Halogen Displacement Reactions (Edexcel AS Chemistry): Revision Note

Halogen Displacement Reactions

• The reactivity of the halogens as oxidising agents decreases down Group 7

  • Reactivity Trend:

Cl₂ > Br₂ > I₂

• A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halide ion from its aqueous solution.

• These are redox reactions

  • The more reactive halogen is reduced (gains electrons)

  • The less reactive halide ion is oxidised (loses electrons)

Summary of halogen colours

• Observing the colour change is key to identifying the products

• The colours can be faint in water

• So, adding an immiscible organic solvent (like cyclohexane) makes the result much clearer:

Displacement Reactions

1. Chlorine with bromide ions (Cl₂ + Br^-)

• Chlorine is more reactive than bromine

  • So, chlorine will displace bromide ions

• The ionic equation is:

Cl₂ (aq) + 2Br⁻ (aq) → 2Cl^- (aq) + Br₂ (aq)

• Observations:

  • In aqueous solution: The colourless solution turns orange

  • With an organic solvent: The organic layer turns orange

• Redox analysis:

  • Reduction: Chlorine's oxidation state changes from 0 to -1

  • Oxidation: Bromine's oxidation state changes from -1 to 0

2. Chlorine with iodide ions (Cl₂ + I^-)

• Chlorine is more reactive than iodine

  • So, chlorine will displace iodide ions

• The ionic equation is:

Cl₂ (aq) + 2I^- (aq) → 2Cl^- (aq) + I₂ (aq)

• Observations:

  • In aqueous solution: The colourless solution turns brown

  • With an organic solvent: The organic layer turns purple / violet

• Redox analysis:

  • Reduction: Chlorine's oxidation state changes from 0 to -1

  • Oxidation: Iodine's oxidation state changes from -1 to 0

3. Bromine with Iodide Ions (Br₂ + I^-)

• Bromine is more reactive than iodine

  • So, bromine will displace iodide ions

• The ionic equation is:

Br₂ (aq) + 2I^- (aq) → 2Br^- (aq) + I₂ (aq)

• Observations:

  • In aqueous solution: The orange solution turns brown

  • With an organic solvent: The organic layer turns purple / violet

• Redox analysis:

  • Reduction: Bromine's oxidation state changes from 0 to -1

  • Oxidation: Iodine's oxidation state changes from -1 to 0

Non-reactions

• A less reactive halogen cannot displace a more reactive halide ion

• For example, adding bromine water to a solution of chloride ions will result in no reaction:

Br₂ (aq) + 2Cl^- (aq) → No Reaction

• The solution would simply remain orange.