# Explain why the Born–Haber cycle for potassium fluoride gives a value for lattice enthalpy that differs from the value calculated using a purely ionic model, and state what this difference indicates about the bonding in potassium fluoride.

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

> The experimental lattice enthalpy of potassium fluoride, derived from its Born–Haber cycle, is −826 kJ mol⁻¹, whereas the theoretical value calculated assuming a perfect ionic model is −808 kJ mol⁻¹.

## Mark scheme (4 marks)

1. The Born–Haber cycle value is described as the experimental lattice enthalpy because it is derived from measurable thermodynamic data (Hess's law), whereas the theoretical value assumes perfectly spherical, non-polarising ions.
2. The experimental value is larger in magnitude than the theoretical value, meaning more energy is released when the lattice forms than the purely ionic model predicts.
3. The discrepancy arises because the real bonding has some covalent character: the F⁻ ion (small, high charge density) polarises the electron cloud of the K⁺ ion to a slight degree, causing partial sharing of electron density.
4. The fact that the experimental lattice enthalpy exceeds the theoretical value indicates that potassium fluoride has predominantly ionic bonding with a small degree of covalent character.

## Key terms

- [Born–Haber cycle](https://www.gradenine.co.uk/glossary/born-haber-cycle)
- [lattice enthalpy](https://www.gradenine.co.uk/glossary/lattice-enthalpy)
- [ionic model](https://www.gradenine.co.uk/glossary/ionic-model)

## 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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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-born-haber-cycle-for-fcfed400) · Published by Druglandscape Ltd.