3.4.12Movement and Interactions
Factors Affecting The Position Of Equilibrium (HT Only) — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 7 parts Lesson 53 of 59
Part 1 of 5Le Chatelier's principle (HT only)
The principle
- The principle If a change is made to the conditions of a system at equilibrium, the system shifts so as to oppose the change.
- Equilibrium is re-established, but with a different mixture of reactants and products.
Shift to the right
- Shift to the right More product is formed, so the yield increases.
Shift to the left
- Shift to the left More reactant is formed, so the yield of product decreases.
Part 2 of 5Concentration
1Add more reactant
- Add more reactant The system is pushed away from equilibrium by the extra reactant.
2Opposes by using it up
- Opposes by using it up The forward reaction speeds up relative to the reverse.
3Shift to the right
- Shift to the right More product forms.
- Removing product has the same effect;
- removing reactant shifts the equilibrium to the left.
- Removing product has the same effect;
Part 3 of 5Temperature
Changing temperature
Temperature raised
- Temperature raised The system opposes by absorbing heat, so equilibrium shifts in the endothermic direction.
Temperature lowered
- Temperature lowered The system opposes by releasing heat, so equilibrium shifts in the exothermic direction.
Example
- Example If the forward reaction is exothermic, raising the temperature favours the reverse reaction and the yield of product falls.
Part 4 of 5Pressure (gases only)
1
Count the moles of gas molecules on each side of the balanced equation. Solids and liquids are ignored.
2
If pressure is increased, the equilibrium shifts towards the side with fewer gas molecules, which lowers the pressure.
3
If pressure is decreased, the equilibrium shifts towards the side with more gas molecules.
4
If both sides have the same number of gas molecules, a pressure change has no effect on the position of equilibrium.
Part 5 of 5Worked example: N2(g) + 3H2(g) ⇌ 2NH3(g), forward reaction exothermic
| Change | Shift | Effect on yield of ammonia |
|---|---|---|
| Increase pressure (4 mol gas to 2 mol) | Right | Increases |
| Increase temperature | Left (endothermic direction) | Decreases |
| Add more nitrogen | Right | Increases |
| Remove ammonia as it forms | Right | Increases |
| Add a catalyst | No shift | No change; equilibrium is reached sooner |
Key terms
| Term | Meaning |
|---|---|
| Le Chatelier's principle | A system at equilibrium responds to a change in conditions by opposing that change. |
| Yield | The amount of product obtained from a reaction. |
| Shift to the right | A change in the equilibrium mixture that gives more product. |
| Shift to the left | A change in the equilibrium mixture that gives more reactant. |
| Endothermic direction | The direction of reaction that takes in heat energy. |
Avoid these mistakes
1
Saying that raising the temperature always increases the yield. It favours the endothermic direction only.
2
Counting solids or liquids when working out the effect of pressure; only gas molecules count.
3
Claiming that a catalyst changes the position of equilibrium. It speeds up both directions equally.
4
Explaining the shift without saying it opposes the change.
5
Mixing up left and rightthe left side is the reactants and the right side is the products.
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