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
chlorine dissolves
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
excellent solvent
used in nail varnish remover. Ethanol is another good solvent used in many perfumes, and perhaps the
most common
solvent we use in science is water.
Some substances are soluble in water but many are 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 experiment is shown below. 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 solvent 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 above 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 below 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.
Equations for this displacement reaction are:
However be careful as you can get caught out if you are not careful! Consider the reaction shown on the right. Here
we have a solution of sodium chloride in the boiling tube with iodine solution being added to it. Equations for this reaction are: