The Effect of Pressure Changes on Equilibrium (AQA GCSE Chemistry: Combined Science)

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The effect of pressure changes on equilibrium

Higher tier only

  • We can predict the effect of changes in pressure on systems in equilibrium
  • Changes in pressure only affects gases
  • In gaseous reactions:
    • An increase in pressure will favour the reaction that produces the least number of molecules
    • A decrease in pressure will favour the reaction that produces the greatest number of molecules
  • 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 equilibrium when the pressure is changed
    • Increasing the pressure will increase the rate of the forward reaction and backward reaction equally which is why the position of equilibrium is unchanged

Worked example

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

2NO2 (g)   ⇌   N2O4 (g)

dark brown     colourless

What will the colour change be if the pressure is increased? Explain your answer.

Answer:

  • The number of gas molecules produced by the forward reaction = 1
  • The number of gas molecules produced by the reverse reaction = 2
  • An increase in the pressure will favour the reaction that produces the least number of molecules
    • This is the forward reaction
  • So, the equilibrium shifts to the right
    • This means that the mixture will appear increasingly colourless as the concentration of N2O4 increases

Examiner Tip

Changes in pressure affect those systems that contain a gas only.

Use the balanced equation to determine which side has the most molecules of gas.

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.