AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The effect of pressure changes on equilibrium (HT only): an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction
As a higher-tier concept, Le Chatelier's principle predicts how a system at dynamic equilibrium responds to a change in pressure.
Study notes
From the checked teaching notes for this specification point.
The idea
As a higher-tier concept, Le Chatelier's principle predicts how a system at dynamic equilibrium responds to a change in pressure. Increasing pressure favours the direction that reduces the total number of gas molecules, because fewer molecules occupy less volume and so relieve the applied pressure. To apply this, count the gas molecules on each side of the symbol equation using the balancing numbers, ignoring solids and liquids. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the left side has 1 + 3 = 4 molecules and the right side has 2, so raising pressure shifts the equilibrium position to the right, increasing the yield of ammonia. If both sides have equal gas molecule counts, pressure has no effect on the position of equilibrium.
What earns the marks
- State that increasing pressure shifts the equilibrium position towards the side with fewer gas molecules.
- Count gas molecules on each side of the symbol equation using the balancing numbers, ignoring solids and liquids.
- Apply the rule to a named equation, for example N₂(g) + 3H₂(g) ⇌ 2NH₃(g), where 4 molecules on the left shift to 2 on the right.
- Explain that the shift relieves the increase in pressure by reducing the number of gas molecules.
- Recognise that if the number of gas molecules is equal on both sides, changing pressure does not change the equilibrium position.
Common mistakes and how to put them right
Counting atoms rather than molecules.
Put it right: Count the balancing number of each gaseous species, so 3H₂ contributes three molecules.
Including solids or liquids in the molecule count.
Put it right: Only gases affect pressure, so ignore species such as CaCO₃(s) or H₂O(l).
Assuming increased pressure always shifts equilibrium to the right.
Put it right: Identify which side has fewer gas molecules first; the shift may be to the left.
Exam tips
- Write the gas molecule totals above each side of the equation before deciding the direction of the shift.
- Use the phrase 'shifts to the side with fewer gas molecules' and then name that side, for example 'to the right, towards ammonia'.
- If the equation is unfamiliar, count only species labelled (g) and check whether the totals differ.
Try the lesson
The start of the The effect of pressure changes on equilibrium (HT only): an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction lesson, made from checked teaching notes for this specification point.
Have a go first
In the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), a chemist increases the pressure. In which direction does the equilibrium position shift, and why?
Listen
As a higher-tier concept, Le Chatelier's principle predicts how a system at dynamic equilibrium responds to a change in pressure. Increasing pressure favours the direction that reduces the total number of gas molecules, because fewer molecules occupy less volume and so relieve the applied pressure. To apply this, count the gas molecules on each side of the symbol equation using the balancing numbers, ignoring solids and liquids. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the left side has 1 + 3 = 4 molecules and the right side has 2, so raising pressure shifts the equilibrium position to the right, increasing the yield of ammonia. If both sides have equal gas molecule counts, pressure has no effect on the position of equilibrium. A strong answer earns marks for these points. State that increasing pressure shifts the equilibrium position towards the side with fewer gas molecules. Count gas molecules on each side of the symbol equation using the balancing numbers, ignoring solids and liquids. Apply the rule to a named equation, for example N₂(g) + 3H₂(g) ⇌ 2NH₃(g), where 4 molecules on the left shift to 2 on the right. Explain that the shift relieves the increase in pressure by reducing the number of gas molecules. A common slip: Counting atoms rather than molecules: for example treating 3H₂ as six particles. Instead, count the balancing number of each gaseous species, so 3H₂ contributes three molecules. Exam tip: Write the gas molecule totals above each side of the equation before deciding the direction of the shift.
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