# Explain why the experimentally determined enthalpy change of combustion of ethanol, measured using a simple calorimeter, is always less exothermic than the accepted standard enthalpy change of combustion value listed in a data booklet.

> IB DP Chemistry Higher Level (2023 syllabus) — R1.1 Measuring enthalpy changes · Explain · 4 marks

## Mark scheme (4 marks)

1. Heat is lost from the calorimeter/system to the surroundings/environment during the experiment.
2. Incomplete combustion of ethanol occurs, producing carbon monoxide or carbon (soot) rather than carbon dioxide, releasing less energy than complete combustion.
3. Some ethanol evaporates from the wick/spirit lamp before combustion, meaning less fuel is burned than the mass measured, so the calculated energy per mole is underestimated.
4. The experimental value is less exothermic (less negative) than the standard value because the combined effect of these sources of error means less heat is transferred to the water per mole of ethanol than is released under ideal/standard conditions.

## Key terms

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

## 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)
- [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-experimentally-determined-enthalpy-2a3672cc) · Published by Druglandscape Ltd.