8.2.2The Periodic Table
The Effect of Changing Conditions on Equilibrium — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 4 parts Lesson 7 of 23 Unit quiz
Section 1 of 4Le Chatelier's Principle
The principle gives a qualitative way to predict the effect of a change on a system at equilibrium.
Principle
- When a change is made to a system at equilibrium, the system moves to oppose the change.
Origin
- Henri Le Chatelier, a French engineer of the 19th century, formulated the principle.
Shift right
- A shift to the right means the forward reaction is favoured, giving more products.
Shift left
- A shift to the left means the reverse reaction is favoured, giving more reactants.
Section 2 of 4Factors That Shift Equilibrium
Temperature, pressure and concentration can all move the position of equilibrium.
Factors That Shift Equilibrium
Temperature
- The effect depends on whether the forward reaction is exothermic or endothermic.
Pressure
- Changing pressure affects only reactions in which the number of gas molecules differs on each side.
Concentration
- Adding a reactant shifts the position of equilibrium to the right, using up some of the added reactant.
Catalysts
- A catalyst does not change the position of equilibrium; it only changes how quickly equilibrium is reached.
Section 3 of 4Making Predictions
A systematic approach gives a reliable prediction.
1
Identify whether the forward reaction is exothermic or endothermic, and count the gas molecules on each side.
2
Decide which change is being made, such as an increase in temperature or an increase in pressure.
3
Apply the principle: the system partly opposes the change by favouring the reaction that reduces its effect.
4
State the direction of the shift and explain its effect on the yield of the product in the question.
Section 4 of 4Exam Checklist & Tips
8.2.2 – Key Points for the Exam
- Le Chatelier's principle A rule stating that a system at equilibrium opposes any change made to it.
- Position of equilibrium The relative amounts of reactants and products present when equilibrium has been reached.
- Yield The amount of product obtained from a reaction.
- Shift A movement of the position of equilibrium to favour the forward or reverse reaction.
Common Mistakes
- Stating that the system stops the change completely; it only partly opposes the change.
- Saying a catalyst increases the yield at equilibrium; a catalyst does not change the position of equilibrium.
- Confusing a shift in the position of equilibrium with a change in the rate of reaction.
- Predicting pressure effects by counting atoms instead of gas molecules.
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