# A chemist studies the reaction between nitrogen monoxide and hydrogen gas. The overall equation for the reaction is: 2NO(g) + 2H₂(g) → N₂(g) + 2H₂O(g). The rate equation for this reaction is: rate = k[NO]²[H₂]. Explain what the rate equation tells us about the mechanism of this reaction, and explain why the overall stoichiometric equation cannot be used on its own to determine the rate equation.

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

> The reaction between nitrogen monoxide and hydrogen is thought to proceed via a two-step mechanism. In the first step, two molecules of NO combine with one molecule of H₂. In the second step, an intermediate reacts with a further molecule of H₂.

## Mark scheme (5 marks)

1. The rate equation shows the reaction is second order with respect to NO and first order with respect to H₂
2. The overall order of the reaction is third order (2 + 1 = 3)
3. The rate-determining (slowest) step must involve two molecules of NO and one molecule of H₂
4. The rate equation can only be determined experimentally, not from the overall equation
5. The stoichiometric equation represents the overall change and not the individual steps, so it gives no information about the mechanism or which species are involved in the slow 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)
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