# Explain why the standard electrode potential of the Cl₂/Cl⁻ half-cell is more positive than that of the I₂/I⁻ half-cell, and predict which species acts as the oxidising agent when a mixture of Cl₂ and I⁻ ions is present in aqueous solution.

> IB DP Chemistry Higher Level (2023 syllabus) — R3.2 Electron transfer reactions · Explain · 4 marks

> The standard electrode potentials are: Cl₂(g) + 2e⁻ → 2Cl⁻(aq), E° = +1.36 V; I₂(aq) + 2e⁻ → 2I⁻(aq), E° = +0.54 V.

## Mark scheme (4 marks)

1. Cl₂ has a greater tendency to be reduced / gain electrons than I₂, as shown by its more positive E° value.
2. This is because Cl has greater electronegativity / nuclear attraction for electrons than I, so Cl₂ more readily accepts electrons to form Cl⁻.
3. Cl₂ acts as the oxidising agent (and is itself reduced to Cl⁻).
4. I⁻ is oxidised to I₂ because the species with the more negative (less positive) E° half-reaction is reversed and acts as the reducing agent; the overall cell EMF is positive (1.36 − 0.54 = +0.82 V), confirming the reaction is spontaneous.

## Key terms

- [standard electrode potential](https://www.gradenine.co.uk/glossary/standard-electrode-potential)
- [oxidising agent](https://www.gradenine.co.uk/glossary/oxidising-agent)

## Related

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