Revision notes for AQA GCSE Chemistry The effect of changing concentration (HT only). Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
The effect of changing concentration (HT only)
What you'll learn
- What dynamic equilibrium means in a reversible reaction.
- How changing concentration makes an equilibrium mixture adjust.
- How to predict whether the equilibrium shifts towards reactants or products.
- How to interpret simple concentration data and graphs.
Start point: reversible reactions
A reversible reaction is a reaction where products can react to reform the original reactants. We show this using the equilibrium arrow ⇌, not the one-way arrow →.
For example, in the Haber process:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
The forward reaction is the reaction as written from left to right: nitrogen and hydrogen forming ammonia.
The reverse reaction is the reaction from right to left: ammonia breaking down to reform nitrogen and hydrogen.
Dynamic equilibrium
For a reversible reaction to reach equilibrium, it must usually be in a closed system. This means no substances can enter or leave.
Definition
Dynamic equilibrium
A reversible reaction is at dynamic equilibrium when the forward and reverse reactions happen at the same rate, so the concentrations of reactants and products stay constant.
The word dynamic matters. The reactions have not stopped. Particles are still reacting in both directions, but the two directions balance each other.
Common Mistake
Equilibrium does not mean equal amounts
At equilibrium, the concentrations of reactants and products are constant, not necessarily equal. You might have lots of product and very little reactant, or the other way around.
What is concentration?
Definition
Concentration
Concentration is the amount of substance dissolved or present in a certain volume. It is often measured in moles per cubic decimetre, written as mol/dm³.
A higher concentration means there are more particles in the same volume. In a reaction mixture, this usually means more frequent successful collisions involving that substance.
This bit is Higher Tier because you need to reason about how the whole equilibrium mixture responds when one concentration changes.
The key idea: changing concentration disturbs equilibrium
If the concentration of one reactant or product is changed, the system is no longer at equilibrium.
The forward and reverse reactions are no longer happening at the same rate. The mixture then changes until a new equilibrium is reached.
Definition
Equilibrium position
The equilibrium position describes the relative amounts of reactants and products present at equilibrium. If it “shifts right”, more products form. If it “shifts left”, more reactants form.
Key Idea
The concentration rule
When a concentration is changed, the equilibrium shifts in the direction that reduces the effect of that change.
This is an application of Le Châtelier’s principle: if a change is made to a system at equilibrium, the system responds to oppose the change as far as possible.
Increasing the concentration of a reactant
Suppose you increase the concentration of a reactant.
For a reaction like:
A(aq) + B(aq) ⇌ C(aq) + D(aq)
adding more A(aq) gives the forward reaction extra reactant to use. The equilibrium shifts to the right, towards the products.
So:
- more products are formed
- the added reactant is partly used up
- the system reaches a new equilibrium
The added reactant does not usually return exactly to its original concentration. It falls from the sudden increase, but the final concentration may still be higher than before.
Example
Adding a reactant in the Haber process
For the equilibrium:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Predict the effect of increasing the concentration of H₂(g).
-
Identify which side H₂(g) is on. It is a reactant, so it is on the left-hand side of the equation.
-
Increasing a reactant makes the system shift away from that added reactant. The equilibrium shifts to the right, towards the products.
-
In the forward reaction, N₂(g) and H₂(g) react to form NH₃(g), so more ammonia is produced until a new equilibrium is reached.
Decreasing the concentration of a product
Now suppose you decrease the concentration of a product.
For:
A(aq) + B(aq) ⇌ C(aq) + D(aq)
removing some C(aq) means there is less product than before. The system responds by forming more product. So the equilibrium shifts to the right.
That means more reactants react until equilibrium is reached again.
This is exactly the kind of statement you need for this topic: if the concentration of a product is decreased, more reactants will react to form products.
The four concentration changes
You can predict all common cases by asking: “Which side has been changed, and should the system use it up or replace it?”
| Change made | Equilibrium shifts | Overall effect |
|---|---|---|
| Increase a reactant | Towards products | More products form |
| Decrease a reactant | Towards reactants | More reactant is formed |
| Increase a product | Towards reactants | More reactants form |
| Decrease a product | Towards products | More product is formed |
Tip
Use the side-of-the-equation test
If a substance is increased, the equilibrium shifts away from that side. If a substance is decreased, the equilibrium shifts towards that side to replace some of it.
What concentration-time graphs show
On a concentration-time graph, the substance you directly change shows an immediate vertical jump or drop. The other substances do not jump instantly; they change gradually as the equilibrium shifts.
The flat parts of the graph show equilibrium, because the concentrations are constant. After the change, the graph eventually becomes flat again at the new equilibrium.
Common Mistake
Mixing up the instant change and the response
Only the substance that was added or removed changes instantly. The shift in equilibrium happens afterwards and causes the other concentrations to change gradually.
Interpreting given data
In exam questions, you may be given a table or graph and asked to predict or explain what happens. Focus on the pattern of changes, not just one number.
Example
Interpreting a concentration table
For the equilibrium:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
The table shows concentrations before and after a change.
| Time | N₂ concentration in mol/dm³ | H₂ concentration in mol/dm³ | NH₃ concentration in mol/dm³ |
|---|---|---|---|
| At equilibrium | 0.40 | 0.60 | 0.50 |
| Immediately after change | 0.40 | 1.00 | 0.50 |
| New equilibrium | 0.32 | 0.75 | 0.66 |
Explain what change was made and how the equilibrium responded.
-
Compare the first two rows. Only H₂ changes immediately, increasing from 0.60 mol/dm³ to 1.00 mol/dm³, so H₂(g) was added.
-
H₂(g) is a reactant. Increasing a reactant shifts the equilibrium to the right, towards the products.
-
Check the new equilibrium row. N₂(g) decreases, H₂(g) falls from its instant high value, and NH₃(g) increases. This matches a shift to the right, forming more ammonia.
Why the system does not just “undo” the change completely
The equilibrium shift reduces the effect of the change, but it does not usually reverse it completely.
For example, if you add H₂(g) to the Haber equilibrium, some of the added H₂(g) reacts with N₂(g) to form more NH₃(g). However, the final H₂(g) concentration may still be higher than it was at the original equilibrium.
The important GCSE idea is the direction of the shift and which substances increase or decrease as the new equilibrium forms.
Key Idea
What you must be able to predict
If a reactant concentration is increased, more products are formed. If a product concentration is decreased, more reactants react to form products. In both cases, the equilibrium shifts towards the products.
Exam technique
In the exam
-
Write down which side of the equation the changed substance is on: reactant side or product side.
-
Decide whether it was increased or decreased: increased means shift away; decreased means shift towards.
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State the result clearly using the reaction: say which substances are used up and which are formed until a new equilibrium is reached.
Self review
Check yourself
-
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), what happens to the amount of NH₃(g) if N₂(g) is added?
-
For A(aq) ⇌ B(aq), which way does the equilibrium shift if B(aq) is removed?
-
On a concentration-time graph, what does a sudden vertical jump in one line tell you?
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