Group VII Displacement Reactions (Cambridge O Level Chemistry)

• 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 VII 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

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 VII so is more reactive
• Chlorine will therefore displace bromine from an aqueous solution of the metal bromide
• The least reactive halogen always ends up in the elemental form

potassium bromide + chlorine  →  potassium chloride + bromine

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

Bromine and iodine

• Bromine is above iodine in Group VII 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)

Summary table of displacement reactions