---
title: "Reading Orders and Half-Lives from Rate Graphs"
description: "A concentration-time curve and a rate-concentration graph each give the order away. A half-life that stays the same is the signature of first order."
canonical: https://lightmysky.com/learn/science/reading-orders-and-half-lives-from-rate-graphs-mt_gCToNUUYll
source: https://lightmysky.com/learn/science/reading-orders-and-half-lives-from-rate-graphs-mt_gCToNUUYll.md
retrieved: 2026-09-12
---

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# Reading Orders and Half-Lives from Rate Graphs

A concentration-time curve and a rate-concentration graph each give the order away. A half-life that stays the same is the signature of first order.

Subject: Science · Area: Chemistry · Ages 17 to 18
Page: https://lightmysky.com/learn/science/reading-orders-and-half-lives-from-rate-graphs-mt_gCToNUUYll

## Ready when they can

- Takes tangents from a concentration-time curve to get rates and plots rate against concentration
- Identifies zero, first and second order from the shape of a rate-concentration graph
- Measures successive half-lives and uses their constancy to argue for first order
- Finds the rate constant from a gradient or from a half-life

## Lesson: Reading order from rate graphs

Pick a few times on a concentration-time curve and draw a tangent at each one. The gradient of each tangent is the rate at that moment, and you then plot those rates against the concentration left at each time.

**Example.** Picture the rate against concentration plot for hydrogen peroxide breaking down. A flat line means zero order, a straight line through the start means first order, and a curve bending upward means second order.

Now watch the half-life, the time for the concentration to halve, again and again. If every halving takes the same time, your reaction is first order, and if the times drift, it is not.

**Tip.** For the rate constant, take a gradient: the gradient of the rate against concentration line for a first order reaction, or the gradient of the straight concentration-time line for a zero order one. You can also use k = 0.693 divided by the half-life, which gives k in per second.

**Recap.** Tangents give rates, shapes give orders, steady half-lives mean first order, and gradients or half-lives give k.

## Practice

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

## Needs first

- [Rate Equations and Orders of Reaction](https://lightmysky.com/learn/science/rate-equations-and-orders-of-reaction-mt_1x3ogDKGxC)
- [Compound Growth and Decay](https://lightmysky.com/learn/mathematics/compound-growth-and-decay-mt_xf5FQ2sugf)

## Opens up

- [The Rate-Determining Step and Reaction Mechanisms](https://lightmysky.com/learn/science/the-rate-determining-step-and-reaction-mechanisms-mt_PM1Vo2CkVD)