Higher and foundation tier
One of the many fun reactions you can carry out with halogens are displacement reactions.
Displace means to kick-out or get rid of! Displacement reactions are a good way to demonstrate
the order of reactivity of the halogens. Remember the most reactive
halogen is fluorine and the
reactivity decreases as you decend the group. To make the first three halogens easier to handle,
as they are toxic gases and liquids they are often dissolved in water, for example
in water to form a solution called chlorine water, similarily for
bromine to form bromine water and so on
for the rest of the halogens.
Recall also that a solvent is a liquid that is good at dissolving substances, e.g. acetone is an
used in nail varnish remover. Ethanol is another good solvent used in many perfumes, and perhaps the
solvent we use in science is water.
Some substances dissolve in water but many do not, for example if you add oil to water it simply floats on top of the water. Oil and water do not mix, they are immisicle. Many of the common solvents we use in chemistry do not mix with water, but like oil they simply float on top of the water.
A typical displacement reaction is shown opposite. In the first test-tube we have a solution of sodium iodide dissolved
in water. On top of this is added a few centimetres of cyclohexane. Cyclohexane is a very good
for halogens and given the choice between dissolving in water and dissolving
in cyclohexane, a halogen
will always dissolve in cyclohexane before water. Cyclohexane does not dissolve in water, but like oil simply floats on top of it.
Sodium iodide, being an ionic compound will dissolve in water. This solution contains sodium ions, Na+ and I - .
When chlorine water is added to this test-tube and shaken for around 30 seconds you can see in the image opposite that the cyclohexane layer has changed colour, it has turned a violet/purple colour.
It may be easier to understand what is happening here if we write an equation for the reaction:
In the example opposite we again have 2 halogens on the reactants side of the equation, these are bromine, in the form of bromide from the sodium bromide solution and chlorine from chlorine water. Chlorine is more reactive than bromine so will displace it from the solution, the bromide will be kicked out of solution and will dissolve in the cyclohexane solvent, turning it red/brown.
However be careful as you can get caught out if you are not careful! Consider the reaction shown on the left . Here we have iodine and chlorine as the two halogens present on the reactant side of the equation. Chlorine is the more reactive of the two so in this case iodine will NOT be able to displace chlorine from its solution. No reaction will happen. In order to displace the chlorine (chloride in the solution) we will need to add the more reactive halogen fluorine.