When one condition of a system at equilibrium is changed, the position of equilibrium shifts to oppose the change. You can use this to predict whether the amount of product rises or falls. Questions usually name the change exactly, such as “pressure is increased”, so the task is to apply one rule to the right equation.
This is the core skill of reversible changes and equilibrium. Check the current Cambridge IGCSE Chemistry 0620 page, and ask your teacher which predictions your route expects.
The six predictions
| Change | The position shifts towards… |
|---|---|
| Increase concentration of a substance | the side that uses it up |
| Decrease concentration of a substance | the side that makes more of it |
| Increase pressure (gases) | the side with fewer gas molecules |
| Decrease pressure (gases) | the side with more gas molecules |
| Increase temperature | the endothermic direction |
| Decrease temperature | the exothermic direction |
A catalyst appears on this list only to be ruled out: it does not shift the position. That is explained in separating rate change from equilibrium position.
How to apply them, step by step
- Write the balanced equation with state symbols and the energy sign of the forward reaction.
- Identify the stated change: concentration, pressure or temperature.
- Choose the matching rule. For pressure, count gas molecules on each side. For temperature, find the endothermic direction.
- State the direction as “towards the products” or “towards the reactants”.
- State the consequence for the amount or yield of the named substance.
Worked example
Sulfur dioxide and oxygen form sulfur trioxide in a reversible reaction. The forward reaction is exothermic.
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
Predict the effect on the equilibrium position of (a) increasing pressure, (b) increasing temperature, (c) adding more oxygen.
(a) Pressure. Left side gas molecules: 2 + 1 = 3. Right side: 2. Higher pressure favours the side with fewer gas molecules. The position shifts towards the products, so more SO₃ forms.
(b) Temperature. The forward reaction is exothermic, so the reverse is endothermic. Higher temperature favours the endothermic direction. The position shifts towards the reactants, so less SO₃ forms.
(c) Oxygen. More O₂ is a higher reactant concentration. The system uses it up, so the position shifts towards the products, forming more SO₃.
Notice that (a) and (b) push in opposite directions. That is exactly why industry needs a compromise, which is the subject of interpreting a yield-rate compromise.
The mistake to watch for
A frequent slip is to apply the pressure rule when the gas counts match.
Mistaken answer: “For H₂(g) + I₂(g) ⇌ 2HI(g), increasing pressure shifts the position towards the products because the pressure rule says so.”
Both sides have 2 gas molecules. The pressure rule has nothing to decide.
The correction: count first. The left has 1 + 1 = 2 gas molecules and the right has 2. Changing pressure does not shift the position here, though it may still change how quickly equilibrium is reached.
Check yourself
1. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), what happens to the position if pressure is increased?
Show answer
Left: 1 + 3 = 4 gas molecules. Right: 2. Higher pressure favours fewer gas molecules, so the position shifts towards the products and more NH₃ forms.
2. For N₂O₄(g) ⇌ 2NO₂(g), the forward reaction is endothermic. What does cooling the mixture do to the amount of NO₂?
Show answer
Cooling favours the exothermic direction, which is the reverse reaction here. The position shifts towards N₂O₄, so the amount of NO₂ decreases and the mixture becomes paler.
3. In the equilibrium H₂(g) + I₂(g) ⇌ 2HI(g), extra hydrogen is added. Which way does the position shift?
Show answer
Extra H₂ raises a reactant concentration. The system uses it up, so the position shifts towards the products, forming more HI.
Where this leads next
The next lesson, separating rate change from equilibrium position, tackles the most common confusion in this topic. You can test the equation side with the equation balance reasoning trainer.
If your predictions are still a coin toss on unfamiliar equations, our teachers in online one-to-one Chemistry tuition can build the checklist with you on fresh examples.