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3.3.3Physical Chemistry

The Position of Equilibrium — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 9 of 9 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Part 1 of 5Le Chatelier's Principle

3.3.3 The Position of Equilibrium

The principle lets us predict the effect of changing conditions.

Statement
  • When a change is made to a system at equilibrium, the system responds to oppose that change.
Position
  • The position of equilibrium describes the relative amounts of reactants and products when equilibrium is reached.
Shift right
  • If the position moves right, the forward reaction is favoured and more product forms.
Shift left
  • If the position moves left, the reverse reaction is favoured and more reactant forms.

Part 2 of 5Temperature and Pressure

Temperature and Pressure
Raising temperature
  • The system opposes the rise by favouring the endothermic direction, which absorbs heat.
Lowering temperature
  • The system favours the exothermic direction, which releases heat.
Raising pressure
  • The position shifts towards the side with fewer gas molecules, reducing the pressure.
Lowering pressure
  • The position shifts towards the side with more gas molecules.
Equal molecules
  • If both sides have the same number of gas molecules, a pressure change does not alter the position.
ChangeEffect on position
Higher temperatureFavours endothermic direction
Higher pressureFavours side with fewer gas molecules
CatalystNo change in position

Part 3 of 5Applying It to the Haber Process

Applying It to the Haber Process
Equation
  • N₂(g) + 3H₂(g) ⇌ 2NH₃(g), with the forward reaction exothermic.
Pressure
  • There are 4 gas molecules on the left and 2 on the right, so high pressure favours ammonia.
Temperature
  • Because the forward reaction is exothermic, a higher temperature lowers the yield of ammonia.
Compromise
  • A moderate temperature, high pressure and an iron catalyst give a reasonable yield at an acceptable rate.

Part 4 of 5Catalysts and Examples

Catalysts and Examples
Catalyst
  • A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but the position is unchanged.
NO₂ and N₂O₄
  • In 2NO₂(g) ⇌ N₂O₄(g), higher pressure favours colourless N₂O₄, so the brown colour fades.
Exam approach
  • State the change, say which direction opposes it, and state the effect on the colour or yield.

Part 5 of 5Exam Checklist & Tips

3.3.3 – Key Points for the Exam

  • Position of equilibrium The relative amounts of reactants and products in an equilibrium mixture.
  • Le Chatelier's principle A system at equilibrium responds to oppose any change made to it.
  • Catalyst A substance that speeds up a reaction without being used up or changing the position of equilibrium.
  • Yield The amount of product obtained.

Common Mistakes

  • Claiming a catalyst increases the yield at equilibrium; it only speeds up reaching it.
  • Forgetting to count gas molecules only when explaining pressure effects.
  • Saying a temperature rise always favours the forward reaction.
  • Describing the effect without referring to the energy change or number of gas molecules.

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3 & IGCSE —

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