The effect of pressure changes on equilibrium (HT only) — AQA GCSE Chemistry
Test yourself on The effect of pressure changes on equilibrium (HT only) with AQA GCSE practice questions.
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The effect of pressure changes on equilibrium (HT only) explained
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.
Read the full explanation
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.
Your focus
- Count gas molecules on each side of a balanced symbol equation.
- Predict the direction of equilibrium shift for an increase or decrease in pressure.
- Explain why pressure has no effect when gas molecule numbers are equal on both sides.
The effect of pressure changes on equilibrium (HT only) exam tips
Marking 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.
- 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.
Examiner 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.
Common Mistakes
- Counting atoms rather than molecules when comparing sides; the correction is to count the number of gaseous molecules shown by the coefficients.
- Including solids or liquids in the molecule count; the correction is to count only species in the gaseous state.
- Assuming pressure always shifts equilibrium to the right; the correction is to compare the number of gas molecules on each side before predicting the direction.