# Explain why the standard enthalpy of combustion of hydrogen is considered a more environmentally favourable fuel than petrol, and identify one limitation of using hydrogen as a fuel despite its high energy output.

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

> Hydrogen gas is increasingly discussed as an alternative to fossil fuels. Its standard enthalpy of combustion is approximately −286 kJ mol⁻¹, and its only combustion product is water.

## Mark scheme (4 marks)

1. Combustion of hydrogen produces only water (no carbon dioxide or other greenhouse gases / pollutants), so it does not contribute to the enhanced greenhouse effect / climate change.
2. Petrol combustion produces carbon dioxide (and other pollutants such as CO or NOₓ), which are greenhouse gases / contribute to air pollution / climate change.
3. Hydrogen has a very high specific energy / energy per unit mass (energy density by mass), making it an efficient fuel.
4. One limitation: hydrogen is difficult / expensive to store and transport safely because it is a gas at room temperature and has a very low density / requires high-pressure tanks or cryogenic conditions; OR most hydrogen is currently produced from fossil fuels (steam reforming), so its production is not truly carbon-neutral.

## 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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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-standard-enthalpy-of-d89caa56) · Published by Druglandscape Ltd.