Short on time? This page is the recap. For explanations, a concentration–time graph and worked examples, use the full study guide.
Recap
- A stress upsets an equilibrium: the forward and reverse rates are no longer equal.
- Le Châtelier’s principle: the system shifts in the direction that partly counteracts the stress, then settles at a new equilibrium.
- The shift is partial. Added species are not completely used up; removed species are not completely replaced.
- A shift shows up in measurable properties: colour, pH and temperature.
Key relationships
| Stress | Shift | Notes |
|---|---|---|
| Add a reactant or product | Away from it (uses it up) | Precipitating or reacting a species away counts as removing it |
| Remove a reactant or product | Towards it (replaces it) | |
| Heat | Endothermic direction | Write heat into the equation; K changes |
| Cool | Exothermic direction | K changes |
| Smaller gas volume (higher pressure) | Side with fewer gas molecules | No shift if numbers are equal |
| Larger gas volume | Side with more gas molecules | |
| Dilute a solution | Side with more dissolved particles | Do not count water as the solvent |
| Catalyst, extra solid or pure liquid, unreactive gas at constant volume | No shift |
Assumptions behind the predictions
- The system was at equilibrium before the stress.
- Only one condition changes at a time; the others stay fixed (for example, temperature stays constant when the volume changes).
- You know the sign of ΔH for temperature questions, or can deduce it from an observation.
Mistakes to avoid
- Saying K changes when you add a species. Only temperature changes K.
- Counting solids, pure liquids or solvent water when you count particles.
- Forgetting the instant effect. Compressing a coloured gas darkens it at once, before any shift.
- Saying the system returns to the old concentrations. It reaches a new equilibrium.
- Assuming heat always shifts right. Check the sign of ΔH.
- Describing the shift but not the observation when the question asks what you would see.
Quick self-check
- For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH < 0, which way does the system shift when the volume is decreased? (Right: 3 gas molecules → 2.)
- Same system: which way does it shift when it is heated? (Left: the reverse reaction is endothermic.)
- Does adding solid CaCO₃ to CaCO₃(s) ⇌ CaO(s) + CO₂(g) change the CO₂ pressure at equilibrium? (No: the solid is not in the expression.)
Next: practice questions.