Reversible Reactions and Equilibrium

A reversible reaction can go both ways ( $\rightleftharpoons$ ). In a closed system it reaches dynamic equilibrium: forward and backward rates are equal, so concentrations stay constant. Changing conditions shifts the equilibrium to oppose the change: higher pressure favours fewer gas moles; higher temperature favours the endothermic direction. The Haber process ( $\mathrm{N_2+3H_2\rightleftharpoons2NH_3}$ ) uses about $450\,^\circ\mathrm{C}$ , $200$ atm and an iron catalyst as a compromise between yield and rate.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 45 minRevision 1 · updated 2026-10-30

Section 1 of 14

Learning objectives

Describe reversible reactions and dynamic equilibrium.
Predict the effect of changing temperature, pressure and concentration.
Explain the conditions used in the Haber and Contact processes.

Pause and think

At equilibrium in a sealed bottle of fizzy drink, no bubbles appear. Has everything stopped?

Section 2 of 14

Core idea

Start hereRunning up a down escalator

Walk up a down escalator at exactly its speed and you stay in the same place, though you're working hard. Dynamic equilibrium is like that: both reactions keep going, but nothing seems to change.

The big idea

In a closed system a reversible reaction reaches a state where forward and backward rates are equal. If conditions change, the equilibrium shifts to oppose the change.

1. Shifting the position

Le Chatelier's principle

Drawing diagram…

The system opposes whatever you do to it.

Worked exampleNitrogen dioxide

$\mathrm{2NO_2(g)\rightleftharpoons N_2O_4(g)}$ , $\Delta H=-57$ kJ/mol. $\mathrm{NO_2}$ is brown; $\mathrm{N_2O_4}$ is colourless.

Goal: Predict the colour change on heating.
Strategy: Find the endothermic direction.

$\text{forward: exothermic}$
$\Delta H$ negative.
$\text{heating favours backward}$
Endothermic direction absorbs the added heat.
$[\mathrm{NO_2}]\uparrow$
More brown gas.

AnswerThe mixture gets darker brown.

✓ CheckCooling in ice makes it paler ✓.

2. The Haber process

$\mathrm{N_2}$ from air, $\mathrm{H_2}$ from methane.
$450\,^\circ\mathrm{C}$ : compromise between yield (low T better) and rate.
$200$ atm: higher yield and rate, limited by cost and safety.
Iron catalyst; unreacted gases recycled; ammonia removed by cooling to a liquid.

Common misconceptionCatalyst and yield A catalyst never changes the yield at equilibrium. It just gets there faster.

Pause and think

Why does recycling unreacted nitrogen and hydrogen matter so much?

Summary

Equal rates, constant concentrations.
System opposes changes.
Haber: 450 °C, 200 atm, Fe.

Dynamic equilibrium: Forward and backward rates equal in a closed system.
Closed system: Nothing can enter or leave.
Position of equilibrium: The relative amounts of reactants and products.

HistoryBread from air. Fritz Haber developed ammonia synthesis in 1909. Fertilisers made this way now help feed about half the world's population.

Fun factWeather predictor. Old "weather cats" were coated with cobalt chloride: blue in dry weather, pink in damp air, because water shifts the equilibrium.

Keep reading for free

Create a free account to read the rest of this lesson: key formulae, worked examples, explore the graph, common mistakes, exam tip, guided practice and more. Your answers are checked and your progress is saved.

Create free account Sign in