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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

The first electron affinity of chlorine is exothermic because the incoming electron is attracted to the neutral chlorine atom by the nucleus, releasing energy. In contrast, the second electron affinity of oxygen is endothermic because the electron is added to the already negatively charged O⁻ ion. The added electron experiences electrostatic repulsion from the negative charge, requiring energy to be supplied. The nuclear attraction is insufficient to overcome this repulsion, so the overall energy change is positive.

Examiner tips

Use the term ‘electron affinity’ and specify ‘first’ or ‘second’.
Explain the role of electrostatic attraction vs repulsion.
Show the sign of the energy change (exothermic vs endothermic).

Common mistakes

Confusing the sign of the electron affinity values.
Forgetting that the second electron is added to an anion.

Mark scheme (4 marks)

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.
For the second electron affinity of oxygen, the electron is being added to an already negatively charged ion (O⁻).
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.
The nuclear attraction is insufficient to overcome the repulsion, so the overall energy change is positive (endothermic).

Key terms in this question

electron affinity · exothermic · endothermic