AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to interpret appropriate given data to predict the…
Full specification wording
The effect of changing concentration (HT only): Students should be able to interpret appropriate given data to predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium.
This skill requires you to use data, such as equilibrium concentrations or amounts, to predict how a change in concentration of a reactant or product affects a reversible reaction at equilibrium.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 4 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This skill requires you to use data, such as equilibrium concentrations or amounts, to predict how a change in concentration of a reactant or product affects a reversible reaction at equilibrium. First identify the change: an increase or decrease in the concentration of a reactant or product. Then apply Le Chatelier's principle. If a reactant concentration is increased, the forward reaction is favoured and equilibrium shifts to the right, producing more products. If a product concentration is increased, the reverse reaction is favoured and equilibrium shifts to the left, producing more reactants. If a reactant or product is removed, the equilibrium shifts in the direction that replaces it. Use the data to support your prediction, for example by comparing the relative amounts before and after the change.
What earns the marks
- Identify whether the changed substance is a reactant or a product.
- State whether its concentration has increased or decreased.
- Apply Le Chatelier's principle: the system shifts to oppose the change.
- Predict the direction of the shift: right if a reactant is added or a product removed; left if a product is added or a reactant removed.
- Use the given data to justify the prediction, for example by referring to changes in equilibrium amounts or concentrations.
- Describe the effect on the relative amounts of reactants and products at the new equilibrium.
Common mistakes and how to put them right
Mixing up the direction of the shift when a reactant is added.
Put it right: Adding a reactant shifts equilibrium to the right, not the left.
Ignoring the data and giving a generic answer.
Put it right: The prediction must be linked to the specific reaction and the information provided.
Thinking that adding a catalyst changes the position of equilibrium.
Put it right: A catalyst speeds up both directions equally and does not shift the equilibrium.
Exam tips
- Underline the substance whose concentration changes and label it as reactant or product before predicting.
- Use the data to compare the new equilibrium with the original, rather than simply stating the direction of the shift.
- Write a clear chain of reasoning: change → effect on rates → direction of shift → new equilibrium.
Try the lesson
The start of the The effect of changing concentration (HT only): Students should be able to interpret appropriate given data to predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium. lesson, made from checked teaching notes for this specification point.
Have a go first
A reaction P(g) + Q(g) ⇌ R(g) is at equilibrium. Data show that the concentration of P is increased while the temperature stays constant. What is the predicted effect on the amount of R at equilibrium?
Listen
This skill requires you to use data, such as equilibrium concentrations or amounts, to predict how a change in concentration of a reactant or product affects a reversible reaction at equilibrium. First identify the change: an increase or decrease in the concentration of a reactant or product. Then apply Le Chatelier's principle. If a reactant concentration is increased, the forward reaction is favoured and equilibrium shifts to the right, producing more products. If a product concentration is increased, the reverse reaction is favoured and equilibrium shifts to the left, producing more reactants. If a reactant or product is removed, the equilibrium shifts in the direction that replaces it. Use the data to support your prediction, for example by comparing the relative amounts before and after the change. A strong answer earns marks for these points. Identify whether the changed substance is a reactant or a product. State whether its concentration has increased or decreased. Apply Le Chatelier's principle: the system shifts to oppose the change. Predict the direction of the shift: right if a reactant is added or a product removed; left if a product is added or a reactant removed. Exam tip: Underline the substance whose concentration changes and label it as reactant or product before predicting.
Key words
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Data interpretation
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Le Chatelier's principle
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Equilibrium shifts
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Concentration changes
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See every definition
- Data interpretation
- Using given data, such as equilibrium concentrations or amounts, to predict how a change in concentration of a reactant or product affects a reversible reaction at equilibrium.
- Le Chatelier's principle
- The principle applied to predict the direction of an equilibrium shift, stating that the system shifts to oppose the change.
- Equilibrium shifts
- The movement of equilibrium to the right if a reactant is added or a product removed, or to the left if a product is added or a reactant removed.
- Concentration changes
- An increase or decrease in the concentration of a reactant or product, which must first be identified before applying Le Chatelier's principle.
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