A more reactive halogen displaces a less reactive halogen from its halide solution, for example Cl2(aq) + 2KBr(aq) → 2KCl(aq) + Br2(aq); the ionic equation is Cl2(aq) + 2Br−(aq) → 2Cl−(aq) + Br2(aq).
A more reactive halogen displaces a less reactive halogen from a solution of its halide. Reactivity decreases down Group 17, so chlorine can displace bromine and iodine, and bromine can displace iodine, but iodine cannot displace either of the others. Because electrons are transferred, displacement is a redox reaction, and it is a standard demonstration of the halogen reactivity order in the Redox chapter of 4541.
The balanced equation
The general pattern is: more reactive halogen + halide of less reactive halogen → halide of more reactive halogen + less reactive halogen. With correct formulae and state symbols:
Cl2(aq) + 2KBr(aq) → 2KCl(aq) + Br2(aq)
Cl2(aq) + 2KI(aq) → 2KCl(aq) + I2(aq)
Br2(aq) + 2KI(aq) → 2KBr(aq) + I2(aq)
The ionic equation removes the spectator potassium ion and shows the real change, the halogen molecule takes electrons from the halide ions:
Cl2(aq) + 2Br−(aq) → 2Cl−(aq) + Br2(aq)
The two half-equations make the redox clear. Chlorine is reduced (it gains electrons) and bromide ions are oxidised (they lose electrons):
Cl2(aq) + 2e− → 2Cl−(aq)
2Br−(aq) → Br2(aq) + 2e−
Conditions required
No heating or catalyst is needed. A halogen dissolved in water, chlorine water or bromine water, is simply added to a solution of a halide salt at room temperature. The halogen added must be higher in the group (more reactive) than the halogen in the salt, otherwise no reaction occurs. An organic solvent such as 1,1,1-trichloroethane is often added afterwards to make the colour of the displaced halogen easier to see.
What you observe
When colourless chlorine water is added to a colourless potassium bromide solution, the solution turns yellow to brown as bromine is set free. When chlorine water is added to potassium iodide solution, it turns brown and a dark solid of iodine may appear; with starch, the mixture turns blue-black. If an organic solvent is shaken in, bromine gives an orange-brown layer and iodine a purple layer, confirming which halogen was displaced. No colour change means the added halogen is less reactive than the one in the salt.
Where it appears in the SPM exam
In 4541/1 you predict whether a displacement occurs from the halogen reactivity order or identify the colour change. In 4541/2 you write the balanced equation, the ionic equation and the two half-equations, identify the oxidising and reducing agents, and explain the trend in reactivity down Group 17. The organic-solvent layer colours and the starch test for iodine are common observation questions in the practical paper too.
How we teach it
Our teachers make sure you assign the redox roles precisely: the more reactive halogen is the oxidising agent and is reduced, while the less reactive halide ion is the reducing agent and is oxidised. Students most often lose marks by predicting an impossible reaction, by forgetting the spectator ion in the ionic equation, or by describing the colours without linking them to the halogen freed. Anchoring every answer to “reactivity decreases down the group” and “oxidising agent is reduced” keeps the whole topic coherent.
Quick summary
A more reactive halogen displaces a less reactive halogen from its halide solution in a redox reaction, with reactivity falling from chlorine to iodine. Learn the balanced equations, the ionic equation and half-equations, the colour changes and organic-solvent layers, and the reactivity trend, and this dependable redox topic is fully covered.
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