# Explain why the standard enthalpy of combustion of octane (C₈H₁₈) is a negative value, and suggest why liquid octane is considered a more energy-dense fuel than gaseous hydrogen (H₂) on a per-unit-volume basis.

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

## Mark scheme (4 marks)

1. Combustion of octane is exothermic, so energy is released to the surroundings.
2. More energy is released in forming bonds in the products (CO₂ and H₂O) than is required to break bonds in the reactants (octane and O₂).
3. Liquid octane has a much higher density than gaseous hydrogen, so a given volume of liquid octane contains far more molecules / moles of fuel.
4. Therefore, more energy is released per unit volume from octane than from hydrogen, even though hydrogen has a higher energy density per unit mass.

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-standard-enthalpy-of-a1198441) · Published by Druglandscape Ltd.