Equilibrium (AQA GCSE Chemistry: Combined Science)

• A reversible reaction is one which occurs in both directions
• When the rate of the forward reaction equals the rate of the reverse reaction, the overall reaction is said to be in a state of equilibrium
  • Equilibrium is dynamic i.e. the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate
• The concentration of reactants and products remains constant
  • This is true if there is no other change to the system, such as temperature and pressure
• It only occurs in a closed system
  • This is so none of the participating chemical species can leave the reaction vessel and nothing else can enter

The difference between an open and closed system

Equilibrium can only be reached in a closed system

• 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:

The rate of the forward and reverse reaction during the progress of a reaction

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