AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
For gaseous reactions at equilibrium: an increase in pressure causes the…
Full specification wording
The effect of pressure changes on equilibrium (HT only): For gaseous reactions at equilibrium: 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 a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.
Pressure changes affect gaseous equilibria by shifting the position towards the side with fewer gas molecules when pressure rises, and towards the side with more gas molecules when pressure falls.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 2
Study notes
From the checked teaching notes for this specification point.
The idea
Pressure changes affect gaseous equilibria by shifting the position towards the side with fewer gas molecules when pressure rises, and towards the side with more gas molecules when pressure falls. Count the molecules of gas on each side using the coefficients in the balanced symbol equation, ignoring solids and liquids. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the left side has four molecules and the right side has two, so increasing pressure shifts equilibrium to the right and increases ammonia yield. Decreasing pressure shifts it to the left. The rule applies only to gases; changing pressure has no effect on the position when the number of gas molecules is equal on both sides.
What earns the marks
- Count the total number of gas molecules on each side of a balanced symbol equation using the coefficients.
- State that increasing pressure shifts the equilibrium position towards the side with fewer gas molecules.
- State that decreasing pressure shifts the equilibrium position towards the side with more gas molecules.
- Recognise that solids and liquids are not counted when comparing numbers of gas molecules.
- Identify when a pressure change has no effect because the number of gas molecules is equal on both sides.
- Link the predicted shift to the effect on the yield of a specified product.
Common mistakes and how to put them right
Counting atoms rather than molecules when comparing sides.
Put it right: Count the number of gaseous molecules shown by the coefficients.
Including solids or liquids in the molecule count.
Put it right: Count only species in the gaseous state.
Assuming pressure always shifts equilibrium to the right.
Put it right: Compare the number of gas molecules on each side before predicting the direction.
Exam tips
- Write the number of gas molecules above each side of the equation before deciding the direction of shift.
- Use the phrase 'side with fewer gas molecules' or 'side with more gas molecules' to make the reasoning explicit.
- Check whether the question asks about pressure increase or decrease and answer that specific change.
Try the lesson
The start of the The effect of pressure changes on equilibrium (HT only): For gaseous reactions at equilibrium: 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 a decrease in pressure causes the equilibrium position to shift towards the side with the larger 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
For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the pressure is increased. In which direction does the equilibrium position shift?
Listen
Pressure changes affect gaseous equilibria by shifting the position towards the side with fewer gas molecules when pressure rises, and towards the side with more gas molecules when pressure falls. Count the molecules of gas on each side using the coefficients in the balanced symbol equation, ignoring solids and liquids. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the left side has four molecules and the right side has two, so increasing pressure shifts equilibrium to the right and increases ammonia yield. Decreasing pressure shifts it to the left. The rule applies only to gases; changing pressure has no effect on the position when the number of gas molecules is equal on both sides. A strong answer earns marks for these points. Count the total number of gas molecules on each side of a balanced symbol equation using the coefficients. State that increasing pressure shifts the equilibrium position towards the side with fewer gas molecules. State that decreasing pressure shifts the equilibrium position towards the side with more gas molecules. Recognise that solids and liquids are not counted when comparing numbers of gas molecules. A common slip: Counting atoms rather than molecules when comparing sides. Instead, count the number of gaseous molecules shown by the coefficients. Exam tip: Write the number of gas molecules above each side of the equation before deciding the direction of shift.
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