GCSE Chemistry revision

Energy changes and reversible reactions

Reversible reactions and dynamic equilibrium

AQA 4.6.2.2 Foundation & Higher

Your specification

AQA student objectives

Learning pathway

ALL 🎯

MOST 🎯🎯

SOME 🎯🎯🎯

Revision summary

Read on screen, then print for Cornell-style active revision.

Equilibrium is reached when the rate of the forward reaction equals the rate of the backward reaction.
At equilibrium, the concentrations of reactants and products remain constant because the two reactions effectively cancel each other out.
Both the forward and backward reactions are still occurring at equilibrium — there is no overall change, but the reactions have not stopped.

1. What is Equilibrium? AQA

Equilibrium is reached when the rate of the forward reaction equals the rate of the backward reaction.
At equilibrium, the concentrations of reactants and products remain constant because the two reactions effectively cancel each other out.
Both the forward and backward reactions are still occurring at equilibrium — there is no overall change, but the reactions have not stopped.

The concentrations of reactants and products being constant at equilibrium does not mean they are equal to one another.
There could be a higher concentration of products than reactants, or vice versa, and the reaction would still be at equilibrium provided the rates are equal.

The position of equilibrium describes the relative concentrations of reactants and products when a reaction is at equilibrium.
If there are more products than reactants, the equilibrium is said to lie to the right.
If there are more reactants than products, the equilibrium is said to lie to the left.

3. Position of Equilibrium AQA

The position of equilibrium describes the relative concentrations of reactants and products when a reaction is at equilibrium.
If there are more products than reactants, the equilibrium is said to lie to the right.
If there are more reactants than products, the equilibrium is said to lie to the left.

Changing conditions such as temperature can shift the position of equilibrium to the left or to the right.
Adding heat to a reaction that has an endothermic forward reaction will shift the equilibrium to the right, producing more products.
Cooling the reaction will shift the equilibrium to the left, increasing the concentration of reactants.

4. How Conditions Affect the Position of Equilibrium AQA

Changing conditions such as temperature can shift the position of equilibrium to the left or to the right.
Adding heat to a reaction that has an endothermic forward reaction will shift the equilibrium to the right, producing more products.
Cooling the reaction will shift the equilibrium to the left, increasing the concentration of reactants.