halogen displacement reactions header

Higher and foundation tier

Competition for electrons- halogen displacement reactions

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.

Carrying out a displacement reaction

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.

chlorine water displacing iodide from solution

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:

sodium iodide(aq) + chlorine(aq) sodium chloride(aq) + iodine(aq)
2NaI(aq) + Cl2(aq) 2NaCl(aq) + I2(aq)

On the reactants side of the word equation we have two halogens present, iodine in the form of iodide in the solution and chlorine from the chlorine water. In a displacement reaction the more reactive halogen will kick-out or displace the less reactive halogen from its compounds, in this case chlorine will displace iodide from the sodium iodide solution. The chlorine effectively takes the place of the iodide. The iodine is kicked out of the solution and now has a choice of places to go to, either into the cyclohexane solvent or the water solution, it goes into the cyclohexane solvent simply because halogens are more soluble in it than in water.

Another advantage of cyclohexane is that the halogen show up as bright clear colours when dissoloved in it, iodine is purple, chlorine is green/yellow and bromine is red-brown. So the purple colour of the cyclohexane is due to the iodine dissolved in it!

Displacing bromine from a solution

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:

sodium bromide(aq) + chlorine(aq) sodium chloride(aq) + bromine(aq)
2NaBr(aq) + Cl2(aq) 2NaCl(aq) + Br2(aq)

Displacement reactions????

halogen displacement reaction 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:

sodium chloride(aq) + iodine(aq) No reaction takes place!
2NaCl(aq) + I2(aq)No reaction takes place!
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.

Key Points

Practice questions

Check your understanding - Questions on halogen displacement reactions.