What mistakes do students make with The effect of changing concentration (HT only)?

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.

What are the exam tips for The effect of changing concentration (HT only) in Chemistry AQA GCSE?

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.

AQA · GCSE · Chemistry

Course version: 8462 · Version from Sept 2018

If the concentration of a product is decreased, more reactants will react…

Full specification wording

The effect of changing concentration (HT only): 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.

Checked against the AQA 8462 specification (opens in a new tab) on 21 September 2026.

Point 3 of 4
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

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.

What earns the marks

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.

Common mistakes and how to put them right

Thinking that decreasing product concentration stops the reaction entirely.
Put it right: The reaction continues in both directions but at unequal rates until equilibrium is re-established.
Assuming the product concentration returns to its original value.
Put it right: It only partially increases, so the new equilibrium differs from the original.
Confusing concentration changes with temperature changes.
Put it right: Changing concentration shifts the position of equilibrium but does not change the value of the equilibrium constant in the way a temperature change does.

Exam tips

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.

Try the lesson

The start of the The effect of changing concentration (HT only): If the concentration of a product is decreased, more reactants will react until equilibrium is reached again. lesson, made from checked teaching notes for this specification point.

Have a go first

A reversible reaction reaches equilibrium in a closed container. Some of the product is then removed so that its concentration decreases. According to Le Chatelier's principle, what happens to the position of equilibrium?

Tip: press 1 to 4 to choose

Listen

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. A strong answer earns marks for these points. 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. Exam tip: State clearly that the forward reaction is favoured and that equilibrium shifts to the right.

See it

A vertical flowchart with seven nodes connected by downward arrows. The first node reads 'Product concentration decreased' and leads to 'System no longer at equilibrium'. This leads to 'Forward reaction relatively faster than reverse reaction', which leads to 'More reactants react to replace removed product'. That leads to 'Forward and reverse rates become equal again', then to 'New equilibrium reached', and finally to 'Product concentration partially restored'. — It shows the sequence of steps by which decreasing a product's concentration shifts the position of equilibrium until a new equilibrium is reached.

Put it in order

Put these steps in the order they happen.

1. Product concentration partially restored
2. New equilibrium reached
3. Forward reaction relatively faster than reverse reaction
4. Forward and reverse rates become equal again
5. Product concentration decreased
6. System no longer at equilibrium
7. More reactants react to replace removed product

Key words

Say what each word means before you turn the card.

Le Chatelier's principle

Try answering aloud or in your head before turning the card.

Dynamic equilibrium

Try answering aloud or in your head before turning the card.

See every definition

Le Chatelier's principle: The principle used to explain how a system at equilibrium opposes a change, such as making more product when product concentration is decreased.
Dynamic equilibrium: The state reached when the rates of the forward and reverse reactions become equal again.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A vertical flowchart with seven nodes connected by downward arrows. The first node reads 'Product concentration decreased' and leads to 'System no longer at equilibrium'. This leads to 'Forward reaction relatively faster than reverse reaction', which leads to 'More reactants react to replace removed product'. That leads to 'Forward and reverse rates become equal again', then to 'New equilibrium reached', and finally to 'Product concentration partially restored'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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