Halogen 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 and 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

Determining a reactivity series

A reactivity series can be produced by attempting some displacement reactions. Different combinations of halogen solution and salt solution are tested. The table shows the results of these tests. A halogen cannot displace itself from a solution of one of its salts, so these three tests were not done.

In the table below, the following formula names are used:

  • KCl - potassium chloride solution
  • KBr - potassium bromide solution
  • KI - Potassium iodide solution
KCl solutionKBr solutionKl solutionReactions
ChlorineNot doneSolution darkensSolution darkens2
BromineNo visible reactionNot doneSolution darkens1
IodineNo visible reactionNo visible reactionNot done0

Example

Use the results in the table to deduce an order of reactivity, starting with the most reactive halogen.

The order of reactivity is chlorine > bromine > iodine. This is because chlorine could displace bromine and iodine, bromine could only displace iodine, but iodine could not displace chlorine or bromine.

Question

Astatine is placed below iodine in group 7. Predict whether astatine will displace iodine from potassium iodide solution.

Astatine is less reactive than iodine, so it will not displace iodine from potassium iodide solution.

Explaining the trend

When an element in group 7 takes part in a reaction, its atom's outer shells gain an electron and form negatively charged ions, called anions. The less easily these anions form, the less reactive the halogen.

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

Displacement reactions as redox reactions - Higher

A balanced equation for the reaction between chlorine and potassium bromide solution can be written in terms of the ions involved:

Cl2(aq) + 2K+(aq) + 2Br-(aq) → 2K+(aq) + 2Cl-(aq) + Br2(aq)

Notice that potassium ions, K+, appear on both sides of the equation. They do not take part in the reaction and are called spectator ions. The equation can be rewritten without them:

Cl2(aq) + 2Br-(aq) → 2Cl-(aq) + Br2(aq)

This equation is an example of a balanced ionic equation. It can be split into two half equations:

Cl2(aq) + 2e- → 2Cl-(aq), (reduction)

2Br-(aq) → Br2(aq) + 2e-, (oxidation)

Notice that:

  • chlorine atoms (in chlorine molecules) gain electrons - they are reduced
  • bromide ions lose electrons and form molecules - they are oxidised

Reduction and oxidation happen at the same time, so the reactions are called redox reactions.

Displacement reactions are just one example of redox reactions. Electrolysis reactions are also redox reactions.

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