# Explain why the electron affinity of chlorine is exothermic whilst the second electron affinity of oxygen is endothermic.

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

> Electron affinity values are important steps in energy cycles used to determine lattice enthalpies indirectly. The first electron affinity of chlorine is −349 kJ mol⁻¹, whilst the second electron affinity of oxygen is +798 kJ mol⁻¹.

## Mark scheme (4 marks)

1. The first electron affinity of chlorine is exothermic because the incoming electron is attracted to / experiences an electrostatic attraction from the nucleus of the neutral chlorine atom.
2. For the second electron affinity of oxygen, the electron is being added to an already negatively charged ion (O⁻).
3. There is electrostatic repulsion between the negatively charged O⁻ ion and the incoming (second) electron, so energy must be supplied to force the electron onto the ion.
4. The nuclear attraction is insufficient to overcome the repulsion, so the overall energy change is positive (endothermic).

## Key terms

- [electron affinity](https://www.gradenine.co.uk/glossary/electron-affinity)
- [exothermic](https://www.gradenine.co.uk/glossary/exothermic)
- [endothermic](https://www.gradenine.co.uk/glossary/endothermic)

## Related

- [Revision notes for IB DP Chemistry Standard 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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