The effect of pressure changes on equilibrium (HT only) — AQA GCSE Chemistry
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The effect of pressure changes on equilibrium (HT only) explained
The effect of pressure changes on equilibrium (HT only) covers 2 specification points, including An increase in pressure causes the equilibrium… and Students should be able to interpret….
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Open a point below for its explanation and revision guidance.
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. 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.
Students should be able to interpret appropriate given data to predict the effect of pressure changes on given reactions at equilibrium.
Given data such as percentage yield at different pressures, you compare values to predict how a pressure change affects an equilibrium. If yield rises as pressure increases, the forward reaction must produce fewer gas molecules than the reactants, so the equilibrium shifts right on compression. If yield falls as pressure increases, the forward reaction produces more gas molecules, so the equilibrium shifts left on compression. For example, data showing ammonia yield increasing from 15% at 100 atm to 30% at 300 atm indicates that the forward reaction reduces the number of gas molecules, consistent with N₂(g) + 3H₂(g) ⇌ 2NH₃(g). You must also recognise when data show no significant change, indicating equal numbers of gas molecules 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.
Show all 6 objectives
- Interpret yield or concentration data across a range of pressures.
- Deduce the relative number of gas molecules on each side from the data trend.
- Predict the effect of a pressure change on the position of equilibrium and product yield.
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.
- Compare yield or concentration values at two or more pressures to identify the trend.
- Deduce from the trend whether the forward reaction increases or decreases the number of gas molecules.
- Predict the direction of equilibrium shift for a stated increase or decrease in pressure.
- Use the balanced symbol equation to confirm the deduction from the data.
- Recognise data showing little or no change with pressure and relate this to equal gas molecule numbers.
- State the effect of the predicted shift on the yield of a named 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.
- 💡Quote two pressure values and their corresponding yields to support your prediction.
- 💡State the direction of shift and then the effect on yield in a single clear sentence.
- 💡If the data show no trend, say so and explain that the number of gas molecules is equal on both sides.
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.
- Treating a small change in yield as no change without checking the scale; the correction is to compare values carefully and consider whether the difference is significant.
- Assuming that higher pressure always means higher yield; the correction is to use the data trend to decide the direction of shift.
- Forgetting to link the data trend back to the number of gas molecules in the equation; the correction is to state the molecule count on each side as justification.