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 · View as Markdown

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

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.

Model answer (4 marks)

1. Heat is lost to the surroundings – the calorimeter, air and bench absorb part of the energy released, so only a fraction heats the water.
2. The fuel does not burn completely – incomplete combustion produces CO or soot, which releases less heat than CO₂.
3. Some fuel evaporates from the wick before it burns – this loss overestimates the mass of fuel actually combusted, making the calculated ΔH less exothermic.
4. The standard enthalpy of combustion is measured at 298 K, 100 kPa and standard states, whereas the experiment is carried out at higher temperature and non‑standard conditions, so the experimental value is systematically lower.

Examiner tips

  • Use the exact terms ‘heat loss’, ‘incomplete combustion’, ‘evaporation of fuel’, and ‘standard conditions’ – these are the key phrases the mark scheme looks for.
  • Show the four points in order, each as a separate sentence or bullet, to demonstrate clear structure.

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 in this question

standard enthalpy of combustion · calorimetry

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