AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The effect of pressure changes on equilibrium (HT only): For gaseous reactions at equilibrium:
For gaseous reactions at equilibrium, changing pressure shifts the position of equilibrium, which is a Higher Tier only concept.
Study notes
From the checked teaching notes for this specification point.
The idea
For gaseous reactions at equilibrium, changing pressure shifts the position of equilibrium, which is a Higher Tier only concept. Count the total number of moles of gas on each side of the balanced equation. Increasing pressure favours the side with fewer moles of gas, because that reduces the pressure. Decreasing pressure favours the side with more moles of gas. If the number of moles of gas is equal on both sides, pressure has no effect on the position of equilibrium. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g), there are 4 moles of gas on the left and 2 on the right, so high pressure favours ammonia formation. Use the balanced equation to count moles carefully, including coefficients.
What earns the marks
- Count the total number of moles of gas on the reactant side and on the product side using the balanced equation and its coefficients.
- Compare the two totals and state which side has fewer moles of gas.
- Predict the direction of shift when pressure is increased, linking it to the side with fewer gas moles.
- Predict the direction of shift when pressure is decreased, linking it to the side with more gas moles.
- Recognise and state that if the number of gas moles is equal on both sides, changing pressure does not change the position of equilibrium.
Common mistakes and how to put them right
Counting atoms instead of moles of gas molecules.
Put it right by counting the number of gas molecules or moles shown by the coefficients in the equation.
Forgetting to include coefficients when counting moles.
Put it right by using the balanced equation, for example 3H₂ means three moles of hydrogen gas.
Assuming pressure always shifts equilibrium to the right.
Put it right by comparing the mole totals on each side before predicting the direction.
Exam tips
- Write the mole count for each side before stating the direction of shift.
- Use the phrase 'fewer moles of gas' or 'more moles of gas' to make your reasoning clear.
- Remember this is Higher Tier content; always check state symbols to ensure you are only counting gaseous moles.
Try the lesson
The start of the The effect of pressure changes on equilibrium (HT only): For gaseous reactions at equilibrium: lesson, made from checked teaching notes for this specification point.
Have a go first
A gaseous reaction at equilibrium has three molecules of gas on the left-hand side of the equation and two molecules of gas on the right-hand side. What happens to the position of equilibrium if the pressure of the system is increased?
Listen
For gaseous reactions at equilibrium, changing pressure shifts the position of equilibrium, which is a Higher Tier only concept. Count the total number of moles of gas on each side of the balanced equation. Increasing pressure favours the side with fewer moles of gas, because that reduces the pressure. Decreasing pressure favours the side with more moles of gas. If the number of moles of gas is equal on both sides, pressure has no effect on the position of equilibrium. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g), there are 4 moles of gas on the left and 2 on the right, so high pressure favours ammonia formation. Use the balanced equation to count moles carefully, including coefficients. A strong answer earns marks for these points. Count the total number of moles of gas on the reactant side and on the product side using the balanced equation and its coefficients. Compare the two totals and state which side has fewer moles of gas. Predict the direction of shift when pressure is increased, linking it to the side with fewer gas moles. Predict the direction of shift when pressure is decreased, linking it to the side with more gas moles. A common slip: Counting atoms instead of moles of gas molecules. Instead, counting the number of gas molecules or moles shown by the coefficients in the equation. Exam tip: Write the mole count for each side before stating the direction of shift.
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