# A student measures the enthalpy change of combustion of ethanol by burning it beneath a copper calorimeter containing water. The value obtained is significantly less exothermic than the standard enthalpy change of combustion listed in a data booklet. Explain two reasons why the experimental value differs from the data booklet value.

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

## Mark scheme (4 marks)

1. Heat is lost to the surroundings (e.g. to the air, the calorimeter itself, or the bench), so not all the heat released by combustion is transferred to the water.
2. Incomplete combustion of ethanol occurs (due to limited oxygen supply), producing carbon monoxide or carbon (soot) rather than carbon dioxide, releasing less energy than complete combustion.
3. Ethanol evaporates from the wick or spirit lamp during the experiment, so the mass of fuel burned is overestimated, making the calculated enthalpy value less exothermic than the true value.
4. The standard enthalpy change of combustion is defined under standard conditions (100 kPa, 298 K, with reactants and products in their standard states), whereas the experiment is not carried out under these exact conditions, contributing to the discrepancy.

## Key terms

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

## 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/a-student-measures-the-enthalpy-change-8dc4842b) · Published by Druglandscape Ltd.