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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

1. Heat is lost to the surroundings, so not all of the energy released by combustion is absorbed by the water.
2. Incomplete combustion can occur, producing CO or soot instead of CO₂, which releases less energy.
3. The copper calorimeter and thermometer absorb part of the heat, reducing the temperature rise of the water.
4. Because of these losses the temperature change of the water is smaller, giving a calculated enthalpy change that is less exothermic than the true value.

Examiner tips

  • Use the word ‘explain’ to show cause‑effect links; include all four points in order.
  • Show the effect on the temperature rise and the sign of ΔH.
  • Mention the calorimeter’s heat capacity explicitly.
  • Keep each point concise and use correct terminology (e.g., ‘incomplete combustion’, ‘heat capacity’).

Common mistakes

  • Failing to mention heat loss to the surroundings; only citing incomplete combustion. Confusing the sign of ΔH (writing less exothermic as more positive). Omitting the calorimeter’s heat capacity as a source of loss.

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 in this question

enthalpy change of combustion · calorimeter · exothermic

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