AQA AS Chemistry

Revision Notes

1.8.2 Le Chatelier's principle

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 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 temperature, pressure or concentration

Effects of concentration

  • How the equilbrium shifts with concentration changes:

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)

Explain what happens to the position of equilibrium when

  1. 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.

Effects of pressure

  • Changes in pressure only affect reactions where the reactants or products are gases
  • How the equilibrium shifts with pressure changes:

Effects of pressure table, IGCSE & GCSE Chemistry revision notes

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)

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

Effects of temperature

  • How the equilbrium shifts with temperature changes:

Effects of temperature table, IGCSE & GCSE Chemistry revision notes

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

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.

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 equilibrium faster
  • Catalysts therefore have no effect on the position of the equilibrium once this is reached
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