# 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

## 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

- [Hess's law](https://www.gradenine.co.uk/glossary/hess-s-law)
- [standard enthalpy change of formation](https://www.gradenine.co.uk/glossary/standard-enthalpy-change-of-formation)

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-hess-s-law-is-used-75bb6f20) · Published by Druglandscape Ltd.