A-Level Chemistry May/June 2024 Q5(b): The reaction of persulfate ions, S2O82-, with iodide ions is catalysed by Fe2+ ions. Wr…

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  The reaction is catalysed by Fe²⁺ ions in a two-step process involving the Fe²⁺/Fe³⁺ redox system. The Fe²⁺ is first oxidised to Fe³⁺ by the persulfate ions, and then the Fe³⁺ is reduced back to Fe²⁺ by the iodide ions, regenerating the catalyst.

  Step 1: $2\text{Fe}^{2+}(\text{aq}) + \text{S}_2\text{O}_8^{2-}(\text{aq}) \rightarrow 2\text{Fe}^{3+}(\text{aq}) + 2\text{SO}_4^{2-}(\text{aq})$

  Step 2: $2\text{Fe}^{3+}(\text{aq}) + 2\text{I}^{-}(\text{aq}) \rightarrow 2\text{Fe}^{2+}(\text{aq}) + \text{I}_2(\text{aq})$

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  How the marks are awarded

  • M1 — The first mark is for the correct balanced equation showing the reaction between Fe²⁺ ions and persulfate ions (S₂O₈²⁻) to produce Fe³⁺ ions and sulfate ions (SO₄²⁻).
  • M2 — The second mark is for the correct balanced equation showing the reaction between Fe³⁺ ions and iodide ions (I⁻) to regenerate the Fe²⁺ catalyst and produce iodine (I₂).

  Common mistakes

  • Writing unbalanced equations, particularly for the second step, e.g., Fe³⁺ + I⁻ → Fe²⁺ + I₂, which is not balanced for iodine atoms.
  • Reversing the two steps, incorrectly showing Fe²⁺ reacting with I⁻ first.
  • Failing to show the regeneration of the Fe²⁺ catalyst in the second equation.
  • Incorrectly identifying the products, for example, writing S or SO₃²⁻ instead of SO₄²⁻ from the reduction of the persulfate ion.

  Examiner tip: For homogeneous catalysis questions involving transition metals, always ensure your proposed steps form a cycle that consumes a reactant, regenerates the catalyst, and forms a product in each step.

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