Reversible Reactions (Oxford AQA IGCSE Chemistry)

Reversible Reactions

• 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

• 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

• 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