The effect of changing conditions on equilibrium (HT only): Students should be able to make qualitative predictions about the effect of changes on systems at equilibrium when given appropriate information.

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Yes. This guide is linked to the The effect of changing conditions on equilibrium (HT only): Students should be able to make qualitative predictions about the effect of changes on systems at equilibrium when given appropriate information. topic in the AQA GCSE specification.

Assessed at Higher Tier only, qualitative prediction means deciding the direction in which an equilibrium shifts, not calculating exact concentrations.

Study notes

From the checked teaching notes for this specification point.

The idea

For example: In 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), increasing pressure favours the forward reaction because three gas molecules form two.

What earns the marks

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.

Common mistakes and how to put them right

Predicting the shift without referring to the given information.
Put it right: Explicitly using the stated ΔH sign or gas molecule count in the explanation.
Assuming that increasing pressure always favours the forward reaction.
Put it right by comparing the number of gas molecules on each side of the equation.
Treating a qualitative prediction as a calculation of new concentrations.
Put it right by stating only the direction of shift and the reason.

Exam tips

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.

Try the lesson

The start of the The effect of changing conditions on equilibrium (HT only): Students should be able to make qualitative predictions about the effect of changes on systems at equilibrium when given appropriate information. lesson, made from checked teaching notes for this specification point.

Have a go first

For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), which way does the position of equilibrium shift if the pressure is increased?

Tip: press 1 to 4 to choose

Listen

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. A strong answer earns marks for these points. 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. A common slip: Predicting the shift without referring to the given information. Instead, explicitly using the stated ΔH sign or gas molecule count in the explanation. Exam tip: Underline the change described in the question and the data provided before writing your prediction.

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