Halogen Displacement Reactions (Edexcel International AS Chemistry): Revision Note

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:

  Cl₂ (aq) + 2NaBr (aq) → 2NaCl (aq) + Br₂ (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:

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

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

  Cl₂ (aq) + 2I^- (aq) → 2Cl^- (aq) + I₂ (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) + Br₂ (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) + I₂ (aq)