AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
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 tables of equilibrium yields or graphs of concentration against time, to work out how a change in concentration affects a reversible reaction at…
- 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 tables of equilibrium yields or graphs of concentration against time, to work out how a change in concentration affects a reversible reaction at equilibrium. You must identify which substance has been added or removed, then apply Le Chatelier's principle. If a reactant concentration is increased, equilibrium shifts to the right to reduce the change, producing more products. If a product is removed, equilibrium shifts to the right. For example, in the Haber process N₂(g) + 3H₂(g) ⇌ 2NH₃(g), data showing a higher yield of ammonia when more nitrogen is added indicates a shift to the right. Always link your prediction to the data provided.
What earns the marks
- Identify from the data whether a reactant or product concentration has increased or decreased.
- Apply Le Chatelier's principle: the equilibrium shifts to oppose the change.
- Increasing a reactant concentration shifts equilibrium to the right, increasing product yield.
- Increasing a product concentration shifts equilibrium to the left, increasing reactant yield.
- Decreasing a product concentration shifts equilibrium to the right, increasing product yield.
- Use the reaction equation to name the substances whose concentrations change and the direction of the shift.
Common mistakes and how to put them right
Ignoring the data and giving a generic answer.
Put it right: Quote or refer to the specific trend or value in the data.
Mixing up reactants and products when reading a table or graph.
Put it right: Check the equation and label each substance carefully.
Assuming a change in concentration permanently changes the rate of reaction.
Put it right: The system adjusts until the rates of the forward and reverse reactions are equal again, establishing a new equilibrium position.
Exam tips
- Read the axes and headings of tables or graphs carefully before predicting the effect.
- Use the phrase 'the equilibrium shifts to the right/left' and then state which substance increases or decreases.
- Check that your prediction is consistent with the data trend, for example a higher product yield when more reactant is added.
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 reversible reaction reaches equilibrium: A + B ⇌ C + D. The concentration of reactant A is then increased. According to Le Chatelier's principle, in which direction does the equilibrium shift?
Listen
This skill requires you to use data, such as tables of equilibrium yields or graphs of concentration against time, to work out how a change in concentration affects a reversible reaction at equilibrium. You must identify which substance has been added or removed, then apply Le Chatelier's principle. If a reactant concentration is increased, equilibrium shifts to the right to reduce the change, producing more products. If a product is removed, equilibrium shifts to the right. For example, in the Haber process N₂(g) + 3H₂(g) ⇌ 2NH₃(g), data showing a higher yield of ammonia when more nitrogen is added indicates a shift to the right. Always link your prediction to the data provided. A strong answer earns marks for these points. Identify from the data whether a reactant or product concentration has increased or decreased. Apply Le Chatelier's principle: the equilibrium shifts to oppose the change. Increasing a reactant concentration shifts equilibrium to the right, increasing product yield. Increasing a product concentration shifts equilibrium to the left, increasing reactant yield. A common slip: Ignoring the data and giving a generic answer. Instead, quote or refer to the specific trend or value in the data. Exam tip: Read the axes and headings of tables or graphs carefully before predicting the effect.
Key words
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Le Chatelier's principle
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Interpreting equilibrium data
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Concentration changes and equilibrium position
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Reversible reactions and dynamic equilibrium
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See every definition
- Le Chatelier's principle
- The equilibrium shifts to oppose the change when a reactant or product concentration changes.
- Interpreting equilibrium data
- Using data, such as tables of equilibrium yields or graphs of concentration against time, to work out how a change in concentration affects a reversible reaction at equilibrium.
- Concentration changes and equilibrium position
- Increasing a reactant concentration shifts equilibrium to the right, increasing product yield, while increasing a product concentration shifts equilibrium to the left, increasing reactant yield.
- Reversible reactions and dynamic equilibrium
- The system adjusts until the rates of the forward and reverse reactions are equal again, establishing a new equilibrium position.
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