# Explain why the electron affinity of oxygen has two values — a first electron affinity that is exothermic and a second electron affinity that is endothermic — and discuss how both values are nevertheless incorporated into a Born–Haber cycle to give a valid lattice enthalpy for an ionic oxide such as magnesium oxide.

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

## Mark scheme (4 marks)

1. The first electron affinity is exothermic because the incoming electron is attracted to the nuclear charge of the neutral oxygen atom, releasing energy.
2. The second electron affinity is endothermic because the incoming electron is being added to an already negatively charged O⁻ ion, so electrostatic repulsion must be overcome, requiring energy input.
3. Hess's law allows both electron affinities to be included in the Born–Haber cycle regardless of their sign, because the cycle is simply an application of conservation of energy / enthalpy along different pathways between the same initial and final states.
4. The large exothermic lattice enthalpy of MgO (energy released when gaseous Mg²⁺ and O²⁻ ions form the lattice) is sufficiently large in magnitude to compensate for the endothermic second electron affinity, making the overall formation of MgO energetically feasible.

## Key terms

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

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