The effect of changing conditions on equilibrium (HT only) — AQA GCSE Chemistry
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The effect of changing conditions on equilibrium (HT only) explained
The effect of changing conditions on equilibrium (HT only) covers 4 specification points, including The relative amounts of all the reactants and… and If a system is at equilibrium and a change is….
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Open a point below for its explanation and revision guidance.
The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.
At equilibrium, the mixture contains both reactants and products, but the proportions of each are not fixed. They depend on the conditions under which the reaction takes place, especially temperature, pressure and concentration. Changing a condition alters the position of equilibrium, so the relative amounts of reactants and products settle at new constant values. For example, in the Haber process for ammonia, nitrogen and hydrogen react reversibly to form ammonia; at a higher pressure the equilibrium mixture contains a greater proportion of ammonia, while at a higher temperature the proportion of ammonia decreases. The relative amounts are therefore a consequence of the chosen conditions, not a universal constant.
If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change.
When a system is at equilibrium and a condition such as temperature, pressure or concentration is changed, the equilibrium position shifts in the direction that opposes the change. This is a qualitative principle used to predict the response of a reversible reaction. If the temperature is increased, the system favours the endothermic direction, which absorbs the added energy. If the pressure is increased, the system favours the direction producing fewer gas molecules, which reduces the pressure. If the concentration of a reactant is increased, the system favours the forward reaction to use up some of that reactant. For example, adding more nitrogen to the Haber process shifts the equilibrium towards ammonia production.
The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle.
Le Chatelier’s Principle states that when a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts in the direction that opposes the change. For example, in the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), increasing pressure shifts equilibrium to the right because there are fewer gas molecules on the product side, partially reducing the pressure rise. Increasing temperature shifts equilibrium in the endothermic direction, so for this exothermic forward reaction it shifts left. Adding a catalyst does not change the position of equilibrium; it only speeds up the attainment of equilibrium. The principle applies to changes in concentration, pressure and temperature, and predictions must state both the direction of shift and the resulting effect on yield.
Students should be able to make qualitative predictions about the effect of changes on systems at equilibrium when given appropriate information.
Qualitative predictions describe the direction in which an equilibrium shifts and whether the yield of a product increases or decreases, without calculating numerical equilibrium constants. Given a balanced equation and information about a change, you apply Le Chatelier’s Principle. For example, for the equilibrium 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), increasing pressure shifts equilibrium to the right because there are three gas molecules on the left and two on the right, so the yield of SO₃ increases. Increasing temperature shifts equilibrium in the endothermic direction; if the forward reaction is exothermic, the yield of SO₃ decreases. Predictions must be justified by reference to the change and the opposing shift, and should state the effect on the amount of a named substance.
Your focus
- Describe how the relative amounts of reactants and products at equilibrium depend on reaction conditions.
- Explain how a change in temperature or pressure alters the position of equilibrium.
- Apply the concept to an industrial equilibrium such as the Haber process.
Show all 12 objectives
- State that a system at equilibrium responds to counteract a change in conditions.
- Predict the direction of equilibrium shift when temperature, pressure or concentration is changed.
- Explain the response of an equilibrium system using the counteracting-change principle.
- State Le Chatelier’s Principle and describe how a system at equilibrium responds to a change in conditions.
- Use a balanced equation to predict the direction of equilibrium shift when concentration, pressure or temperature is changed.
- Explain why a catalyst does not change the position of equilibrium but does change the rate of attainment of equilibrium.
- Make qualitative predictions about the direction of equilibrium shift when given a change in conditions.
- State and justify the effect of a change on the yield of a named substance in an equilibrium system.
- Interpret a balanced equation to support predictions about pressure and temperature changes.
The effect of changing conditions on equilibrium (HT only) exam tips
Marking Points
- States that the amounts of reactants and products at equilibrium are not fixed but depend on the reaction conditions.
- Identifies temperature, pressure and concentration as conditions that can affect the position of equilibrium.
- Explains that changing a condition shifts the position of equilibrium and changes the relative amounts of reactants and products.
- Applies the idea to an industrial example such as the Haber process, linking pressure or temperature to the proportion of ammonia formed.
- Recognises that the equilibrium mixture still contains both reactants and products, but in different relative amounts.
- Uses the term position of equilibrium correctly to describe the balance between reactants and products.
- States that a change in conditions disturbs an equilibrium and the system responds to counteract the change.
