Equilibrium (CIE IGCSE Chemistry)

Extended tier only

• Reversible reactions occur in both the forward and backward directions
• A reversible reaction can reach equilibrium in a closed system
  • This is so none of the participating chemical species can leave the reaction vessel and nothing else can enter

Equilibrium can only be reached in a closed vessel which prevents reactants or products from escaping the system

• At equilibrium:
  • The rate of the forward reaction is equal to the rate of the reverse reaction
  • The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure)
• Equilibrium is dynamic
  • This means that the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate

• An example of a reaction reaching equilibrium is the reaction between H₂ and N₂ in the Haber process:
  • At the start of the reaction, only nitrogen and hydrogen are present
    • This means that the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest
  • As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease
    • So, the rate of the forward reaction will decrease
  • However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase
    • Ammonia will decompose to reform hydrogen and nitrogen
  • In a closed system, the two reactions are interlinked and none of the gases can escape
  • So, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:

At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction

Extended tier only

• The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction
  • This balance is framed in an important concept known as Le Chatelier's Principle,
• This principle states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change
• The principle is used to predict changes to the position of equilibrium when there are changes in:
  • Temperature
  • Pressure 
  • Concentration
• Knowing the energy changes, states and concentrations involved allows us to use the principle to manipulate the outcome of reversible reactions
• For example, if pressure is increased, the position of equilibrium moves in the direction which has the smallest amount of gaseous molecules
• The position of equilibrium is said to shift to the right when the forward reaction is favoured
  • This means that there is an increase in the amount of products formed
• The position of equilibrium is said to shift to the left when the reverse reaction is favoured
  • So, there is an increase in the amount of reactants formed

How temperature affects equilibrium

• We can predict the effect of changes in temperature on systems in equilibrium
  • To make this prediction it is necessary to know whether the reaction is exothermic or endothermic
• If the temperature is raised:
  • The yield from the endothermic reaction increases
  • The yield from the exothermic reaction decreases
• If the temperature is lowered:
  • The yield from the endothermic reaction decreases
  • The yield from the exothermic reaction increases
• When a change in temperature is made to a system, the system will oppose the change
  • E.g. If the temperature is increased, the system will oppose the change by decreasing the temperature
  • It will do this by favouring the endothermic reaction

The effects of temperature on equilibrium

How pressure affects equilibrium

• Changes in pressure only affect reactions where the reactants or products are gases
• We can predict the effect of changes in pressure on systems in equilibrium
  • To make this prediction, the balanced symbol equation must be known

The effects of pressure on equilibrium

How concentration affects equilibrium

• The effect of changing concentration can be thought of as a balance, with the reactants on the left and the products on the right
  • If the concentration of a reactant increases, then the equilbrium shifts to the right to balance this balance

 The effects of concentration on equilibrium

How catalysts affects equilibrium

• The presence of a catalyst:
  • Does not affect the position of equilibrium
  • Increases the rate at which equilibrium is reached
• This is because the catalyst increases the rate of both the forward and backward reactions by the same amount by providing an alternative pathway requiring lower activation energy
• As a result, the equilibrium concentration of reactants and products is the same as it would be without the catalyst

Diagram showing the effect of a catalyst on the time taken for equilibrium to be established