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Properties and reactions of elements

Displacement reactions

The reactivity of the halogens decreases as we move down the group. This can be shown by looking at displacement reactions.

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.

If you look at the equation, you can see that the Cl and Br have swapped places.

Chlorine + sodium bromide → sodium chloride + bromine

Cl2(aq) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq)

Reactivity series

This type of reaction happens with all of the halogens. A more reactive halogen displaces a less reactive halogen from a solution of one of its salts.

If you test different combinations of the halogens and their salts, you can work out a reactivity series for group 7. The most reactive halogen displaces all of the other halogens from solutions of their salts, and is itself displaced by none of the others. The least reactive halogen displaces none of the others, and is itself displaced by all of the others. It works just the same whether you use sodium salts or potassium salts.

The animation shows what happens when chlorine, bromine and iodine are added to various halogen salts. Note carefully which products are present in the test tube after each reaction.

Use your notes to put the halogens into their order of reactivity, with the most reactive first. You should get them in the order they appear from the top to the bottom of group 7.

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