# Explain why the enthalpy change of combustion of a fuel determined by a simple calorimetry experiment using a spirit burner is consistently less exothermic than the accepted standard enthalpy of combustion value listed in a data booklet.

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

> A student burns ethanol in a spirit burner beneath a copper calorimeter containing water. The temperature rise of the water is recorded and used to calculate the enthalpy change of combustion. The value obtained is significantly less exothermic than the standard enthalpy of combustion of ethanol.

## Mark scheme (4 marks)

1. Heat is lost to the surroundings (e.g. to the air, the calorimeter itself, or the bench) rather than being entirely transferred to the water.
2. Incomplete combustion of the fuel occurs, producing carbon monoxide or carbon (soot) rather than carbon dioxide, releasing less energy than complete combustion.
3. Evaporation of the fuel from the wick before combustion (or between readings) means some fuel is lost without burning, so the mass of fuel burned is overestimated, making the calculated enthalpy value less exothermic.
4. The standard enthalpy of combustion is defined under standard conditions (298 K, 100 kPa, all reactants and products in their standard states), which differ from the actual experimental conditions, contributing to a systematic discrepancy.

## Key terms

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

## 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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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-enthalpy-change-of-efa9844d) · Published by Druglandscape Ltd.