# A chemist investigates the reaction between propanone and iodine in acidic solution. The overall equation for the reaction is: CH₃COCH₃ + I₂ → CH₃COCH₂I + HI. The rate equation for this reaction is: rate = k[CH₃COCH₃][H⁺]. Explain what the rate equation tells us about the mechanism of this reaction, and explain why iodine does not appear in the rate equation.

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

> The proposed mechanism for this reaction involves two steps. The first step is the acid-catalysed formation of an enol intermediate from propanone, which is slow. The second step is the rapid reaction of the enol intermediate with iodine.

## Mark scheme (5 marks)

1. The rate equation shows the reaction is first order with respect to propanone
2. The rate equation shows the reaction is first order with respect to H⁺ (hydrogen ions / acid)
3. Only the species in the rate-determining (slowest) step appear in the rate equation
4. Iodine does not appear in the rate equation because it is not involved in the rate-determining step
5. The overall order of the reaction is 2 (second order overall), consistent with two species 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-chemist-investigates-the-reaction-between-2c65b4e0) · Published by Druglandscape Ltd.