Le Chatelier's Principle

Position of the equilibrium

The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture.
When the concentration of reactants increases, the position of equilibrium shifts to the left
When the concentration of products increases, the position of equilibrium shifts to the right

Le Chatelier’s principle

Le Chatelier’s principle says that if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change
The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration

Effects of concentration

Effects of concentration table

Change	How the equilibrium shifts
Increase in concentration of a reactant	Equilibrium shifts to the right
Decrease in concentration of a reactant	Equilibrium shifts to the left
Increase in concentration of a product	Equilibrium shifts to the left
Decrease in concentration of a product	Equilibrium shifts to the right

Worked example

Changes in equilibrium position

Use the reaction below:

CH₃COOH (l) + C₂H₅OH (l) $⇌$ CH₃COOC₂H₅ (l) + H₂O (l)

Explain what happens to the position of equilibrium when:

More CH₃COOC₂H₅ (l) is added
Some C₂H₅OH (l) is removed

Answer

Answer 1: More CH₃COOC₂H₅ (l) is added
- The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed
- The reaction moves in this direction to oppose the effect of added ethyl ethanoate, so the ethyl ethanoate decreases in concentration
Answer 2: Some C₂H₅OH (l) is removed
- The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed
- The reaction moves in this direction to oppose the removal of ethanol so more ethanol (and ethanoic acid) are formed from ethyl ethanoate and water

Worked example

Changes in equilibrium position

Use the reaction below:

Ce⁴⁺ (aq) + Fe²⁺ (aq) $⇌$ Ce³⁺ (aq) + Fe³⁺ (aq)

Explain what happens to the position of equilibrium when:

Water is added to the equilibrium mixture.

Answer

There is no effect as the water dilutes all the ions equally so there is no change in the ratio of reactants to products

Effects of pressure

Changes in pressure only affect reactions where the reactants or products are gases

Effects of pressure table

Change	How the equilibrium shifts
Increase in pressure	Equilibrium shifts in the direction that produces a smaller number of molecules of gas to decrease the pressure again
Decrease in pressure	Equilibrium shifts in the direction that produces a larger number of molecules of gas to increase the pressure again

Worked example

Changes in pressure

Predict the effect of increasing pressure on the following reactions:

N₂O₄ (g) $⇌$ 2NO₂ (g)
CaCO₃ $⇌$ CaO (s) + CO₂ (g)

Answer

Answer 1:
- The equilibrium shifts to the left as there are fewer gas molecules on the left
- This causes a decrease in pressure
Answer 2:
- The equilibrium shifts to the left as there are no gas molecules on the left but there is CO₂ on the right
- This causes a decrease in pressure

Worked example

Changes in pressure

Predict the effect of decreasing pressure on the following reaction:

2NO₂ (g) $⇌$ 2NO (g) + O₂ (g)

Answer

The equilibrium shifts to the right as there is a greater number of gas molecules on the right
This causes an increase in pressure

Effects of temperature

Effects of temperature table

Change	How the equilibrium shifts
Increase in temperature	Equilibrium moves in the endothermic direction to reverse the change
Decrease in temperature	Equilibrium moves in the exothermic direction to reverse the change

Worked example

Changes in temperature

Predict the effect of increasing the temperature on the following reaction:

H₂ (g) + CO₂ (g) $⇌$ H₂O (g) + CO (g) ΔH = +410.2 kJ mol^-1

Answer

The reaction will absorb the excess energy
Since the forward reaction is endothermic, the equilibrium will shift to the right

Worked example

Changes in temperature

For the following reaction, increasing the temperature increases the amount of CO₂ (g) at constant pressure.

Ag₂CO₃ (s) $⇌$ Ag₂O (s) + CO₂ (g)

Explain whether the reaction is endothermic or exothermic.

Answer

The reaction absorbs the excess energy
Since more CO₂ (g) is formed, the equilibrium has shifted towards the right
Therefore, the reaction is endothermic
Remember: Endothermic reactions favour the products

Effects of catalysts

A catalyst is a substance that increases the rate of a chemical reaction (they increase the rate of the forward and reverse reaction equally)
Catalysts only cause a reaction to reach its equilibrium faster
Catalysts therefore have no effect on the position of the equilibrium once this is reached

Le Chatelier's Principle (CIE A Level Chemistry)

Revision Note