# A chemist investigates the reaction between bromate(V) ions and bromide ions in acidic solution. The overall equation for the reaction is shown below.

BrO₃⁻(aq) + 5Br⁻(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O(l)

Experiments show that the rate equation for this reaction is:

rate = k[BrO₃⁻][Br⁻][H⁺]²

Explain what the rate equation tells us about the reaction mechanism, and explain why the overall equation cannot represent the mechanism for this reaction.

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

> The rate equation for the reaction between bromate(V) ions and bromide ions in acidic solution is:
> 
> rate = k[BrO₃⁻][Br⁻][H⁺]²
> 
> The overall equation is:
> 
> BrO₃⁻(aq) + 5Br⁻(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O(l)

## Mark scheme (5 marks)

1. The reaction is fourth order overall (or: the overall order is 4, found by summing the individual orders: 1 + 1 + 2 = 4)
2. The rate-determining step (slow step) involves the species whose concentrations appear in the rate equation: one BrO₃⁻ ion, one Br⁻ ion, and two H⁺ ions
3. The rate equation is determined by (only) the slowest step in the mechanism / the rate-determining step
4. The overall equation shows 5 Br⁻ ions and 6 H⁺ ions reacting, but only 1 Br⁻ and 2 H⁺ appear in the rate equation, so the overall equation cannot represent a single elementary step / the mechanism
5. A single-step mechanism would require the simultaneous collision of 12 particles (BrO₃⁻ + 5Br⁻ + 6H⁺), which is statistically improbable / effectively impossible

## 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)
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