The effect of changing concentration (HT only) — AQA GCSE Chemistry
Verified courseTeaching note checked against the official specification.
The effect of changing concentration (HT only) explained
The effect of changing concentration (HT only) covers 4 specification points, including If the concentration of one of the reactants… and If the concentration of a reactant is….
Read the full explanation
Open a point below for its explanation and revision guidance.
If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.
When the concentration of a reactant or product in a dynamic equilibrium is changed, the system is temporarily no longer at equilibrium. The rates of the forward and reverse reactions become unequal, so the concentrations of all substances change until a new equilibrium is established. For example, in the equilibrium H₂(g) + I₂(g) ⇌ 2HI(g), adding more H₂ increases the forward reaction rate, so H₂ and I₂ are used up and more HI is formed until equilibrium is restored. Removing a product has a similar effect, shifting equilibrium to replace it. The new equilibrium may have different concentrations from the original, but the system remains dynamic: both forward and reverse reactions continue at equal rates.
If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.
Increasing the concentration of a reactant disturbs an equilibrium by making the forward reaction faster than the reverse reaction. The system responds by shifting equilibrium to the right, so more products are formed until the rates of the forward and reverse reactions become equal again. For example, in the equilibrium CH₃COOH(aq) + C₂H₅OH(aq) ⇌ CH₃COOC₂H₅(aq) + H₂O(l), adding more ethanoic acid increases the rate of the forward reaction, producing more ethyl ethanoate until a new equilibrium is established. The final yield of product is greater than before the change, although some of the added reactant remains. This is a qualitative prediction: the direction of shift and the increase in product formation are stated without numerical calculation.
If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.
Le Chatelier's principle predicts how a system at equilibrium responds to a change in concentration. If the concentration of a product is decreased, the system is no longer at equilibrium. The forward reaction, which forms products from reactants, becomes relatively faster than the reverse reaction. As a result, more reactants react to replace some of the removed product. This continues until the rates of the forward and reverse reactions become equal again and a new equilibrium is established. For example, in the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), removing ammonia by condensing it shifts the position of equilibrium to the right, so more nitrogen and hydrogen react and more ammonia is formed overall.
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. 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.
Your focus
- Explain why changing the concentration of a reactant or product means the system is no longer at equilibrium.
- Describe how the concentrations of all substances change until a new equilibrium is reached.
- Relate changes in concentration to the rates of the forward and reverse reactions in a dynamic equilibrium.
Show all 12 objectives
- Explain why increasing the concentration of a reactant leads to more product formation.
- Describe how the system returns to equilibrium after a reactant concentration is increased.
- Predict the effect of increasing a reactant concentration on the yield of a named product using a balanced equation.
- 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 Le Chatelier's principle to a named reversible reaction when a product is removed.
- Interpret given equilibrium data to identify changes in concentration.
- Predict the direction of equilibrium shift when a reactant or product concentration changes.
- Justify predictions using Le Chatelier's principle and the supplied data.
The effect of changing concentration (HT only) exam tips
Marking Points
- State that changing the concentration of a reactant or product disturbs the equilibrium so the system is temporarily not at equilibrium.
- Explain that the rates of the forward and reverse reactions become unequal immediately after the change.
- Describe that the concentrations of all substances change until a new equilibrium is reached.
- Use a balanced equation to show how adding a reactant increases the forward reaction and leads to more product formation.
- Recognise that at the new equilibrium both forward and reverse reactions continue at equal rates, but the concentrations may differ from the original equilibrium.
- State that increasing the concentration of a reactant increases the rate of the forward reaction relative to the reverse reaction.
- Explain that equilibrium shifts to the right, so more products are formed.
- Describe that the shift continues until the forward and reverse reaction rates become equal again at a new equilibrium.
- Use a balanced equation to identify the reactant whose concentration is increased and the product whose yield increases.
- Recognise that the new equilibrium has a greater amount of product than the original equilibrium, but not all added reactant is converted.
- A decrease in product concentration means the reverse reaction is initially slower than the forward reaction.
- The forward reaction is favoured, so more reactants are converted into products.
- The position of equilibrium shifts to the right, towards the products.
- The system continues to adjust until the forward and reverse rates are equal again.
- A new equilibrium is reached in which the product concentration is partially restored.
- The shift can be explained using Le Chatelier's principle: the system opposes the change by making more product.
- 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.
Examiner Tips
- 💡Use the phrase the system is no longer at equilibrium to show understanding of the disturbance.
- 💡Link the change in concentration to the relative rates of the forward and reverse reactions.
- 💡State that a new equilibrium is reached and that concentrations of all substances change, not just the one added or removed.
- 💡Name the reactant added and the product whose yield increases, using the balanced equation.
- 💡Use the phrase shifts to the right and link it to more products being formed until equilibrium is reached again.
- 💡Avoid saying the reaction goes to completion; state that a new equilibrium is established.
- 💡State clearly that the forward reaction is favoured and that equilibrium shifts to the right.
- 💡Use the phrase 'until equilibrium is reached again' to show understanding that the system is dynamic.
- 💡Link the explanation to Le Chatelier's principle by saying the system acts to oppose the decrease in product concentration.
- 💡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.
Common Mistakes
- Thinking that only the concentration of the added substance changes; in fact the concentrations of all substances change as the system re-equilibrates.
- Believing that equilibrium stops when a concentration is changed; the system is temporarily not at equilibrium but re-establishes equilibrium.
- Assuming the new equilibrium has the same concentrations as before; the position of equilibrium shifts, so concentrations differ.
- Thinking that all of the added reactant is converted into product; some remains at the new equilibrium.
- Believing that the equilibrium constant changes when a reactant concentration is increased; the position of equilibrium shifts but the equilibrium constant is unchanged at constant temperature.
- Stating that the reverse reaction stops; both reactions continue, but the forward reaction is initially faster.
- Thinking that decreasing product concentration stops the reaction entirely; the reaction continues in both directions but at unequal rates until equilibrium is re-established.
- Assuming the product concentration returns to its original value; it only partially increases, so the new equilibrium differs from the original.
- Confusing concentration changes with temperature changes; changing concentration shifts the position of equilibrium but does not change the value of the equilibrium constant in the way a temperature change does.
- Mixing up the direction of the shift when a reactant is added; adding a reactant shifts equilibrium to the right, not the left.
- Ignoring the data and giving a generic answer; the prediction must be linked to the specific reaction and the information provided.
- Thinking that adding a catalyst changes the position of equilibrium; a catalyst speeds up both directions equally and does not shift the equilibrium.