d8) · Published by Druglandscape Ltd.