# A student investigates the reaction between hydrogen peroxide and iodide ions in acidic solution. The overall equation for the reaction is shown below.

H₂O₂ + 2H⁺ + 2I⁻ → I₂ + 2H₂O

Experimental results show that when the concentration of hydrogen peroxide is doubled (keeping all other concentrations constant), the rate of reaction doubles. When the concentration of iodide ions is doubled (keeping all other concentrations constant), the rate of reaction also doubles. The concentration of H⁺ ions has no effect on the rate of reaction.

Explain what these results tell us about the rate equation for this reaction and about the mechanism by which the reaction occurs.

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

## Mark scheme (5 marks)

1. The reaction is first order with respect to hydrogen peroxide (H₂O₂)
2. The reaction is first order with respect to iodide ions (I⁻)
3. The reaction is zero order with respect to H⁺ ions
4. The rate equation is: rate = k[H₂O₂][I⁻]
5. The rate equation shows the mechanism cannot be a single step matching the overall equation; the rate-determining (slowest) step must involve one molecule of H₂O₂ and one ion of I⁻ only (H⁺ is not involved in the rate-determining step)

## 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-student-investigates-the-reaction-between-9dee2a4e) · Published by Druglandscape Ltd.