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Cambridge IGCSE Chemistry · 0620

Chapter 6: Chemical Reactions — Part 4

Topic 6.3 · Reversible reactions and equilibrium

Fundamentals of Reversibility

  • In reversible reactions, the products can react to reform the original reactants; this is shown by the symbol ?.
  • Hydrated salts contain water of crystallisation, while anhydrous salts do not.
  • Copper(II) sulfate: Blue (hydrated) ? White (anhydrous) + Water.
  • Cobalt(II) chloride: Pink (hydrated) ? Blue (anhydrous) + Water.
  • Direction is changed by heating (to remove water) or adding water.

Exam Traps

  • Do not confuse the colours — CuSO4 is blue when hydrated and white when anhydrous (not the reverse).

Dynamic Equilibrium

A reversible reaction in a closed system reaches equilibrium when:

  1. The rate of the forward reaction equals the rate of the reverse reaction.
  2. The concentrations of reactants and products remain constant.

Exam Traps

  • Do not say the reaction stops at equilibrium — both forward and reverse reactions continue at equal rates.
  • Do not say concentrations are zero at equilibrium — they remain constant but not necessarily equal.

Le Chatelier's Principle

  • If a condition of a system at equilibrium is changed, the position of equilibrium shifts to counteract the change.
    • Temperature: Increasing temperature shifts equilibrium in the endothermic direction. Decreasing temperature shifts it in the exothermic direction.
    • Pressure: Increasing pressure shifts equilibrium to the side with fewer gaseous molecules. Decreasing pressure shifts it to the side with more gaseous molecules.
    • Concentration: Increasing reactant concentration shifts equilibrium to the right (more products).
    • Catalysts: Have no effect on the position of equilibrium but allow it to be reached faster.

Exam Traps

  • Do not say a catalyst increases yield — it lowers Ea but does not shift the equilibrium position.

Industrial Processes

The Haber Process (Ammonia Manufacture):

  • Equation: N2(g) + 3H2(g) ? 2NH3(g) (Exothermic forward).
  • Conditions: 450°C, 20,000 kPa (200 atm), and an iron catalyst.
  • Explanations: 450°C is a compromise between a high yield (favoured by low temp) and a fast rate. High pressure favours the side with fewer molecules (the product) but is limited by cost and explosion risk.

The Contact Process (Sulfur Trioxide Manufacture):

  • Equation: 2SO2(g) + O2(g) ? 2SO3(g) (Exothermic forward).
  • Conditions: 450°C, 200 kPa (2 atm), and a vanadium(V) oxide catalyst.
  • Explanations: Similar temperature compromise to Haber. Pressure is kept relatively low because sulfur trioxide is an acidic gas, making high pressure an extreme explosion risk.

Exam Traps

  • Do not say the Haber process uses the lowest possible temperature — yield would be high but rate too slow without compromise.
  • Do not apply Haber pressure reasoning to the Contact process — industrial pressure is much lower for safety reasons.

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