AQA GCSE Chemistry

Revision Notes

6.2.7 The Effect of Pressure Changes on Equilibrium

Higher Tier Only

The Effect of Pressure Changes on Equilibrium

  • Le Chatelier’s Principle can be used to predict the effect of changes in pressure on systems in equilibrium
  • Changes in pressure only affects gases so firstly you have to identify all gaseous reactants and products
  • The following table summarises how a pressure change alters the position of equilibrium:

Effect of Pressure Changes on an Equilibrium Table

Effect of Pressure on Equilibrium, downloadable IGCSE & GCSE Chemistry revision notes

  • If there are the same number of moles of gases on either side of the equation, then there is NO effect on the position of equilbrium when the presure is changed
  • Increasing the pressure will increase the rate of the forward reaction and backward reaction equally which is why the position of equilbrium is unchanged

Worked Example

Nitrogen dioxide molecules can dimerise and form dinitrogen tetroxide in the following equilbrium reaction:

2NO2 (g)   ⇌   N2O4 (g)

dark brown     colourless


    • Number of gas molecules on the left side = 2
    • Number of gas molecules on the right side = 1
    • An increase in the pressure will cause the equilibrium to shift in the direction that produces the smaller number of molecules of gas so the equilibrium shifts to the right

Exam Tip

Changes in pressure affects those systems that contain a gas only. Use the balanced equation to determine which side has the most molecules of gas

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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