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 · View as Markdown

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

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

Model answer (4 marks)

Cl₂ has a greater tendency to be reduced than I₂, as shown by its more positive E° value. This is because Cl is more electronegative and has a greater nuclear attraction for electrons, so Cl₂ more readily accepts electrons to form Cl⁻.
Cl₂ acts as the oxidising agent (it is reduced to Cl⁻). I⁻ is oxidised to I₂ because the species with the less positive E° 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.

Examiner tips

  • Use the phrase ‘greater tendency to be reduced’ to link E° to oxidising ability.
  • Explain the electronegativity reason briefly – it shows understanding of why E° differs.
  • State the direction of the reaction clearly – Cl₂ is reduced, I⁻ is oxidised.
  • Show the cell EMF calculation to prove spontaneity.

Common mistakes

  • Confusing which species is oxidised/reduced – students often say I₂ is reduced.
  • Failing to mention the electronegativity explanation or giving a vague reason for the E° difference.
  • Not showing the EMF calculation or leaving the sign of the EMF unclear.

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 in this question

standard electrode potential · oxidising agent

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