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 · View as Markdown

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

Model answer (4 marks)

1. Heat is lost to the surroundings (air, calorimeter, bench) so not all heat from combustion raises the water temperature.
2. Incomplete combustion (limited O₂) produces CO or soot instead of CO₂, releasing less energy.
3. Ethanol evaporates from the wick or lamp, so the mass of fuel actually burned is less than calculated, making the enthalpy value appear less exothermic.
4. The standard enthalpy of combustion is defined at 298 K, 1 bar, with reactants and products in their standard states; the experiment is carried out at different temperature, pressure and non‑standard states, contributing to the discrepancy.

Examiner tips

  • Use the exact wording from the mark scheme; list each reason clearly. Show that each point explains a source of error. Keep the answer concise – 4 short sentences is enough for full marks.

Common mistakes

  • Confusing heat loss with incomplete combustion. Assuming the calorimeter is perfectly insulated. Forgetting to mention the standard‑state definition of the data booklet value.

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 in this question

enthalpy change of combustion · calorimeter

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