Explain why the experimentally determined enthalpy change of combustion of ethanol, measured using a simple calorimeter, is always less exothermic than the accepted standard enthalpy change of combustion value listed in a data booklet.

IB DP Chemistry Higher 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 from the calorimeter and the surrounding environment during the experiment, so the measured temperature rise is smaller than it would be in an isolated system.
2. Incomplete combustion of ethanol can occur, producing CO or soot instead of CO₂, which releases less heat than complete combustion.
3. Some ethanol evaporates from the wick or spirit lamp before it burns, meaning less fuel is actually combusted than the mass measured, so the calculated energy per mole is underestimated.
4. These combined errors mean the experimentally determined enthalpy change of combustion is less exothermic (less negative) than the standard value listed in the data booklet.

Examiner tips

  • Use the word ‘explain’ to describe each error and its effect on the measured value.
  • Show the logical chain: loss of heat → lower ΔT → less negative ΔH; incomplete combustion → lower heat release; evaporation → less fuel burned → lower ΔH.
  • Include the final statement that links all errors to the overall less exothermic result.
  • Use correct terminology: ‘exothermic’, ‘heat loss’, ‘incomplete combustion’, ‘evaporation’.”]

Mark scheme (4 marks)

  1. Heat is lost from the calorimeter/system to the surroundings/environment during the experiment.
  2. Incomplete combustion of ethanol occurs, producing carbon monoxide or carbon (soot) rather than carbon dioxide, releasing less energy than complete combustion.
  3. Some ethanol evaporates from the wick/spirit lamp before combustion, meaning less fuel is burned than the mass measured, so the calculated energy per mole is underestimated.
  4. The experimental value is less exothermic (less negative) than the standard value because the combined effect of these sources of error means less heat is transferred to the water per mole of ethanol than is released under ideal/standard conditions.

Key terms in this question

standard enthalpy change of combustion · calorimeter

Related

More Measuring enthalpy changes questions

▶ Try answering this question with AI marking (free) →