The effect of changing concentration (HT only) — AQA GCSE Combined Science
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The effect of changing concentration (HT only) explained
A reversible reaction in a closed system reaches dynamic equilibrium when forward and reverse rates are equal, keeping all concentrations constant.
Read the full explanation
Changing the concentration of one reactant or product makes the rates unequal, temporarily disrupting equilibrium. The system then shifts to oppose the change until a new equilibrium is established. For example, in H₂(g) + I₂(g) ⇌ 2HI(g), adding H₂ increases the forward rate. More HI forms, and H₂ and I₂ concentrations fall until rates match again. Every substance's concentration changes during this adjustment. Note that predicting these effects is a Higher Tier only skill.
If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.
At dynamic equilibrium, the forward and reverse reaction rates are equal. Increasing the concentration of a reactant gives more reactant particles per unit volume, so the forward reaction rate increases while the reverse rate is momentarily unchanged. The forward reaction therefore dominates, forming more products. As products accumulate, the reverse reaction speeds up until the two rates are equal again and a new equilibrium is reached. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g), adding more N₂ increases the forward rate, so more NH₃ forms until equilibrium is re-established. The new equilibrium mixture contains more product than before, but not all the added reactant is converted. This concept is assessed at Higher Tier only.
If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.
Le Chatelier's principle explains how a system at equilibrium responds to a change in concentration. If a product's concentration is decreased, the system is no longer at equilibrium. The forward reaction becomes relatively faster than the reverse reaction, so more reactants react to form products. This continues until the forward and reverse reaction rates become equal again and a new equilibrium is established. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g), removing ammonia as it forms lowers the product concentration. The equilibrium shifts to the right, increasing the yield of ammonia. The position of equilibrium moves in the direction that opposes the change, so decreasing a product concentration shifts equilibrium towards the products.
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 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.
Your focus
- Describe what happens to a system at equilibrium when the concentration of one reactant or product is changed.
- Explain why the system is no longer at equilibrium immediately after a concentration change.
- Predict how the concentrations of all substances change as the system reaches a new equilibrium.
Show all 12 objectives
- Explain why increasing the concentration of a reactant increases the rate of the forward reaction.
- Describe how the system adjusts until equilibrium is reached again.
- Predict the effect of increasing reactant concentration on the amount of product formed at the new equilibrium.
- Describe how decreasing the concentration of a product affects the position of equilibrium.
- Explain the effect in terms of the relative rates of the forward and reverse reactions.
- Apply the principle to a given reversible reaction to predict the direction of the shift.
- Interpret tables or graphs showing equilibrium concentrations or yields.
- Predict the direction of equilibrium shift when a reactant or product concentration changes.
- Justify predictions by linking data trends to Le Chatelier's principle.
The effect of changing concentration (HT only) exam tips
Marking Points
- States that at equilibrium the forward and reverse reaction rates are equal and the concentrations of all substances remain constant.
- Explains that changing the concentration of one reactant or product makes the forward and reverse rates unequal, so the system is no longer at equilibrium.
- Describes the shift in the direction that reduces the effect of the change until a new equilibrium is reached.
- Recognises that the concentrations of all substances, not just the one changed, alter during the shift to the new equilibrium.
- Applies the idea to a named reversible reaction, such as H₂(g) + I₂(g) ⇌ 2HI(g) or N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
- Uses the terms 'dynamic equilibrium' and 'closed system' correctly when explaining the behaviour.
- States that increasing the concentration of a reactant increases the rate of the forward reaction.
- Explains that the reverse reaction rate is initially unchanged, so the forward reaction is faster than the reverse.
- Concludes that more products are formed until the rates become equal again and equilibrium is re-established.
- Recognises that the new equilibrium mixture contains a greater amount of product than the original equilibrium mixture.
- Applies the explanation to a given reversible reaction, identifying which substance is the added reactant and which substances are products.
- Uses the idea that the system shifts to reduce the effect of the added reactant.
- Decreasing the concentration of a product means the reverse reaction is initially slower than the forward reaction.
- The forward reaction is favoured, so more reactants react to form products.
- The position of equilibrium shifts to the right, towards the products.
- A new equilibrium is reached when the rates of the forward and reverse reactions are equal again.
- The shift opposes the change, so the system replaces some of the removed product.
- In N₂(g) + 3H₂(g) ⇌ 2NH₃(g), removing NH₃ shifts equilibrium to the right and increases NH₃ yield.
- 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.
Examiner Tips
- 💡When asked to explain the effect of a concentration change, always compare the forward and reverse rates before and after the change.
- 💡Use the phrase 'the equilibrium shifts to oppose the change' and then state the direction (forward or reverse) for the specific reaction given.
- 💡Remember that predicting the effects of concentration changes on equilibrium is assessed at Higher Tier only.
- 💡Link your answer to the reaction equation: name the reactant whose concentration increases and the product whose amount increases.
- 💡Use the phrase 'the equilibrium shifts to the right' or 'in the forward direction' and then explain why in terms of rates.
- 💡As this is a Higher Tier only topic, be prepared to explain the shift in terms of unequal forward and reverse reaction rates.
- 💡State the direction of the shift clearly, for example 'equilibrium shifts to the right', rather than only saying 'more products form'.
- 💡Link the shift to the relative rates of the forward and reverse reactions before the new equilibrium is reached.
- 💡Use the reaction equation to identify reactants and products before predicting the direction of the shift.
- 💡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.
Common Mistakes
- Thinking that only the concentration of the substance you changed alters. Correction: the shift changes the concentrations of all reactants and products until the new equilibrium is reached.
- Believing that equilibrium means the reaction has stopped. Correction: equilibrium is dynamic; both forward and reverse reactions continue at equal rates.
- Assuming the system returns to the original concentrations. Correction: a new equilibrium position is established with different concentrations from before the change.
- Thinking that all the added reactant is converted into product. Correction: only some is converted; a new equilibrium is reached with some added reactant remaining.
- Believing that the reverse reaction stops. Correction: the reverse reaction continues but is initially slower than the forward reaction.
- Confusing rate with yield. Correction: increasing reactant concentration increases both the rate at which equilibrium is approached and the equilibrium yield of products, but these are distinct ideas.
- Thinking equilibrium shifts to the left when a product is removed; correction: removing a product shifts equilibrium to the right to replace it.
- Believing the reaction stops when equilibrium is disturbed; correction: both reactions continue, but at different rates until equilibrium is re-established.
- Assuming that removing a product permanently stops the reverse reaction; correction: the reverse reaction slows down initially but speeds up again as the product concentration builds back up to the new equilibrium level.
- Ignoring the data and giving a generic answer; correction: quote or refer to the specific trend or value in the data.
- Mixing up reactants and products when reading a table or graph; correction: check the equation and label each substance carefully.
- Assuming a change in concentration permanently changes the rate of reaction; correction: the system adjusts until the rates of the forward and reverse reactions are equal again, establishing a new equilibrium position.