Unit Overview

Chemical equilibria (AS)

CIE A-Level Chemistry· 5 min read 📊 n/a

1. Unit at a glance

Many chemical reactions do not go to completion, instead reaching a steady balanced state called equilibrium. This unit follows a logical learning arc: you will first build the conceptual foundation of what dynamic equilibrium is, then learn to predict how equilibrium responds to changing conditions, and finally quantify equilibrium positions using mathematical constants.

This unit is split into 3 connected sub-topics:

Dynamic equilibrium characteristics

Learn the defining properties of dynamic equilibrium in closed reversible systems.

★⏱ 3 min

Le Chatelier's principle

Predict equilibrium shifts from changes in concentration, temperature, pressure and catalyst presence.

★★⏱ 5 min

Equilibrium constants

Write $K_{c}$ and $K_{p}$ expressions, calculate their values, and interpret what they indicate about equilibrium position.

★★★⏱ 8 min

2. Common Pitfalls

Wrong move:

Claiming dynamic equilibrium can form in an open system

Why:

Open systems allow reactants or products to escape, so a steady state cannot be maintained

Correct move:

Only recognize dynamic equilibrium in closed systems for CIE exam questions

Wrong move:

Including solid or pure liquid concentrations in equilibrium constant expressions

Why:

The concentration of solids and pure liquids is constant at a fixed temperature

Correct move:

Omit all solids and pure liquids from $K_{c}$ and $K_{p}$ expressions

3. Quick Reference Cheatsheet

Concept / Formula	Key Unit Rule
Dynamic equilibrium	Rate of forward reaction = Rate of reverse reaction; concentrations are constant
Le Chatelier's principle	Equilibrium shifts to counteract any external change in reaction conditions
Equilibrium constant general form	$K = \frac{[ p r o d u c t s ] ^{s t o i c hi o m e t r i c coe f f i c i e n t s}}{[ r e a c t an t s ] ^{s t o i c hi o m e t r i c coe f f i c i e n t s}}$
Effect of catalyst	Catalysts speed up reaching equilibrium, do not change equilibrium position or $K$ value
Effect of temperature	Temperature changes alter the value of $K$ , shifting equilibrium in the endothermic direction for a temperature increase

What's Next

Begin this unit with the first sub-topic on dynamic equilibrium characteristics to build the core conceptual foundation required for later topics in this unit. Once you complete all three sub-topics here, you will move on to the next AS unit covering reaction kinetics, which connects equilibrium concepts to the study of reaction rates.