5.3.4Additional lessons
Le Chatelier’s Principle — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 112 of 172
Part 1 of 4The Principle
The principle is used to decide how to improve the yield of a reversible reaction.
Statement
- When a system at equilibrium is disturbed, the position shifts to oppose the change and partly cancel its effect.
Shift right
- A shift to the right means more products form until a new equilibrium is reached.
Shift left
- A shift to the left means more reactants form until a new equilibrium is reached.
Prediction only
- The principle predicts the direction of the shift;
- it does not explain why, and it gives no information about rates.
Part 2 of 4Effects of Each Change
Effects of Each Change
Concentration
- Adding a reactant (or removing a product) shifts the position to the right;
- adding a product shifts it to the left.
Pressure
- Increasing the pressure of a gas mixture favours the side with fewer gas molecules;
- it has no effect if both sides have equal moles of gas.
Temperature
- Raising the temperature favours the endothermic direction and lowering it favours the exothermic direction;
- this also changes K.
Catalyst
- A catalyst speeds up the forward and reverse reactions equally, so the position and K do not change, only the time to reach equilibrium.
| Change | Shift | Effect on K |
|---|---|---|
| Add reactant | Right | None |
| Increase pressure | Side with fewer gas moles | None |
| Increase temperature | Endothermic direction | Changes |
| Add catalyst | No shift | None |
Part 3 of 4Application: The Haber Process
1
The equation is N₂(g) + 3H₂(g) ⇌ 2NH₃(g), with ΔH = −92 kJ mol⁻¹, so the forward reaction is exothermic.
2
Four moles of gas on the left become two on the right, so high pressure favours ammonia and is used at about 200 atm.
3
Low temperature would favour the exothermic forward reaction and give a higher yield, but it makes the reaction too slow.
4
A compromise temperature of about 450 °C is therefore used, giving a reasonable yield at an acceptable rate.
5
An iron catalyst speeds up the process, and ammonia is removed as a liquid so that the position keeps shifting to the right.
Part 4 of 4Exam Checklist & Tips
5.3.4 – Key Points for the Exam
- Le Chatelier's principle A system at equilibrium responds to a disturbance by shifting to oppose it.
- Yield The amount of product obtained compared with the theoretical maximum.
- Compromise conditions Conditions chosen to balance yield against reaction rate and cost.
- Position of equilibrium The relative amounts of reactants and products in the mixture.
Common Mistakes
- Saying that a catalyst increases the yield of an equilibrium reaction.
- Claiming that pressure affects an equilibrium with equal gas moles on both sides.
- Thinking that changing the concentration changes the value of K.
- Forgetting that temperature changes both the position and the value of K.
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3 & IGCSE —
Kc
- : Kc! (ΔH < 0) Kc. (ΔH > 0) Kc.
- : ( ).
- : Kc .