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.
Adding chlorine, bromine and iodine to halogen salts
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 solution||KBr solution||Kl solution||Reactions|
|Chlorine||Not done||Solution darkens||Solution darkens||2|
|Bromine||No visible reaction||Not done||Solution darkens||1|
|Iodine||No visible reaction||No visible reaction||Not done||0|
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.
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.
When an element in group 7 takes part in a reaction, its atoms 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:
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)
Cl2(aq) + 2e- → 2Cl-(aq), (reduction)
2Br-(aq) → Br2(aq) + 2e-, (oxidation)
Displacement reactions are just one example of redox reactions. The reaction of a group 1 metal with a group 7 element is also a redox reaction. For example, the reaction of potassium with iodine to form potassium iodide:
2K(s) + I2(s) → 2KI(s)
In this reaction, potassium atoms lose electrons to form positively-charged potassium ions (oxidation) and iodine atoms gain electrons to form negatively-charged iodide ions (reduction).