---
title: "The Rate-Determining Step and Reaction Mechanisms"
description: "Most reactions run in several steps and the slowest one sets the pace. The orders in the rate equation say which species turn up in that step, which makes kinetics evidence about mechanism."
canonical: https://lightmysky.com/learn/science/the-rate-determining-step-and-reaction-mechanisms-mt_PM1Vo2CkVD
source: https://lightmysky.com/learn/science/the-rate-determining-step-and-reaction-mechanisms-mt_PM1Vo2CkVD.md
retrieved: 2026-09-12
---

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# The Rate-Determining Step and Reaction Mechanisms

Most reactions run in several steps and the slowest one sets the pace. The orders in the rate equation say which species turn up in that step, which makes kinetics evidence about mechanism.

Subject: Science · Area: Chemistry · Ages 17 to 18
Page: https://lightmysky.com/learn/science/the-rate-determining-step-and-reaction-mechanisms-mt_PM1Vo2CkVD

## Ready when they can

- Explains why only the slow step, and the steps before it, can affect the measured rate
- Matches a proposed mechanism to a rate equation and rejects one that does not fit
- Uses the orders to say which species, and how many of each, appear in the rate-determining step
- Explains how a catalyst can change the mechanism rather than just push the same one along

## Lesson: The slow step sets the pace

Most reactions run in small steps, and the slowest step sets the pace for all of them. Only that slow step and the steps before it can shape the measured rate, since later steps finish too quickly to matter.

**Example.** Take nitrogen dioxide with carbon monoxide: the measured rate is rate = k[NO2]2 and CO has order zero. That tells you two NO2 particles meet in or before the slow step, while CO only joins in a later fast step.

To test a mechanism, write down the rate equation it predicts and hold it next to the measured one. If the two disagree, the mechanism is wrong and you throw it out. Passing the test only keeps a mechanism alive, since a different route can predict the same equation.

**Tip.** A catalyst opens a different route with a lower barrier, and the new route can have a new slow step and even a new rate equation. The catalyst is remade at the end, so it never gets used up.

**Recap.** The slow step and earlier steps fix the rate, orders count who is there, mechanisms must match the data, and catalysts open new routes.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Reading Orders and Half-Lives from Rate Graphs](https://lightmysky.com/learn/science/reading-orders-and-half-lives-from-rate-graphs-mt_gCToNUUYll)
- [Rates of Reaction: Collision Theory and What Changes a Rate](https://lightmysky.com/learn/science/rates-of-reaction-collision-theory-and-what-changes-a-rate-mt_Ma2bOotr9g)

## Opens up

- [Steady-State and Pre-Equilibrium Approximations](https://lightmysky.com/learn/science/steady-state-and-pre-equilibrium-approximations-mt_9ESWVl28-3)
- [The Arrhenius Equation and Finding Activation Energy](https://lightmysky.com/learn/science/the-arrhenius-equation-and-finding-activation-energy-mt_QBQDSNwbXK)
- [Transition Metals: Variable Oxidation State, Colour and Catalysis](https://lightmysky.com/learn/science/transition-metals-variable-oxidation-state-colour-and-catalysis-mt_UIVC6d7wak)