Group 7 - displacement reactions

A more reactive halogen can displace a less reactive halogen from solutions of its salts. For example, chlorine is more reactive than iodine. A solution of chlorine can displace iodine from potassium iodide solution:

chlorine + potassium iodide → potassium chloride + iodine

Cl2(aq) + 2KI(aq) → 2KCl(aq) + I2(aq)

The reaction mixture turns darker as iodine solution forms.

The slideshow shows what happens when solutions of chlorine, bromine and iodine are added to various potassium halide salts.

Chlorine water is added to three test tubes containing potassium chloride, potassium bromide and potassium iodide.

Adding chlorine, bromine and iodine to halogen salts

Chlorine water is added to three solutions


Write a balanced equation for the displacement reaction of bromine solution with sodium iodide solution.

Br2(aq) + 2NaI(aq) → 2NaBr(aq) + I2(aq)


Can iodine displace chlorine from sodium chloride solution? Explain your answer.

No, because iodine is less reactive than chlorine.

Explaining the trend in reactivity

When a group 7 element takes part in a reaction, its atoms each gain one electron. These atoms form negatively charged ions. The ions have a stable arrangement of electrons, with a complete outer shell.

Going down group 7:

  • the atoms become larger
  • the outer shell becomes further from the nucleus
  • the force of attraction between the nucleus and the outer shell decreases
  • an outer electron is gained less easily
  • the halogen becomes less reactive

The periodic table, containing all the known elements, arranged in groups and periods

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