# Explain why the energy density of ethanol (C₂H₅OH) is lower than that of octane (C₈H₁₈) when both values are expressed per gram of fuel.

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

> Biofuels such as ethanol are increasingly blended with petrol to reduce dependence on fossil fuels. The energy densities of ethanol and octane, expressed per gram, differ significantly.

## Mark scheme (4 marks)

1. Octane contains a greater proportion of C–H bonds (by mass) than ethanol, and C–H bonds release more energy per bond on combustion than C–O bonds.
2. C–H bonds are more energetically favourable to combust than C–O bonds; the C–O bond in ethanol is already partially oxidised, so less energy is released when it is further oxidised to CO₂.
3. Ethanol has a lower molar mass relative to the number of C–H bonds it contains compared to octane, meaning less chemical energy is stored per gram.
4. Therefore, per gram, octane releases more energy on complete combustion, giving it a higher energy density than ethanol.

## Key terms

- [energy density](https://www.gradenine.co.uk/glossary/energy-density)

## 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)
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