# Explain why the enthalpy change of combustion of methane determined experimentally using a simple calorimeter is always less exothermic than the accepted standard enthalpy change of combustion.

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

> The standard enthalpy change of combustion of methane is −890 kJ mol⁻¹. When a student burns methane under a copper calorimeter containing water, the experimental value obtained is consistently less exothermic than this accepted value.

## Mark scheme (4 marks)

1. Heat is lost to the surroundings / environment rather than all being transferred to the water in the calorimeter.
2. Incomplete combustion of methane occurs, producing carbon monoxide or soot/carbon rather than solely carbon dioxide, releasing less energy than full combustion.
3. The calorimeter itself (copper vessel) and the thermometer absorb some of the heat, so not all heat goes into the water.
4. As a result of these heat losses, the measured temperature rise of the water is smaller than it would be under ideal conditions, so the calculated enthalpy change has a smaller magnitude / is less negative.

## Key terms

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

## 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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