Introduction to Le Châtelier’s Principle

Learning Objective 7.9.A Identify the response of a system at equilibrium to an external stress, using Le Châtelier’s principle.

Quick Notes

Le Châtelier’s Principle: When a system at equilibrium is stressed, it will shift to counteract the stress and restore equilibrium.

Stresses include: concentration changes, temperature changes, pressure/volume changes, and dilution.

The response and shift affects measurable and macroscopic properties like pH, color, or temperature.

Full Notes

What Is Le Châtelier’s Principle?

When a system at equilibrium is subjected to a change in conditions (called a “stress”), the system shifts in a direction that reduces or offsets that stress, eventually re-establishing equilibrium.

Matt’s Exam Tip

Try to avoid thinking of equilibrium systems as 'trying' to do anything. Le Chatelier's Principle is observational, it doesn't explain why such shifting occurs. The reason is because the rate for one reaction direction increases (or decreases) more than the other, meaning both rates are no longer the same - equilibrium is broken. After a given amount of time, the rates will become the same again and equilibrium will be re-established however now the equilibrium quantities will have changed - hence we describe the system as having 'shifted'.

Effect of Changing Concentration

Increasing reactant concentration shifts equilibrium right, (product concentration increases).
Increasing product concentration shifts equilibrium left, (reactant concentration increases).
No effect on the value of K.

Example:
A(g) + B(g) ⇌ C(g)
If more A is added then equilibrium shifts right, concentration of C increases.

Effect of Changing Temperature

Increasing temperature favours the endothermic direction (+ΔH).
Decreasing temperature favours the exothermic direction (-ΔH).
K changes with temperature

For an exothermic reaction (releases heat, -ΔH): Increasing temperature shifts equilibrium toward reactants, so K decreases.
For an endothermic reaction (absorbs heat, +ΔH): Increasing temperature shifts equilibrium toward products, so K increases.

For Example:
In the Haber Process:
Forward reaction is exothermic (-ΔH).

Haber process showing exothermic forward direction (−ΔH) N₂(g) + 3H₂(g) ⇌ 2NH₃(g); ΔH = – kJ/mol

Increasing temperature shifts equilibrium left, reducing NH₃ yield.
Decreasing temperature shifts equilibrium right, increasing NH₃ yield.

Changing Pressure (for Gaseous Equilibria)

Increasing pressure shifts equilibrium towards the side with fewer gas molecules.
Decreasing pressure shifts equilibrium towards the side with more gas molecules.
Has no effect if there is the same number of gas molecules on both sides
No effect on the value of K.

For Example:
In the Haber Process:

Haber process: 4 moles (N₂ + 3H₂) ⇌ 2 moles (NH₃)

Higher pressure shifts equilibrium right, increasing NH₃ yield.

Catalysts

Catalysts do not affect equilibrium position or K.

The rates of forward and reverse directions are increased equally.

They simply help the system reach equilibrium faster.

Heterogeneous Equilibria

Le Châtelier’s Principle also applies to equilibria involving different phases.

Example:
X(g) ⇌ X(aq)
Increasing [X(aq)] shifts equilibrium to the left (less dissolves).
Increasing [X(g)] shifts it to the right (more dissolves).