Explain why Hess's law is used to determine the standard enthalpy change of formation of ethanol rather than measuring it directly by experiment.

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)

Directly forming ethanol from its elements requires precise stoichiometric proportions of carbon and hydrogen that cannot be achieved practically in a laboratory.

Hess’s law states that the enthalpy change of a reaction depends only on the initial and final states, not on the pathway.

Therefore an indirect cycle is constructed using reactions with measurable enthalpy changes, such as the standard combustion enthalpies of ethanol, carbon and hydrogen.

The standard enthalpy of formation of ethanol is then calculated by algebraically combining these measured values according to Hess’s law, e.g. ΔHf°(ethanol)=ΣΔHc°(elements)−ΔHc°(ethanol).

Examiner tips

  • Use the word ‘directly forming ethanol’ to show you understand the practical limitation.
  • Explain Hess’s law in one sentence and link it to the use of a cycle.
  • Show the algebraic combination of combustion enthalpies explicitly.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Claiming the reaction can be carried out directly without explaining the stoichiometric issue.
  • Confusing the enthalpy of formation with the enthalpy of combustion.
  • Omitting the algebraic step that combines the measured values.

Mark scheme (4 marks)

  1. The direct formation reaction requires carbon and hydrogen to react in specific stoichiometric proportions to form ethanol, which cannot be carried out under practical laboratory conditions.
  2. Hess's law states that the enthalpy change of a reaction is independent of the pathway taken, depending only on the initial and final states.
  3. An indirect/alternative cycle is constructed using measurable enthalpy changes, such as the standard enthalpies of combustion of ethanol, carbon, and hydrogen.
  4. The standard enthalpy of formation is then calculated by algebraically combining these measurable enthalpy changes according to Hess's law (e.g. ΔHf° = ΣΔHc°(elements) − ΔHc°(ethanol)).

Key terms in this question

Hess's law · standard enthalpy change of formation

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