GCSE Chemistry revision

The effect of changing conditions on equilibrium

Reversible reactions and dynamic equilibrium

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Le Chatelier's principle states that if you change the conditions of a reversible reaction at equilibrium, the position of equilibrium will shift to counteract that change.
The position of equilibrium describes the balance between reactants and products — it lies to the left if there are more reactants, and to the right if there are more products.

1. Introduction AQA

Le Chatelier's principle states that if you change the conditions of a reversible reaction at equilibrium, the position of equilibrium will shift to counteract that change.
The position of equilibrium describes the balance between reactants and products — it lies to the left if there are more reactants, and to the right if there are more products.

The production of ammonia from nitrogen and hydrogen (N₂ + 3H₂ ⇌ 2NH₃) is a key example used to illustrate Le Chatelier's principle.
The overall energy change of the forward reaction is -92 kJ mol⁻¹, meaning the forward reaction is exothermic and the backward reaction is endothermic.

2. The Haber Process as an Example AQA

The production of ammonia from nitrogen and hydrogen (N₂ + 3H₂ ⇌ 2NH₃) is a key example used to illustrate Le Chatelier's principle.
The overall energy change of the forward reaction is -92 kJ mol⁻¹, meaning the forward reaction is exothermic and the backward reaction is endothermic.

Changing conditions causes the position of equilibrium to shift in the direction that opposes the applied change (Le Chatelier's Principle):
Temperature: Increasing temperature shifts equilibrium in the endothermic direction; decreasing temperature shifts it in the exothermic direction.
Pressure: Increasing pressure shifts equilibrium toward the side with fewer gas molecules; decreasing pressure shifts it toward the side with more gas molecules.
Concentration: Increasing a reactant's concentration shifts equilibrium away from that reactant to form more products.
Example: In the Haber process (N₂ + 3H₂ ⇌ 2NH₃, where the forward reaction is exothermic and converts 4 moles of gas into 2 moles), increasing the overall pressure shifts equilibrium to the right toward the side with fewer gas molecules (2 moles), producing more NH₃.

3. Effects of Temperature, Pressure, and Concentration AQA

Changing conditions causes the position of equilibrium to shift in the direction that opposes the applied change (Le Chatelier's Principle):
Temperature: Increasing temperature shifts equilibrium in the endothermic direction; decreasing temperature shifts it in the exothermic direction.
Pressure: Increasing pressure shifts equilibrium toward the side with fewer gas molecules; decreasing pressure shifts it toward the side with more gas molecules.
Concentration: Increasing a reactant's concentration shifts equilibrium away from that reactant to form more products.
Example: In the Haber process (N₂ + 3H₂ ⇌ 2NH₃, where the forward reaction is exothermic and converts 4 moles of gas into 2 moles), increasing the overall pressure shifts equilibrium to the right toward the side with fewer gas molecules (2 moles), producing more NH₃.

Adding a catalyst increases the rate of both the forward and reverse reactions by the exact same extent, meaning the system reaches equilibrium faster without altering the position of equilibrium or the final product yield.
Example: In the Contact process (2SO₂ + O₂ ⇌ 2SO₃), using a vanadium(V) oxide catalyst allows sulfur trioxide to be produced much more quickly, but it does not change the total equilibrium percentage yield of SO₃.

4. Effect of a Catalyst on Equilibrium AQA

Adding a catalyst increases the rate of both the forward and reverse reactions by the exact same extent, meaning the system reaches equilibrium faster without altering the position of equilibrium or the final product yield.
Example: In the Contact process (2SO₂ + O₂ ⇌ 2SO₃), using a vanadium(V) oxide catalyst allows sulfur trioxide to be produced much more quickly, but it does not change the total equilibrium percentage yield of SO₃.

If temperature is increased, equilibrium shifts in the endothermic direction; if decreased, it shifts in the exothermic direction.
If pressure is increased, equilibrium shifts to the side with fewer molecules; if decreased, it shifts to the side with more molecules.
If the concentration of a reactant is increased, equilibrium shifts towards the products to reduce the concentration of that reactant.

5. Summary / Key Terms AQA

If temperature is increased, equilibrium shifts in the endothermic direction; if decreased, it shifts in the exothermic direction.
If pressure is increased, equilibrium shifts to the side with fewer molecules; if decreased, it shifts to the side with more molecules.
If the concentration of a reactant is increased, equilibrium shifts towards the products to reduce the concentration of that reactant.