Le Chatelier’s Principle

Section 1 of 7The principle (HT only)

Statement

• Statement (HT only) If a change is made to the conditions of a system at equilibrium, the system shifts so as to oppose that change.
• The principle is named after the French chemist Henri Le Chatelier.

Position of equilibrium

• Position of equilibrium The position of equilibrium describes the relative amounts of reactants and products when the reaction is at equilibrium.
• Shifting right gives more products and a higher percentage yield.
  • Shifting left gives more reactants and a lower yield.

Why it matters

• Why it matters Chemical engineers choose conditions that push the position towards the product side, to make as much product as possible and keep costs low.

Section 2 of 7Changing the concentration

1Add more reactant

• Add more reactant The system is no longer at equilibrium and has an excess of reactant.

2System opposes

• System opposes It tries to remove the extra reactant by turning it into product.

3Shift to the right

• Shift to the right More product forms and the percentage yield rises. Removing product has the same effect.

4Opposite change

• Opposite change Adding more product, or removing reactant, shifts the position to the left.

Section 3 of 7Changing the temperature

Change	System response	Shift if forward reaction is exothermic	Shift if forward reaction is endothermic
Increase temperature	Favours the endothermic direction, which absorbs heat	Left (fewer products)	Right (more products)
Decrease temperature	Favours the exothermic direction, which releases heat	Right (more products)	Left (fewer products)

Section 4 of 7Changing the pressure

Pressure and gases

Gases only

• Gases only Pressure only matters for reactions where gases take part.
  • Count the gas molecules on each side of the balanced equation and ignore solids and liquids.

Increase the pressure

• Increase the pressure The system opposes the change by shifting towards the side with fewer gas molecules, which lowers the pressure.

Decrease the pressure

• Decrease the pressure The system shifts towards the side with more gas molecules, which raises the pressure.

Equal numbers of gas molecules

• Equal numbers of gas molecules If both sides have the same number of gas molecules, a change in pressure does not move the position of equilibrium.

Section 5 of 7Worked examples (HT only)

1

Example 1: 2NO₂(g) ⇌ N₂O₄(g). NO₂ is dark brown and N₂O₄ is colourless. There are 2 gas molecules on the left and 1 on the right.

2

If the pressure is raised, the equilibrium shifts to the right, towards the smaller number of gas molecules. The mixture becomes paler as more colourless N₂O₄ forms.

3

Example 2: ICl(l) + Cl₂(g) ⇌ ICl₃(s) is exothermic in the forward direction. ICl is dark brown and ICl₃ is yellow.

4

If the mixture is heated, the system favours the endothermic reaction, which is the reverse. The equilibrium moves left and the mixture becomes more brown.

5

If more chlorine is added, the position shifts to the right and more yellow solid forms.

Section 6 of 7Key terms

Term	Meaning
Le Chatelier's Principle	When a system at equilibrium is disturbed, it shifts to oppose the change.
Position of equilibrium	The relative amounts of reactants and products present when a reaction is at equilibrium.
Percentage yield	The actual mass of product as a percentage of the maximum possible mass.
Exothermic	A reaction that transfers energy to the surroundings as heat.
Endothermic	A reaction that takes in energy from the surroundings.

Section 7 of 7Exam technique

1

Counting solids and liquids when deciding the effect of pressure. Only gas molecules count.

2

Forgetting that every reversible reaction is exothermic in one direction and endothermic in the other.

3

Saying that a higher pressure speeds up one direction only. It increases the rate of both reactions; the position shifts because of the different numbers of gas molecules.

4

Explaining a shift without linking it to the change being opposed, such as 'to use up the extra reactant'.

5

Confusing a change in the position of equilibrium with a change in rate.