Displacement reactions

Quick question from a student…

How do displacement reactions work …  Also, how does it ‘replace’ the less reactive element????

We’ve studied displacement reactions before, by looking at reactions of metals. More reactive metals will react to remove less reactive metals from compounds and replace them in the compound. In A-Level this is taken further to look at the non-metals – and it works exactly the same way.

In displacement reactions the more reactive element will displace a less reactive element for example…

2KI + Cl2 –> 2KCl + I2

As chlorine is more reactive than Iodine it replaces (or displaces) the iodine in the compound.
You can use this type of experiment to look at the reactivity of halogens.

I found this page from BBC bitesize which is basic, but might be quite useful.

Halogens dissolved in cyclohexane. You can clearly see (left - right)Chlorine, Bromine, Iodine

Halogens dissolved in cyclohexane. You can clearly see (left – right)
Chlorine, Bromine, Iodine

You can easily see the halogens given off in these reactions if you put a layer of cyclohexane on top of the solution (this is because the halogens are very soluble in the cyclohexane layer and so will dissolve very easily into the organic solvent which will then change colour depending on the dissolved halogen) Don’t forget you can always test for halides very easily.

Add dilute nitric acid then a few drops of silver nitrate

For more info visit Chemstuff’s tests for ions page.

Why does this all happen?

The reactivity can be explained by electronegativity

The reason this happens is the cation is very keen to give it’s electron away, it doesn’t really care where it goes to. As you go up group VII the electronegativity increases so it is more electron withdrawing and so will accept an electron more readily than a less reactive halogen.