Group VII Displacement Reactions (Cambridge International IGCSE Science)

Spec point — Describe and explain the displacement reactions of halogens with other halide ions

Group VII displacement reactions

Extended only

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

• The reactivity of Group 7 non-metals increases as you move up the group

• Out of the three commonly used halogens, chlorine, bromine and iodine, chlorine is the most reactive and iodine is the least reactive

  • This is because a more reactive halogen will oxidise the halide ions of a less reactive halogen:

    • The more reactive halogen gains electrons to form halide ions
    • The less reactive halide ions lose electrons to form the halogen element

Colour of halogens in aqueous solutions

Halogen displacement reactions

Chlorine and bromine

• If you add chlorine solution to colourless potassium bromide solution, the solution becomes orange as bromine is formed
• Chlorine is above bromine in Group 7 so is more reactive
• Chlorine will therefore displace bromine from an aqueous solution of the metal bromide
• The less reactive halogen is displaced and forms the halogen element

potassium bromide + chlorine → potassium chloride + bromine

2KBr (aq) + Cl₂ (aq) → 2KCl (aq) + Br₂ (aq)

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

Bromine and iodine

• Bromine is above iodine in Group 7 so is more reactive
• Bromine will therefore displace iodine from an aqueous solution of metal iodide
• The solution will turn brown as iodine is formed

magnesium iodide + bromine → magnesium bromide + iodine

MgI₂ (aq) + Br₂ (aq) → MgBr₂ (aq) + I₂ (aq)

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

Summary table of displacement reactions

Exam Hint Iodine solid, solution and vapour are different colours. Solid iodine is dark grey-black, iodine vapour is purple and aqueous iodine is brown.