- Explains that an increase in temperature favours the endothermic direction, while a decrease favours the exothermic direction.
- Explains that an increase in pressure favours the direction producing fewer gas molecules, while a decrease favours the direction producing more gas molecules.
- Explains that increasing the concentration of a reactant favours the forward reaction, while increasing the concentration of a product favours the reverse reaction.
- Applies the principle to a named equilibrium, such as the Haber process, to predict the direction of shift.
- Uses the term position of equilibrium to describe the direction in which the system responds.
- State that Le Chatelier’s Principle predicts that a system at equilibrium responds to a change by shifting in the direction that opposes the change.
- Apply the principle to a change in concentration by stating that increasing a reactant shifts equilibrium to the right, increasing product yield.
- Apply the principle to a change in pressure by comparing the number of gas molecules on each side; increasing pressure shifts equilibrium towards the side with fewer gas molecules.
- Apply the principle to a change in temperature by identifying the exothermic and endothermic directions; increasing temperature shifts equilibrium in the endothermic direction.
- Explain that a catalyst does not change the position of equilibrium because it speeds up the forward and reverse reactions equally.
- Use a balanced equation to justify a prediction, for example N₂(g) + 3H₂(g) ⇌ 2NH₃(g) has four gas molecules on the left and two on the right.
- Identify the change described, such as an increase in concentration, pressure or temperature.
- Use the balanced equation to determine the direction of the equilibrium shift that opposes the change.
- State whether the yield or amount of a specified reactant or product increases or decreases.
- Justify the prediction by referring to Le Chatelier’s Principle and the relevant feature of the equation, such as gas molecule numbers or exothermic/endothermic direction.
- Recognise that qualitative predictions do not require numerical calculation of equilibrium concentrations or constants.
Examiner Tips
- 💡Name the specific condition being changed, such as temperature or pressure, and state how the relative amounts of reactants and products change.
- 💡Use a familiar industrial equilibrium such as the Haber process to illustrate the effect of conditions on relative amounts.
- 💡Avoid saying that equilibrium is destroyed; instead say that the position of equilibrium shifts to a new balance.
- 💡Identify the change first, then state the direction that counteracts it, and finally link this to the relative amounts of reactants and products.
- 💡For pressure questions, count the gas molecules on each side of the equation before predicting the shift.
- 💡For temperature questions, identify which direction is endothermic and which is exothermic before predicting the shift.
- 💡Always quote the balanced equation and count gas molecules on each side before predicting a pressure change.
- 💡State the direction of the shift and then link it explicitly to whether the yield of the named substance increases or decreases.
- 💡For temperature changes, identify whether the forward reaction is exothermic or endothermic before predicting the shift.
- 💡Read the given information carefully and identify exactly which condition is changed.
- 💡Write the balanced equation and annotate the number of gas molecules on each side before making a pressure prediction.
- 💡Use comparative language such as increases, decreases or shifts to the right, and avoid vague statements such as it changes.
Common Mistakes
- Thinking that the relative amounts at equilibrium are always the same for a given reaction; the correction is that they depend on the conditions used.
- Confusing the rate of reaction with the position of equilibrium; the correction is that conditions can change both, but the position of equilibrium describes the relative amounts, not the speed.
- Assuming that changing conditions changes only the forward reaction; the correction is that both forward and reverse rates are affected and the system settles at a new equilibrium position.
- Saying that the system acts to increase the change; the correction is that the system responds to counteract or oppose the change.
- Mixing up the temperature rule by saying that heating favours the exothermic direction; the correction is that heating favours the endothermic direction.
- Applying the pressure rule without counting gas molecules; the correction is to compare the number of gas molecules on each side and favour the side with fewer when pressure increases.
- Thinking that increasing pressure always shifts equilibrium to the right; the correct approach is to compare the number of gas molecules on each side of the equation.
- Believing that a catalyst increases yield by shifting equilibrium; a catalyst only increases the rate at which equilibrium is reached.
- Confusing the effect of temperature with the effect of pressure; temperature changes alter the equilibrium position according to whether the forward reaction is exothermic or endothermic, not according to gas molecule numbers.
- Giving only the direction of shift without stating the effect on the yield of a named substance; the prediction should include both.
- Assuming that increasing pressure always increases yield; the correct prediction depends on the number of gas molecules on each side.
- Treating a temperature change as affecting the rate only; temperature also changes the position of equilibrium according to the exothermic or endothermic direction.