Halogen displacement reactions

A more can a less reactive halogen from of its . 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 salts.

Adding chlorine, bromine and iodine to halogen salts

Chlorine water is added to three solutions

Determining a reactivity series

A can be produced by attempting some . 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 . 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 outer shells gain an and form negatively charged , called . 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
• 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 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 . The equation can be rewritten without them:

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

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

Cl2(aq) + 2e- → 2Cl-(aq), (reduction)

2Br-(aq) → Br2(aq) + 2e-, (oxidation)

Notice that:

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

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

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

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