The effect of changing concentration (HT only) — AQA GCSE Chemistry
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The effect of changing concentration (HT only) explained
Increasing the concentration of a reactant disturbs an equilibrium by making the forward reaction faster than the reverse reaction.
Read the full explanation
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.
Your focus
- 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.
The effect of changing concentration (HT only) exam tips
Marking Points
- 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.
Examiner Tips
- 💡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.
Common Mistakes
- 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.