Le Chatelier’s Principle (Oxford AQA International A Level Chemistry)

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 at dynamic equilibrium, the position of the equilibrium moves to minimise this change

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

  • Temperature

  • Pressure

  • Concentration

Effects of temperature

Effects of temperature table

Effects of pressure

• Changes in pressure only affect reactions where the reactants or products are gases

Effects of pressure table

Effects of concentration

Effects of concentration table

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 its equilibrium faster

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

Compromise Conditions

• Many industrial processes require huge amounts of heat and pressure which is very expensive to maintain

• Temperature and pressure become a bigger consideration when a reaction is reversible

• An example of this is the Haber process for the production of ammonia:

N₂ (g) + 3H₂ (g) 2NH₃ (g) ΔH =−92.4 kJ mol^-1

Temperature: 450ºC

• High temperature favours the backward endothermic reaction

  • This results in a higher yield of reactants

• Low temperature favours the forward exothermic reaction

  • This results in a higher yield of products

  • However, the rate of reaction is very slow

• 450ºC is a compromise temperature between low yield of products and speed of production

Pressure: 200 atm

• There are 4 molecules of gaseous reactants and 2 molecules of gaseous products

• Low pressure favours the backward reaction

  • This results in a higher yield of reactants

• High pressure favours the forward reaction

  • This results in a higher yield of products

  • However, high pressures can be dangerous and very expensive equipment is needed

• 200 atm is a compromise pressure between a lower yield of products being made safely and economically

Choosing reaction conditions

• The reaction conditions chosen for the Haber process are not ideal in terms of the yield

• However, they provide a balance between product yield, reaction rate and production cost

• These are called compromise conditions as they are chosen to give a good compromise between the yield, rate and cost