GCSE-readyChanging the position of equilibrium
When conditions change, a reaction at equilibrium shifts to oppose the change. This lets chemists predict how to get more product.
Dynamic equilibrium
Predicting the shift
Temperature up: shifts in the endothermic direction. Temperature down: exothermic direction.
Pressure up (gases): shifts to the side with fewer gas molecules.
Concentration of a reactant up: shifts to the right, making more products.
Catalyst: does not change the position; it only makes equilibrium be reached faster, because it speeds up both reactions equally.
| Change | N2 + 3H2 ⇌ 2NH3 (forward exothermic) |
|---|---|
| Raise temperature | Shifts left: less ammonia |
| Raise pressure | Shifts right (4 gas molecules → 2): more ammonia |
| Add more nitrogen | Shifts right: more ammonia |
| Add a catalyst | No change in position; equilibrium reached sooner |
A colour example
NO2 is brown and N2O4 is colourless.
Heating the mixture shifts it left (endothermic direction), so it turns darker brown. Increasing the pressure shifts it right (2 molecules → 1), so it becomes paler.
For 2SO2(g) + O2(g) ⇌ 2SO3(g), the forward reaction is exothermic. Predict the effect of raising the pressure on the amount of SO3.
- Count gas molecules: left 2 + 1 = 3; right 2.
- Higher pressure favours the side with fewer gas molecules.
- So the equilibrium shifts right.
Answer: More SO3 forms, because the position shifts to the side with fewer gas molecules.
Equilibrium shifts to oppose a change: higher temperature favours the endothermic direction, higher pressure favours fewer gas molecules, more reactant favours the products. A catalyst does not change the position, it only gets there faster.
- Endothermic
- A reaction that takes in heat energy from the surroundings, so the temperature falls. ΔH is positive.
- Catalyst
- A substance that speeds up a reaction without being used up, by providing a pathway with a lower activation energy.
- Reversible reaction
- A reaction that can go both forwards and backwards, shown by ⇌.
- Dynamic equilibrium
- In a reversible reaction in a closed system, the point where the forward and backward reactions happen at the same rate, so amounts stay constant.
- Exothermic
- A reaction that gives out heat energy to the surroundings, so the temperature rises. ΔH is negative.
- Concentration
- The amount of solute in a given volume of solution, in g dm^{-3} or mol dm^{-3}.
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N2 + 3H2 ⇌ 2NH3 is exothermic in the forward direction. What happens to the yield of ammonia if the temperature is lowered?
- It stays the same
- It decreases
- It becomes zero
- It increases
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Answer: It increases
Lower temperature favours the exothermic forward reaction.
For N2(g) + 3H2(g) ⇌ 2NH3(g), what is the effect of raising the pressure?
- No change
- The position shifts right, towards fewer gas molecules
- The reaction stops
- The position shifts left
Show the answer
Answer: The position shifts right, towards fewer gas molecules
Higher pressure favours the side with fewer gas molecules.
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- Pearson Edexcel GCSE Chemistry (1CH0) specification, topic 4.13, 4.14, 4.17
- Pearson Edexcel International GCSE Chemistry (4CH1) specification, topic 3.19C, 3.20C, 3.21C, 3.22C
Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.
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