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2.6.6Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Le Chatelier’s Principle — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 4 parts Lesson 42 of 53 Unit quiz

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Part 1 of 4The Principle

2.6.6 Le Chatelier's Principle

If a system at equilibrium is disturbed, the position shifts to oppose the change.

Statement
  • The equilibrium shifts in the direction that reduces the effect of the imposed change.
Concentration up
  • Raising a reactant's concentration shifts the position to the right to use up the excess.
Concentration down
  • Removing a product shifts the position to the right to replace the lost product.
Pressure
  • Higher pressure shifts the position towards the side with fewer moles of gas.
Temperature
  • A rise in temperature favours the endothermic direction, and a fall favours the exothermic direction.
Added reactant
  • Adding a reactant makes the forward rate briefly exceed the backward rate, so the position moves to the right.

Part 2 of 4Worked Applications

Worked Applications
Haber process
  • N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = -92 kJ mol⁻¹: high pressure and low temperature increase the yield of ammonia.
Dimerisation
  • N₂O₄(g) ⇌ 2NO₂(g) is endothermic, so heating gives more brown NO₂.
Equal moles of gas
  • H₂(g) + I₂(g) ⇌ 2HI(g) has equal moles of gas on each side, so pressure has no effect.
Catalyst
  • A catalyst does not shift the position because it speeds up both directions equally.
Esterification
  • In CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O, removing the ester shifts the position right and raises the yield.

Part 3 of 4Answering Explanation Questions

1
State the change made, such as an increase in pressure or a decrease in temperature.
2
State the direction in which the equilibrium shifts, using left, right, forward or backward.
3
Explain how this opposes the change, for example by reducing the number of gas moles and so the pressure.
4
State the effect on the observation or on the yield of the product.
5
If a catalyst or equal numbers of gas moles are involved, say that the position does not change.
6
The new equilibrium is reached once the forward and backward rates are equal again.

Part 4 of 4Exam Checklist & Tips

2.6.6 – Key Points for the Exam

  • Le Chatelier's principle When a change is applied to an equilibrium, the system shifts to oppose it.
  • Position of equilibrium The relative proportions of reactants and products in the equilibrium mixture.
  • Exothermic A reaction that releases heat to the surroundings, with negative ΔH.
  • Endothermic A reaction that takes in heat from the surroundings, with positive ΔH.

Common Mistakes

  • Writing that a rise in temperature moves the equilibrium in the exothermic direction.
  • Saying a catalyst increases the yield at equilibrium.
  • Applying pressure arguments to a reaction with no gases.
  • Predicting a shift when water is added to an equilibrium with equal numbers of ions on each side;
    • dilution then changes all concentrations equally.

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3 & IGCSE —

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