AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to interpret appropriate given data to predict the…
Full specification wording
The effect of pressure changes on equilibrium (HT only): 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.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 2
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- 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.
Common mistakes and how to put them right
Treating a small change in yield as no change without checking the scale.
Put it right: Compare values carefully and consider whether the difference is significant.
Assuming that higher pressure always means higher yield.
Put it right: 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.
Put it right: State the molecule count on each side as justification.
Exam tips
- 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.
Try the lesson
The start of the The effect of pressure changes on equilibrium (HT only): Students should be able to interpret appropriate given data to predict the effect of pressure changes on given reactions at equilibrium. lesson, made from checked teaching notes for this specification point.
Have a go first
Data for the equilibrium 2NO₂(g) ⇌ N₂O₄(g) show that the right-hand side has fewer gas molecules. A student increases the pressure. What is the predicted effect on the equilibrium position?
Listen
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. A strong answer earns marks for these points. 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. A common slip: Treating a small change in yield as no change without checking the scale. Instead, compare values carefully and consider whether the difference is significant. Exam tip: Quote two pressure values and their corresponding yields to support your prediction.
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