Halogen Displacement Reactions (Edexcel International A Level Chemistry)

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Halogen Displacement

    • The reactivity of halogens is also shown by their displacement reactions with other halide ions in solutions
    • A more reactive halogen can displace a less reactive halogen from a halide solution of the less reactive halogen
    • Eg. The addition of chlorine water to a solution of bromine water:

    Cl(aq) + 2NaBr (aq) → 2NaCl (aq) + Br(aq)

    • The chlorine has displaced the bromine from solution as it is more reactive which can be summarised in the following ionic equation by removing the sodium spectator ions:

    Cl(aq) + 2Br(aq) → 2Cl(aq) + Br(aq)

    • Chlorine can also displace iodine from a solution of iodide ions:

    Cl(aq) + 2I(aq) → 2Cl(aq) + I(aq)

  • We can see these are both redox reactions by taking a look at changes in the oxidation number of each element in the reaction
    • Br and I both change from = -1 → 0 so the bromine and iodine have been oxidised in their respective reactions
    • Cl = 0 → -1 so the chlorine has been reduced in both reactions
    • No change in oxidation number for the sodium

Chlorine with Bromides & Iodides

  • If you add chlorine solution to colourless potassium bromide or potassium iodide solution a displacement reaction occurs:
    • The solution becomes yellow-orange as bromine is formed
    • The solution becomes brown as iodine is formed

  • If an organic solvent is added, such as cyclohexane, the following observations are seen:
    • The organic layer will appear yellow-orange as bromine is formed
    • The organic layer will appear purple as iodine is formed 
    • The organic solvent is useful as the the halogens are more soluble in this layer which helps observe the colour changes more easily
  • Chlorine is above bromine and iodine in Group 7 so it is more reactive
  • Chlorine will displace bromine or iodine from an aqueous solution of the metal halide:

Cl2 (aq) + 2KBr (aq) → 2KCl (aq) + Br(aq)

chlorine + potassium bromide  potassium chloride + bromine

Cl2 (aq) + 2KI (aq) → 2KCl (aq) + I2 (aq)

chlorine + potassium iodide  potassium chloride + iodine

Bromine with Iodides

  • Bromine is above iodine in Group 7 so it is more reactive
  • Bromine will displace iodine from an aqueous solution of the metal iodide

bromine + potassium iodide  potassium bromide + iodine

Br2 (aq) + 2KI (aq) → 2KBr (aq) + I2 (aq)

  • The reaction will turn brown as iodine is formed 
  • If an organic solvent is added the organic layer will appear purple as iodine is formed 
  • We can show that this is a redox reaction by looking at the changes in oxidation numbers:
    • I = -1 → 0 so the iodine has been oxidised 
    • Br = 0 → -1 so the bromine has been reduced 
    • No change in oxidation number for the potassium
  • Rather than writing the full equation, we can also write the ionic equation by removing the potassium spectator ion

Br2 (aq) + 2I- (aq) → 2Br- (aq) + I(aq)

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.