# 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

> 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.

## 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

- [standard enthalpy change of combustion](https://www.gradenine.co.uk/glossary/standard-enthalpy-change-of-combustion)
- [calorimetry](https://www.gradenine.co.uk/glossary/calorimetry)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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