AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
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 explains how a system at equilibrium responds to a change in concentration.
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- 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.
Common mistakes and how to put them right
Thinking equilibrium shifts to the left when a product is removed.
Put it right: Removing a product shifts equilibrium to the right to replace it.
Believing the reaction stops when equilibrium is disturbed.
Put it right: Both reactions continue, but at different rates until equilibrium is re-established.
Assuming that removing a product permanently stops the reverse reaction.
Put it right: The reverse reaction slows down initially but speeds up again as the product concentration builds back up to the new equilibrium level.
Exam tips
- 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.
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 is at equilibrium. The concentration of one of the products is decreased. What happens to the amount of reactants that react?
Listen
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. A strong answer earns marks for these points. 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. A common slip: Thinking equilibrium shifts to the left when a product is removed. Instead, removing a product shifts equilibrium to the right to replace it. Exam tip: State the direction of the shift clearly, for example 'equilibrium shifts to the right', rather than only saying 'more products form'.
See it
Put it in order
Put these steps in the order they happen.
- 1. System is no longer at equilibrium
- 2. Reverse reaction is initially slower than the forward reaction
- 3. Forward reaction is favoured
- 4. More reactants react to form products
- 5. Position of equilibrium shifts to the right, towards the products
- 6. System replaces some of the removed product
- 7. New equilibrium reached when forward and reverse rates are equal again
- 8. Concentration of a product is decreased
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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