AQA GCSE Chemistry

Revision Notes

6.2.3 Equilibrium


  • 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

Equilibrium in open & closed systems, IGCSE & GCSE Chemistry revision notes

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:

Dynamic-Equilibrium, IGCSE & GCSE Chemistry revision notes

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

Exam Tip

Remember equilibrium is only reached in a closed vessel.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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