# Explain why the standard enthalpy of atomisation of chlorine is an endothermic process and why this value must be included when constructing a Born-Haber cycle for potassium chloride, KCl.

> Edexcel A-Level Chemistry (9CH0) — 13.1 Born-Haber cycles and entropy · Explain · 5 marks

> Born-Haber cycles are used to determine lattice enthalpies indirectly using Hess's law. Each step in the cycle represents a specific enthalpy change, and all steps must be included to complete the cycle correctly.

## Mark scheme (5 marks)

1. Atomisation of chlorine involves breaking bonds / breaking the Cl–Cl bond in Cl₂
2. Bond breaking requires energy / is endothermic
3. The standard enthalpy of atomisation produces one mole of gaseous atoms from the element in its standard state
4. The Born-Haber cycle requires gaseous atoms as an intermediate / gaseous Cl atoms are needed before electron affinity can occur
5. Hess's law requires all enthalpy changes in the cycle to be accounted for so that the lattice enthalpy can be calculated correctly

## Key terms

- [standard enthalpy of atomisation](https://www.gradenine.co.uk/glossary/standard-enthalpy-of-atomisation)
- [Born-Haber cycle](https://www.gradenine.co.uk/glossary/born-haber-cycle)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](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)

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