Halogen displacement reactions [Higher tier only]

The reactivity of the halogens – the Group 7 elements – decreases as you move down the group. This can be shown by looking at displacement reactions.

Diagram showing displacement reactions between sodium chloride, sodium bromide, potassium iodide; and chlorine water, bromine water, iodine solution.

Example

When chlorine (as a gas or dissolved in water) is added to sodium bromide solution, the chlorine takes the place of the bromine. Because chlorine is more reactive than bromine, it displaces bromine from sodium bromide.

The solution turns brown. This brown colour is the displaced bromine. The chlorine has gone to form sodium chloride.

In this equation, the Cl and Br have swapped places:

  • chlorine + sodium bromide → sodium chloride + bromine
  • Cl2(aq) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq)

This type of reaction happens with all the halogens.

curriculum-key-fact
A more reactive halogen displaces a less reactive halogen from a solution of one of its salts.