6.2.4Chemical Change: Rate and Extent
The Effect of Changing Conditions on Equilibrium — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 13 of 16
Part 1 of 5Equilibrium recap
Dynamic equilibrium
- Dynamic equilibrium In a closed system a reversible reaction reaches equilibrium when the forward and reverse reactions happen at exactly the same rate.
- Reactions have not stopped; the amounts of reactants and products simply stay constant.
Closed system
- Closed system Nothing can enter or leave, so no gas escapes.
- Equilibrium cannot be reached in an open container.
Position of equilibrium
- Position of equilibrium This describes how far the reaction lies to the right (more products) or to the left (more reactants).
Part 2 of 5Le Chatelier's principle (HT only)
1A condition is changed
- A condition is changed Concentration, temperature or pressure of a system at equilibrium is altered.
2The system is disturbed
- The system is disturbed The forward and reverse rates are no longer equal.
3It opposes the change
- It opposes the change The position shifts in the direction that works against the change you made.
4New equilibrium
- New equilibrium The rates become equal again, with a different mixture of reactants and products.
Part 3 of 5Predicting the shift (HT only)
| Change made | System tries to | Shift |
|---|---|---|
| Add more reactant | Use up the extra reactant | Towards the products |
| Remove some product | Make more product | Towards the products |
| Raise the temperature | Absorb the extra heat | In the endothermic direction |
| Lower the temperature | Release more heat | In the exothermic direction |
| Raise the pressure (gases) | Reduce pressure by making fewer gas molecules | Towards the side with fewer gas molecules |
| Lower the pressure (gases) | Raise pressure by making more gas molecules | Towards the side with more gas molecules |
Part 4 of 5Applying it in industry (HT only)
Choosing conditions
Yield
- Yield Conditions that push the position towards products give a higher yield.
Rate
- Rate Conditions that favour the products are not always fast, so a compromise is needed.
Cost and safety
- Cost and safety High pressures need strong, expensive equipment and are more dangerous.
Part 5 of 5Catalysts
No change in position
- No change in position A catalyst speeds the forward and reverse reactions equally, so equilibrium is reached sooner but the position does not move.
Key terms
| Term | Meaning |
|---|---|
| Dynamic equilibrium | A state in which forward and reverse reactions occur at equal rates in a closed system. |
| Position of equilibrium | The relative amounts of reactants and products at equilibrium. |
| Le Chatelier's principle | When a condition of a system at equilibrium is changed, the system shifts to oppose the change. |
| Closed system | A system where no matter can enter or leave. |
| Yield | The amount of product obtained compared with the maximum possible. |
Avoid these mistakes
1
Saying the reactions stop at equilibrium; both directions continue at equal rates.
2
Claiming that a catalyst moves the position of equilibrium.
3
Forgetting that pressure only affects equilibria involving gases with different numbers of molecules on each side.
4
Describing the shift as 'to the side that gets cooled' instead of in terms of the endothermic or exothermic direction.
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