# A chemist investigates the reaction between peroxodisulfate ions and iodide ions. The overall equation for the reaction is:

S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq)

Experiments show that the rate equation for this reaction is:

rate = k[S₂O₈²⁻][I⁻]

A student proposes the following two-step mechanism:

Step 1: S₂O₈²⁻(aq) + I⁻(aq) → SO₄²⁻(aq) + SO₄I³⁻(aq)   slow
Step 2: SO₄I³⁻(aq) + I⁻(aq) → SO₄²⁻(aq) + I₂(aq)   fast

Explain whether this proposed mechanism is consistent with the rate equation given above.

> Edexcel A-Level Chemistry (9CH0) — 16.1 Rate equations and mechanisms · Explain · 5 marks

## Mark scheme (5 marks)

1. The slow step (rate-determining step) determines the overall rate of reaction
2. The rate equation is derived from the species in the rate-determining step only
3. Step 1 involves one S₂O₈²⁻ and one I⁻, so the predicted rate equation from this step is rate = k[S₂O₈²⁻][I⁻]
4. The predicted rate equation matches the experimentally determined rate equation
5. Therefore the mechanism is consistent with the experimental rate equation

## Key terms

- [rate equation](https://www.gradenine.co.uk/glossary/rate-equation)
- [mechanism](https://www.gradenine.co.uk/glossary/mechanism)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-chemist-investigates-the-reaction-between-a1982505) · Published by Druglandscape Ltd.