The effect of changing conditions on equilibrium (HT only) — AQA GCSE Combined Science
Test yourself on The effect of changing conditions on equilibrium (HT only) with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
The effect of changing conditions on equilibrium (HT only) explained
This Higher Tier only topic explores how the relative amounts of reactants and products at dynamic equilibrium depend on the conditions imposed.
Read the full explanation
At equilibrium in a closed system, forward and reverse reactions continue at equal rates, so concentrations stay constant. According to Le Chatelier's principle, changing temperature, pressure, or concentration shifts the position of equilibrium. For example, if the forward direction is exothermic, an increase in temperature shifts the equilibrium to favour the endothermic reverse reaction, decreasing the relative amount of products. For gases, increasing pressure favours the side with fewer molecules. A catalyst speeds both directions equally and does not change relative amounts.
If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change.
This statement expresses Le Chatelier's principle (assessed at Higher Tier only): when a system at dynamic equilibrium is disturbed by a change in conditions, the equilibrium shifts in the direction that opposes the change. If temperature is increased, the endothermic direction is favoured to absorb the added heat; if pressure is increased, the side with fewer gas molecules is favoured to reduce pressure; if a reactant is added, the forward reaction is favoured to use it up. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g), raising pressure shifts equilibrium right because four gas molecules become two, increasing the ammonia proportion. Students should predict the direction of shift and explain how it counteracts the change.
The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle.
A reversible reaction in a closed system reaches dynamic equilibrium when forward and reverse rates are equal, so concentrations stay constant. Assessed at Higher Tier only, Le Chatelier’s Principle predicts how a system responds to a change: the equilibrium shifts in the direction that opposes the change. If temperature rises, the endothermic direction is favoured. If reactant concentration increases, the forward reaction is favoured. If pressure increases, the side with fewer gas molecules is favoured. A catalyst does not shift equilibrium; it only speeds up its attainment. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g), increasing pressure favours the forward reaction because four gas molecules form two.
Students should be able to make qualitative predictions about the effect of changes on systems at equilibrium when given appropriate information.
Assessed at Higher Tier only, qualitative prediction means deciding the direction in which an equilibrium shifts, not calculating exact concentrations. You are given a reversible equation and information such as whether the forward reaction is exothermic or endothermic, or the number of gas molecules on each side. For a temperature increase, the equilibrium shifts in the endothermic direction; for a decrease, it shifts in the exothermic direction. For a pressure increase, it shifts towards the side with fewer gas molecules. For a concentration increase of a reactant, it shifts towards products. For example, in 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), increasing pressure favours the forward reaction because three gas molecules form two.
Your focus
- Describe how the relative amounts of reactants and products at equilibrium depend on reaction conditions.
- Explain the effect of temperature, pressure and concentration changes on the position of equilibrium.
- Distinguish between the effect of a catalyst on rate and its lack of effect on equilibrium position.
Show all 12 objectives
- State that a system at equilibrium responds to a change by counteracting it.
- Predict the direction of equilibrium shift when temperature, pressure or concentration changes.
- Explain how the predicted shift counteracts the change using a balanced equation.
- Describe dynamic equilibrium and state the conditions under which it occurs.
- Apply Le Chatelier’s Principle to predict the direction of shift when temperature, pressure or concentration changes.
- Explain why a catalyst does not change the position of equilibrium but does change the rate at which equilibrium is reached.
- Interpret given information about a reversible reaction to identify whether a change will shift the equilibrium left or right.
- Construct a qualitative prediction that names the direction of shift and links it to the imposed change.
- Distinguish between predicting the position of equilibrium and calculating quantitative equilibrium values.
The effect of changing conditions on equilibrium (HT only) exam tips
Marking Points
- States that at equilibrium the rates of the forward and reverse reactions are equal and concentrations remain constant in a closed system.
- Explains that the relative amounts of reactants and products are determined by the conditions, not by a fixed ratio.
- Identifies temperature as a condition that changes the position of equilibrium, favouring the endothermic or exothermic direction.
- Identifies pressure as a condition affecting gaseous equilibria by favouring the side with fewer or more gas molecules.
- Explains that changing the concentration of a reactant or product shifts the equilibrium to counteract the change.
- States that a catalyst does not change the position of equilibrium or the relative amounts, only the rate at which equilibrium is reached.
- States that a system at equilibrium responds to a change by shifting in the direction that counteracts the change.
