# A student carries out experiments to find the order of reaction with respect to hydrogen peroxide (H₂O₂) in a decomposition reaction. In experiment 1, the initial concentration of H₂O₂ is 0.10 mol/dm³ and the initial rate is 2.0 × 10⁻³ mol/dm³/s. In experiment 2, the concentration is doubled to 0.20 mol/dm³ and the initial rate doubles to 4.0 × 10⁻³ mol/dm³/s. In experiment 3, the concentration is tripled to 0.30 mol/dm³ and the initial rate triples to 6.0 × 10⁻³ mol/dm³/s. Explain what these results show about the order of reaction with respect to H₂O₂, and state what a rate equation for this reaction would look like.

> AQA A-Level Chemistry (7405) — 3.1.9 Rate equations (A-Level only) · Explain · 5 marks

> Hydrogen peroxide decomposes in the presence of a catalyst. A student collects initial rate data at three different concentrations of hydrogen peroxide, keeping all other conditions constant.

## Mark scheme (5 marks)

1. When concentration doubles, the rate doubles (or equivalent proportional statement from the data)
2. The reaction is first order with respect to H₂O₂
3. First order means rate is proportional to [H₂O₂]¹ (or that the exponent/power of concentration in the rate equation is 1)
4. The rate equation is rate = k[H₂O₂] (accept rate = k[H₂O₂]¹)
5. k is the rate constant (and its value can be determined from the data, e.g. k = rate ÷ [H₂O₂])

## Key terms

- [order of reaction](https://www.gradenine.co.uk/glossary/order-of-reaction)
- [rate equation](https://www.gradenine.co.uk/glossary/rate-equation)
- [initial rate](https://www.gradenine.co.uk/glossary/initial-rate)

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](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-carries-out-experiments-to-1f799044) · Published by Druglandscape Ltd.