Le Chatelier's Principle (CIE A Level Chemistry)

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:

1. More CH₃COOC₂H₅ (l) is added 
2. 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

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:

1. 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

Changes in pressure

Predict the effect of increasing pressure on the following reactions:

1. N₂O₄ (g)  2NO₂ (g)
2. 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

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

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

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