# Potassium bromide, KBr, is an ionic compound. A student constructs a Born-Haber cycle to calculate the lattice enthalpy of potassium bromide. Explain why the first ionisation energy of potassium is an endothermic process and why the electron affinity of bromine is an exothermic process.

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

> Potassium bromide has a lattice enthalpy of −672 kJ mol⁻¹. The first ionisation energy of potassium is +419 kJ mol⁻¹ and the first electron affinity of bromine is −325 kJ mol⁻¹.

## Mark scheme (5 marks)

1. First ionisation energy involves removing an electron from a potassium atom
2. Energy must be supplied / input to overcome the attraction between the electron and the nucleus, making it endothermic
3. First electron affinity of bromine involves adding an electron to a bromine atom
4. The added electron is attracted to the nucleus of bromine, so energy is released, making it exothermic
5. The magnitude of ionisation energy of potassium is greater than the magnitude of electron affinity of bromine, so the overall process of electron transfer from K to Br requires a net energy input (endothermic overall)

## Key terms

- [first ionisation energy](https://www.gradenine.co.uk/glossary/first-ionisation-energy)
- [lattice enthalpy](https://www.gradenine.co.uk/glossary/lattice-enthalpy)
- [endothermic](https://www.gradenine.co.uk/glossary/endothermic)
- [exothermic](https://www.gradenine.co.uk/glossary/exothermic)
- [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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/potassium-bromide-kbr-is-an-ionic-d46817ca) · Published by Druglandscape Ltd.