8.2    Factors Affecting Chemical Equilibria

Position of equilirbium

1) The position of equilibrium refers to the relative amount of products and reactants present in an equilibrium  mixture.

Le Chatelier’s principle

• Le Chatelier’s principle states that if a change is made to a system in equilibrium, the system reacts in such a way so as to oppose the change and a new equilibrium is established
• In other words, “whatever done to the system, the system does the opposite”.

Effect of  change in concentration

A + B ⇌ C + D

• When something is added or removed, the system will does the opposite so as to remove or replace it
• When the concentration of A or B increases, by Le Chatelier’s principle, the position of equilibrium shifts to the right to decrease the concentration More C and D are being formed.
• When the concentration of A or B decreases, by Le Chatelier’s principle, the position of equilibrium shifts to the left to increase the concentration More A and B are being formed.

Effect of change in pressure

2A(g) + B(g) ⇌ C(g) + D(g)

• Changes in pressure will only affect reactions between gases
• When the pressure of a system is altered, the system will respond to neutralise the change
• When the pressure is increased, by Le Chatelier’s principle, the position of equilibrium shifts to the right to reduce the total number of molecules and to decrease the pressure
• When the pressure is decreased, by Le Chatelier’s principle, the position of equilibrium shifts to the left to increase the total number of molecules and to increase the pressure

1. Increased pressure favours the reaction which produces fewer molecules.
2. Decreased pressure favours the reaction which produces more molecules.

• If there are equal number of molecules on each side of the equation, changes in pressure will not affect the position of equilibrium. However, it will still affect the rate of reaction

Effect of  change in temperature

A + B ⇌ C + D       ΔH° = enthalpy change of forward reaction

• If heat is introduced to the system, the system will respond to neutralise the change, either by absorbing or releasing heat
• If ΔH° > 0 (endothermic reaction):
  1. When temperature is increased, by Le Chatelier’s principle, the position of equilibrium shifts to the right to remove the extra heat (the endothermic reaction is favoured).
  2. When temperature is reduced, by Le Chatelier’s principle, the position of equilibrium shifts to left to release heat (the forward reaction, an exothermic reaction is favoured).
• 1. Increased temperature favours endothermic reaction.
  2. Decreased temperature favours exothermic reaction.

Effect of catalyst

• Catalysts will not affect the position of This is because a catalyst increases both the rate of forward and reverse reaction.
• However, catalysts can speed up the time taken to reach equilibrium