- Applies the principle to temperature: an increase favours the endothermic direction, while a decrease favours the exothermic direction.
- Applies the principle to pressure: an increase favours the side with fewer gas molecules, while a decrease favours the side with more.
- Applies the principle to concentration: adding a substance favours the reaction that removes it, while removing a substance favours the reaction that replaces it.
- Uses a balanced equation to justify the predicted direction of shift for a named change.
- State that equilibrium is dynamic: forward and reverse reactions continue at equal rates in a closed system.
- Identify the change imposed on the system, such as temperature, pressure or concentration.
- Apply Le Chatelier’s Principle by stating that the equilibrium shifts to oppose the change.
- For temperature, link the shift to the endothermic direction when temperature increases and the exothermic direction when temperature decreases.
- For pressure, compare the number of gas molecules on each side and favour the side with fewer when pressure increases.
- For concentration, state that increasing a reactant favours the forward reaction and increasing a product favours the reverse reaction.
- Explain that a catalyst increases the rate of both forward and reverse reactions equally and does not change the equilibrium position.
- Select the relevant information from the question, such as the sign of ΔH or the number of gas molecules on each side.
- State the change imposed on the system, for example an increase in temperature or pressure.
- Predict the direction of shift by applying Le Chatelier’s Principle to oppose the change.
- Justify the prediction by referring to the endothermic direction, the exothermic direction, or the side with fewer or more gas molecules.
- Recognise that adding a catalyst does not alter the position of equilibrium and therefore does not change the qualitative prediction.
Examiner Tips
- 💡Use the phrase 'position of equilibrium' when describing how relative amounts change with conditions.
- 💡For gaseous equilibria, count gas molecules on each side of the balanced equation to justify the effect of pressure.
- 💡Name the change first, then state the direction of shift, then explain how that counteracts the change.
- 💡For pressure questions, count gas molecules on each side of the equation before predicting the shift.
- 💡For temperature questions, identify whether the forward reaction is exothermic or endothermic from the information given.
- 💡Remember that applying Le Chatelier's principle is a Higher Tier only skill, so expect it in HT papers.
- 💡Write the balanced equation first and label each side with the number of gas molecules before predicting pressure effects.
- 💡Use the phrase ‘the equilibrium shifts to oppose the change’ and then name the favoured direction explicitly.
- 💡Check whether the reaction is exothermic or endothermic in the direction stated before deciding the effect of temperature.
- 💡As this is a Higher Tier only topic, ensure you can fully explain the opposing shift rather than just stating it.
- 💡Underline the change described in the question and the data provided before writing your prediction.
- 💡Give a direction and a reason in the same sentence, for example ‘the equilibrium shifts to the right because the forward reaction is endothermic’.
- 💡If the question says ‘explain’, include the principle that the system opposes the change rather than only naming the direction.
- 💡Be prepared for these qualitative predictions in Higher Tier exams, as this specific skill is HT only.
Common Mistakes
- Thinking equilibrium means equal amounts of reactants and products; correction: equilibrium means equal rates, and amounts depend on the conditions.
- Believing a catalyst increases the yield of product; correction: a catalyst speeds both forward and reverse reactions equally and does not change relative amounts.
- Assuming the equilibrium position is fixed for a reaction; correction: changing temperature, pressure or concentration changes the relative amounts present.
- Saying the system 'wants' to oppose the change without linking to reaction direction; correction: state which reaction, forward or reverse, is favoured and why.
- Applying pressure rules to equilibria with equal gas molecules on both sides; correction: pressure changes have no effect on the position of equilibrium in that case.
- Confusing the effect of a catalyst with a change in conditions; correction: a catalyst does not shift equilibrium and so does not counteract a change.
- Saying that increasing temperature always favours the forward reaction; correct this by identifying which direction is endothermic and favouring that direction when temperature rises.
- Confusing rate with position of equilibrium; correct this by explaining that a catalyst changes how quickly equilibrium is reached, not where it lies.
- Ignoring the state symbols when applying pressure changes; correct this by counting only gas molecules and comparing the two sides of the equation.
- Predicting the shift without referring to the given information; correct this by explicitly using the stated ΔH sign or gas molecule count in the explanation.
- Assuming that increasing pressure always favours the forward reaction; correct this by comparing the number of gas molecules on each side of the equation.
- Treating a qualitative prediction as a calculation of new concentrations; correct this by stating only the direction of shift and the reason.