The effect of pressure changes on equilibrium (HT only) — AQA GCSE Combined Science
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The effect of pressure changes on equilibrium (HT only) explained
For gaseous reactions at equilibrium, changing pressure shifts the position of equilibrium, which is a Higher Tier only concept.
Read the full explanation
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.
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
As a higher-tier concept, Le Chatelier's principle predicts how a system at dynamic equilibrium responds to a change in pressure. Increasing pressure favours the direction that reduces the total number of gas molecules, because fewer molecules occupy less volume and so relieve the applied pressure. To apply this, count the gas molecules on each side of the symbol equation using the balancing numbers, ignoring solids and liquids. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the left side has 1 + 3 = 4 molecules and the right side has 2, so raising pressure shifts the equilibrium position to the right, increasing the yield of ammonia. If both sides have equal gas molecule counts, pressure has no effect on the position of equilibrium.
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.
As a higher-tier concept, when the pressure on a system at dynamic equilibrium is decreased, the equilibrium position shifts towards the side with the larger number of gas molecules. Producing more gas molecules increases the total volume occupied, which opposes the reduction in pressure. To apply the rule, count the gas molecules on each side of the symbol equation using the balancing numbers, ignoring solids and liquids. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the left side has 1 + 3 = 4 molecules and the right side has 2, so lowering pressure shifts the equilibrium position to the left, reducing the yield of ammonia. If both sides have equal gas molecule counts, pressure changes have no effect on the equilibrium position.
Students should be able to interpret appropriate given data to predict the effect of pressure changes on given reactions at equilibrium.
This higher-tier skill asks you to use supplied data, not memory, to predict how changing pressure shifts a reaction at equilibrium. First count gas moles on each side of the equation, for example N₂(g) + 3H₂(g) ⇌ 2NH₃(g) has 4 mol on the left and 2 mol on the right. Increasing pressure shifts the position of equilibrium towards the side with fewer gas molecules, so here it moves right, raising ammonia yield. Decreasing pressure shifts it towards the side with more gas molecules. If gas moles are equal, pressure has no effect. Given data may include yield tables or graphs at different pressures; read the trend, link it to the mole count, and state the direction of shift and the resulting change in yield or concentration.
Your focus
- Count moles of gas on each side of a balanced gaseous equilibrium equation.
- Predict the direction of equilibrium shift when pressure is increased or decreased.
- Explain why equal gas mole totals mean pressure has no effect on the position of equilibrium.
Show all 12 objectives
- Identify the side of a symbol equation with the smaller number of gas molecules.
- Predict the direction of the equilibrium shift when pressure is increased.
- Justify the predicted shift by referring to the relief of the applied pressure.
- Identify the side of a symbol equation with the larger number of gas molecules.
- Predict the direction of the equilibrium shift when pressure is decreased.
- Justify the predicted shift by referring to the production of more gas molecules.
- Count the gaseous molecules on each side of a given equilibrium equation.
- Use the mole count and given data to predict the direction of equilibrium shift when pressure changes.
- Explain the resulting change in yield or concentration for a specified pressure increase or decrease.
The effect of pressure changes on equilibrium (HT only) exam tips
Marking 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.
- Recognise and state that if the number of gas moles is equal on both sides, changing pressure does not change the position of equilibrium.
- State that increasing pressure shifts the equilibrium position towards the side with fewer gas molecules.
- Count gas molecules on each side of the symbol equation using the balancing numbers, ignoring solids and liquids.
- Apply the rule to a named equation, for example N₂(g) + 3H₂(g) ⇌ 2NH₃(g), where 4 molecules on the left shift to 2 on the right.
- Explain that the shift relieves the increase in pressure by reducing the number of gas molecules.
- Recognise that if the number of gas molecules is equal on both sides, changing pressure does not change the equilibrium position.
- State that decreasing pressure shifts the equilibrium position towards the side with more gas molecules.
- Count gas molecules on each side of the symbol equation using the balancing numbers, ignoring solids and liquids.
- Apply the rule to a named equation, for example N₂(g) + 3H₂(g) ⇌ 2NH₃(g), where 2 molecules on the right shift to 4 on the left.
- Explain that the shift opposes the decrease in pressure by producing more gas molecules.
- Recognise that if the number of gas molecules is equal on both sides, changing pressure does not change the equilibrium position.
- Count the number of gas molecules on each side of the given equation, treating only gaseous species as relevant to pressure.
- State the rule that increasing pressure shifts equilibrium towards the side with fewer gas molecules, and decreasing pressure shifts it towards the side with more gas molecules.
- Apply the rule to the specific reaction in the data, naming the direction of shift, for example forwards or to the right.
- Interpret the given yield, percentage or concentration data to confirm or predict the effect, for example higher pressure giving a greater yield when the forward reaction reduces gas moles.
- Recognise and state that if the number of gas molecules is equal on both sides, a pressure change has no effect on the position of equilibrium.
- Link the predicted shift to a measurable outcome such as changed yield, changed equilibrium concentration or changed amount of product.
Examiner 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.
- 💡Write the gas molecule totals above each side of the equation before deciding the direction of the shift.
- 💡Use the phrase 'shifts to the side with fewer gas molecules' and then name that side, for example 'to the right, towards ammonia'.
- 💡If the equation is unfamiliar, count only species labelled (g) and check whether the totals differ.
- 💡Write the gas molecule totals above each side of the equation before deciding the direction of the shift.
- 💡Use the phrase 'shifts to the side with more gas molecules' and then name that side, for example 'to the left, towards nitrogen and hydrogen'.
- 💡If the equation is unfamiliar, count only species labelled (g) and check whether the totals differ.
- 💡Write the mole count for each side beside the equation before deciding the direction of shift.
- 💡Use the phrase 'shifts to the right' or 'shifts to the left' and then state the effect on yield or concentration.
- 💡When data are given, refer to specific values or the overall trend rather than making a general statement alone.
- 💡Check whether the question says increase or decrease in pressure, and answer that exact change.
Common Mistakes
- Counting atoms instead of moles of gas molecules: correct this by counting the number of gas molecules or moles shown by the coefficients in the equation.
- Forgetting to include coefficients when counting moles: correct this by using the balanced equation, for example 3H₂ means three moles of hydrogen gas.
- Assuming pressure always shifts equilibrium to the right: correct this by comparing the mole totals on each side before predicting the direction.
- Counting atoms rather than molecules: for example treating 3H₂ as six particles. Correction: count the balancing number of each gaseous species, so 3H₂ contributes three molecules.
- Including solids or liquids in the molecule count. Correction: only gases affect pressure, so ignore species such as CaCO₃(s) or H₂O(l).
- Assuming increased pressure always shifts equilibrium to the right. Correction: identify which side has fewer gas molecules first; the shift may be to the left.
- Counting atoms rather than molecules: for example treating 3H₂ as six particles. Correction: count the balancing number of each gaseous species, so 3H₂ contributes three molecules.
- Including solids or liquids in the molecule count. Correction: only gases affect pressure, so ignore species such as CaCO₃(s) or H₂O(l).
- Assuming decreased pressure always shifts equilibrium to the left. Correction: identify which side has more gas molecules first; the shift may be to the right.
- Counting all molecules rather than only gaseous molecules. Correction: count gas moles only, since solids and liquids contribute negligibly to pressure effects.
- Assuming increased pressure always increases yield. Correction: check which side has fewer gas molecules, because the shift favours that side.
- Reversing the direction of shift. Correction: state clearly that higher pressure favours the side with fewer gas molecules and lower pressure favours the side with more.
- Ignoring the data and reciting a general rule. Correction: quote or describe the trend in the given figures to support the prediction.