1.7.2Physical Chemistry
Le Chatelier’s Principle — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 36 of 63
Part 1 of 6The Principle
Statement
- If a change is made to a system at equilibrium, the position of equilibrium shifts to oppose that change.
Position of equilibrium
- This describes the relative amounts of reactants and products in the equilibrium mixture.
Shift right
- Equilibrium moves towards the products, which increases the yield of products.
Shift left
- Equilibrium moves towards the reactants, which decreases the yield of products.
Rate
- The principle predicts the direction of change and says nothing about how fast it occurs.
Part 2 of 6Effect of Changes
Concentration
- Increasing the concentration of a reactant shifts the position to the right, using up the added substance.
Pressure
- Increasing pressure shifts equilibrium towards the side with fewer moles of gas, reducing the pressure.
Temperature
- Increasing temperature shifts the equilibrium in the endothermic direction, absorbing the extra heat.
Catalyst
- A catalyst has no effect on the position of equilibrium;
- it only increases the rate at which equilibrium is reached.
Equal moles of gas
- If both sides have the same number of gas moles, pressure has no effect on the position.
| Change | Equilibrium shifts | Value of K |
|---|---|---|
| Add reactant | Right | Unchanged |
| Increase pressure | Side with fewer gas moles | Unchanged (Kc, Kp) |
| Increase temperature | Endothermic direction | Changes |
| Add catalyst | No shift | Unchanged |
Part 3 of 6Industrial Compromises
Industrial Compromises
Haber process
- N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is exothermic, ΔH = −92 kJ mol⁻¹, with 4 moles of gas on the left and 2 on the right.
High pressure
- Increases yield but is costly and needs strong equipment, so about 200 atm is used.
Temperature
- A low temperature favours the yield but gives a slow rate, so about 450 °C with an iron catalyst is a compromise.
Removal of product
- Ammonia is liquefied and removed, so unreacted gases are recycled to improve overall conversion.
Part 4 of 6Answering Questions
1
Identify the change made and the direction that would oppose it.
2
State which direction the equilibrium shifts, using the terms 'to the right' or 'to the left'.
3
Link the shift to the enthalpy sign or the number of gas moles when explaining temperature or pressure effects.
4
State the effect on the yield of the product and, if asked, on the value of K.
5
For a catalyst, state that the yield is unchanged but equilibrium is reached more quickly.
Part 5 of 6Worked Example
Question For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹, predict the effect on the yield of SO₃ of (a) increasing the pressure and (b) increasing the temperature.
- (a) The left side has 3 moles of gas and the right side has 2 moles of gas.
- Increasing pressure shifts equilibrium towards the side with fewer gas moles, which is the right.
- (b) The forward reaction is exothermic, so the reverse is endothermic.
- Raising temperature shifts the equilibrium to the left, in the endothermic direction.
Answer
- (a) The yield of SO₃ increases.
- (b) The yield of SO₃ decreases.
Part 6 of 6Exam Checklist & Tips
1.7.2 – Key Points for the Exam
- Dynamic equilibrium A state in which forward and reverse reactions occur at equal rates and concentrations stay constant.
- Position of equilibrium The relative proportions of reactants and products at equilibrium.
- Le Chatelier's principle A system at equilibrium shifts to oppose any change imposed on it.
- Yield The amount of product obtained from a reaction.
Common Mistakes
- Saying a catalyst moves the position of equilibrium.
- Forgetting that only temperature changes the value of the equilibrium constant.
- Saying that higher temperature always gives a higher yield; this depends on the sign of ΔH.
- Writing that the equilibrium 'goes faster' to the right, which confuses position with rate.
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3 & IGCSE —
Kc
- : Kc! (ΔH < 0) Kc. (ΔH > 0) Kc.
- : ( ).
- : Kc .