The Effect of Pressure Changes on Equilibrium (AQA GCSE Chemistry)
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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
- 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:
- 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
- This means that the mixture will appear increasingly colourless as the concentration of N2O4 increases
Exam 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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