# Explain why the standard enthalpy of atomisation of chlorine is exactly half the bond enthalpy of the Cl–Cl bond, and explain how these two values are used in a Born–Haber cycle to determine the lattice enthalpy of potassium chloride.

> IB DP Chemistry Higher Level (2023 syllabus) — R1.2 Energy cycles in reactions · Explain · 4 marks

## Mark scheme (4 marks)

1. The standard enthalpy of atomisation is defined as the enthalpy change to produce ONE mole of gaseous atoms, whereas the bond enthalpy of Cl–Cl refers to breaking ONE mole of Cl–Cl bonds, producing TWO moles of Cl(g); therefore atomisation enthalpy = ½ × bond enthalpy.
2. In the Born–Haber cycle, the enthalpy of atomisation of chlorine (½ Cl₂ → Cl(g)) is used as one of the steps that converts the elements in their standard states into gaseous atoms/ions before lattice formation.
3. Hess's law is applied: the sum of all other enthalpy changes in the cycle (atomisation of K, ionisation energy of K, electron affinity of Cl, and standard enthalpy of formation of KCl) equals the negative of the lattice enthalpy.
4. The lattice enthalpy itself represents the enthalpy change when one mole of the ionic solid is formed from its gaseous ions (K⁺(g) + Cl⁻(g) → KCl(s)), and is always exothermic (negative); its magnitude reflects the strength of the ionic attractions in the lattice.

## Key terms

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

## 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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