Explain why the standard enthalpy change of combustion of propan-1-ol is more negative than that of ethanol, and outline TWO assumptions made when using simple calorimetry (a spirit burner and a metal can of water) to determine these values experimentally.

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).

The standard enthalpy changes of combustion of ethanol (C₂H₅OH) and propan-1-ol (C₃H₇OH) are −1371 kJ mol⁻¹ and −2021 kJ mol⁻¹ respectively.

Model answer (4 marks)

Propan‑1‑ol contains one more –CH₂– group than ethanol, so it has one extra C–H bond and one extra C–C bond to break and one extra CO₂ and H₂O to form during complete combustion. The additional bonds released give a larger magnitude of ΔH°comb, making the value more negative.

Assumption 1: all heat released by the combustion is transferred to the water – i.e. no heat loss to the surroundings or to the calorimeter itself.

Assumption 2: the specific heat capacity and density of the solution in the calorimeter are taken as those of pure water (4.18 J g⁻¹ K⁻¹ and 1.00 g cm⁻³).

Examiner tips

  • Use the word ‘more negative’ and explain the extra –CH₂– group; mention bonds broken/formed. Show the two assumptions clearly and state the numerical values for heat capacity and density. Keep the answer concise – 4 marks only.
  • common_mistakes
  • :
  • Saying the difference is due to a ‘larger molecule’ without linking to extra bonds. Forgetting to specify the numerical values for heat capacity and density. Using the wrong sign or not stating ‘more negative’ explicitly.

Mark scheme (4 marks)

  1. Propan-1-ol has more C–H (and C–C) bonds / a greater number of bonds broken and formed during complete combustion, releasing more energy per mole.
  2. One additional –CH₂– unit in propan-1-ol means one additional mole of CO₂ and one additional mole of H₂O is produced per mole of alcohol combusted, accounting for the ~650 kJ mol⁻¹ difference.
  3. First assumption: all heat released by combustion is transferred to the water / no heat loss to the surroundings or the calorimeter itself.
  4. Second assumption: the specific heat capacity and density of the solution / water in the calorimeter are the same as pure water (4.18 J g⁻¹ K⁻¹ and 1.00 g cm⁻³).

Key terms in this question

standard enthalpy change of combustion · calorimetry

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