Equilibrium (AQA GCSE Chemistry: Combined Science)

• We have already seen that a reversible reaction is one that occurs in both directions
• When, during the course of reaction, the rate of the forward reaction equals the rate of the reverse reaction, then 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 (given there is no other change to the system such as temperature and pressure)
• It only occurs in a closed system so that none of the participating chemical species are able to leave the reaction vessel and nothing else can enter

Equilibrium can only be reached in a closed container

• An example of a dynamic equilibrium is the reaction between H₂ and N₂ in the Haber process
• When only nitrogen and hydrogen are present at the beginning of the reaction, 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)
• Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:

Diagram showing when the rates of forward and backward reactions become equal