Le Chatelier's Principle (Edexcel IAL Chemistry)

The Position of Equilibrium

Spec point — 13.7

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

Le Chatelier’s principle

• Le Chatelier’s principle says that if a change is made to a system in dynamic equilibrium, the position of the equilibrium moves to counteract this change

• The principle is used to predict changes to the position of equilibrium when there are changes in:

  • Concentration
  • Pressure
  • Temperature

Effects of concentration

• When the concentration of a reactant is increased:

  • The equilibrium shifts to the right to use up the excess reactant
  • This forms more products

• When the concentration of a reactant is decreased:

  • The equilibrium shifts to the left to replace the lost reactant
  • Products are used up

• When the concentration of a product is increased:

  • The equilibrium shifts to the left to use up the excess product

• When the concentration of a product is decreased:

  • The equilibrium shifts to the right to replace the lost product

• To increase the yield of a product, increase the concentration of reactants or decrease the concentration of products as they form

Worked Example Changes in equilibrium position

Using the reaction below:

CH₃COOH (I) + C₂H₅OH (I) ⇌ CH₃COOC₂H₅ (I) + H₂O (I)

Explain what happens to the position of equilibrium when:

1. More CH₃COOC₂H₅ is added
2. Some C₂H₅OH is removed

Using the reaction below:

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

1. Explain what happens to the position of equilibrium when water is added to the equilibrium mixture

Answers

Answer 1:

• 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 the added ethyl ethanoate, reducing its concentration

Answer2:

• 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 produced from ethyl ethanoate and water

Answer 3:

• There is no effect
• Water dilutes all ions equally, so there is no change in the ratio of reactants to products

Effects of pressure

• Changes in pressure only affect reactions where reactants or products are gases
• If the number of gas moles is equal on both sides, pressure changes have no effect

Worked Example Changes in pressure

Predict the effect of increasing the pressure on the following reactions:

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

Predict the effect of decreasing the pressure on the following reaction:

1. 2NO₂ (g) ⇌ 2NO (g) + O₂ (g)

Answers

Answer 1:

• The equilibrium shifts to the left
• There is 1 mole of gas on the left versus 2 on the right
• This reduces the pressure

Answer 2:

• The equilibrium shifts to the left
• There are no gas molecules on the left but 1 mole of CO₂ gas on the right
• This reduces the pressure

Answer 3:

• The equilibrium shifts to the right
• There are 2 moles of gas on the left versus 3 on the right
• This increases the pressure, opposing the decrease

Effects of temperature

• Increasing temperature shifts the equilibrium in the endothermic direction
• Decreasing temperature shifts the equilibrium in the exothermic direction

Worked Example Changes in temperature

Using the reaction below:

H₂ (g) + CO₂ (g) ⇌ H₂O (g) + CO (g) ΔH = +41.2 kJ mol^-1

1. Predict the effect of increasing the temperature on this reaction.

Using the reaction below:

Ag₂CO₃ (s) ⇌ Ag₂O (s) + CO₂ (g)

1. Increasing the temperature increases the amount of CO₂ (g) at constant pressure. Is this reaction exothermic or endothermic? Explain your answer.

Answers

Answer 1:

• The forward reaction is endothermic (ΔH = +41.2 kJ mol^-1)
• Increasing temperature shifts the equilibrium to the right to absorb the extra energy

Answer 2:

• Increasing temperature shifts the equilibrium to the right (more CO₂ produced)
• The equilibrium shifts in the endothermic direction when temperature increases, so the forward reaction is endothermic