Le Chatelier's Principle (AQA A Level Chemistry): Revision Note

Changes in equilibrium positionUsing 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)

Explain what happens to the position of equilibrium when

3. Water is added to the equilibrium mixture

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 added ethyl ethanoate, so the ethyl ethanoate decreases in concentration.

Answer 2:

• 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.

Answer 3:

• 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 pressurePredict 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:

3. 2NO₂ (g) ⇌ 2NO (g) + O₂ (g)

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

Answer 3:

• 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 temperatureUsing 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)

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

Explain your answer

Answer 1:

• The reaction will absorb the excess energy and since the forward reaction is endothermic, the equilibrium will shift to the right.

Answer 2:

• The reaction will absorb the excess energy and since this causes a shift of the equilibrium towards the right (as more CO₂(g) is formed) this means that the reaction is endothermic.