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3.3.3Stoichiometry

The Position of Equilibrium — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 6 parts Lesson 22 of 28 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 6Le Chatelier's Principle

3.3.3 The Position of Equilibrium
Principle
  • Le Chatelier's principle states that when a condition of a system at equilibrium changes, the system moves to oppose that change.
Shift to the right
  • The position of equilibrium shifts to the right when the forward reaction is favoured, producing more products.
Shift to the left
  • The position shifts to the left when the reverse reaction is favoured, increasing the amount of reactants.
Partial opposition
  • The shift only partly counteracts the change, so the system does not fully return to its original state.
Energy change needed
  • To predict the effect of temperature you must know whether the forward reaction is exothermic or endothermic.

Part 2 of 6Effect of Temperature

1
Raising the temperature moves the equilibrium in the endothermic direction, which absorbs the extra heat energy.
2
Lowering the temperature moves the equilibrium in the exothermic direction, which releases heat to restore the balance.
3
If the forward reaction is exothermic, a rise in temperature shifts the equilibrium to the left, producing more reactants.
4
If the forward reaction is endothermic, a rise in temperature shifts the equilibrium to the right, producing more products.

Part 3 of 6Effect of Pressure

Effect of Pressure
Gases only
  • Changing pressure affects only gaseous substances, because solids and liquids are hardly compressed.
Direction
  • An increase in pressure favours the side with fewer gas molecules, which reduces the total pressure.
Equal moles
  • When both sides contain the same number of gas moles, the position of equilibrium is unchanged.
Lowering pressure
  • A decrease in pressure favours the side with more gas molecules.
SituationEffect of increasing pressure
Fewer gas molecules on the product sideShifts right, more product
More gas molecules on the product sideShifts left, more reactant
Equal numbers of gas molecules on each sideNo change in position
Only solids and liquidsNo effect, because pressure acts mainly on gases

Part 4 of 6Catalysts and Equilibrium

Catalysts and Equilibrium
No shift
  • A catalyst does not change the position of equilibrium, so the equilibrium yield is unchanged.
Speed
  • A catalyst speeds up both the forward and reverse reactions by the same amount, so equilibrium is reached sooner.
Mechanism
  • It provides an alternative pathway with lower activation energy for both directions.
Industry
  • In the Haber process the iron catalyst allows equilibrium to be reached faster, but it does not raise the yield.

Part 5 of 6Worked Example

Question

  • The reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is exothermic in the forward direction.
  • Predict the effect on the position of equilibrium of (a) raising the temperature and (b) increasing the pressure.

  1. (a) The forward reaction is exothermic, so the reverse reaction is endothermic
  2. Raising the temperature favours the endothermic reverse reaction, so the equilibrium moves to the left
  3. (b) The left side has 1 + 3 = 4 gas molecules and the right side has 2 gas molecules
  4. Increasing the pressure favours the side with fewer gas molecules, so the equilibrium moves to the right

Answer

  • (a) The position moves left, giving less ammonia.
  • (b) The position moves right, giving more ammonia.

Part 6 of 6Exam Checklist & Tips

3.3.3 – Key Points for the Exam

  • Le Chatelier's principle A rule stating that a system at equilibrium opposes any change made to its conditions.
  • Position of equilibrium The relative amounts of reactants and products present at equilibrium.
  • Exothermic A reaction that releases energy to the surroundings.
  • Endothermic A reaction that takes in energy from the surroundings.
  • Yield The amount of product obtained from a reaction.

Common Mistakes

  • Saying the system shifts to remove the change completely; it only partly opposes the change.
  • Applying pressure changes to reactions involving only solids or liquids, which have no effect on the position of equilibrium.
  • Predicting a temperature shift without first deciding whether the forward reaction is exothermic or endothermic.
  • Stating that a catalyst increases the yield of product at equilibrium.

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3 & IGCSE —

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