Group VII Displacement Reactions (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

Extended tier 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

Colour of halogens in aqueous solutions

Aqueous solution	Colour
Chlorine	Very pale green but usually appears colourless as it is very dilute
Bromine	Orange but will turn yellow when diluted
Iodine	Brown

 

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 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 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)

Summary table of displacement reactions 

	Chlorine (Cl2)	Bromine (Br2)	Iodine (I2)
Potassium chloride (KCl)	x	No reaction	No reaction
Potassium bromide (KBr)	Chlorine displaces the bromide ions Yellow-orange colour of bromine seen	x	No reaction
Potassium Iodide  (KI)	Chlorine displaces the iodide ions Brown colour of iodine is seen	Bromine displaces the iodide ions Brown colour of iodine is seen	x