---
title: "Rate Equations and Orders of Reaction"
description: "The rate equation says how the rate actually depends on the concentrations, and it comes from experiment rather than from the balanced equation. Each order says how much one concentration matters."
canonical: https://lightmysky.com/learn/science/rate-equations-and-orders-of-reaction-mt_1x3ogDKGxC
source: https://lightmysky.com/learn/science/rate-equations-and-orders-of-reaction-mt_1x3ogDKGxC.md
retrieved: 2026-09-12
---

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# Rate Equations and Orders of Reaction

The rate equation says how the rate actually depends on the concentrations, and it comes from experiment rather than from the balanced equation. Each order says how much one concentration matters.

Subject: Science · Area: Chemistry · Ages 17 to 18
Page: https://lightmysky.com/learn/science/rate-equations-and-orders-of-reaction-mt_1x3ogDKGxC

## Ready when they can

- Writes a rate equation from initial rate data by comparing runs where one concentration changes at a time
- States the order with respect to each reactant and the overall order
- Works out the units of the rate constant for a given overall order
- Explains how a species in the balanced equation can turn out to be order zero

## Lesson: The Data Decides the Order

A rate equation links speed to concentration: rate equals k times each reactant raised to its order. The powers are the orders, and their sum is the overall order. The balanced equation never hands you these powers. Only experiment can.

Compare runs where one concentration changes at a time. Doubling that reactant doubles the rate for first order, quadruples it for second, and leaves it flat for zero. Measure starting slopes before products interfere.

**Example.** Take rate equals k times A times B squared: first order in A, second in B, third overall. The units of k follow the overall order: per second for first, decimetres cubed per mole per second for second, and moles per decimetre cubed per second for zero.

**Tip.** A species in the equation can still be order zero, usually because it reacts after the slow step or sits in such excess that its level barely moves. Rate cares about the slow step, not the paper balance.

**Recap.** Read orders from initial rate runs, add them for the overall order, and set the units of k to match.

## Practice

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

## Needs first

- [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)
- [Direct Proportion as a Formula](https://lightmysky.com/learn/mathematics/direct-proportion-as-a-formula-mt_nhSf9I7Bjz)
- [Free Energy and Whether a Reaction Is Feasible](https://lightmysky.com/learn/science/free-energy-and-whether-a-reaction-is-feasible-mt_o1kZgBYWoM)

## Opens up

- [Deriving the Michaelis-Menten Equation from the Steady State](https://lightmysky.com/learn/science/deriving-the-michaelis-menten-equation-from-the-steady-state-mt_BFp0fwZ8_i)
- [Reading Orders and Half-Lives from Rate Graphs](https://lightmysky.com/learn/science/reading-orders-and-half-lives-from-rate-graphs-mt_gCToNUUYll)
- [Nucleophilic Substitution in Haloalkanes](https://lightmysky.com/learn/science/nucleophilic-substitution-in-haloalkanes-mt_QG-nKlnVvW)