halogen displacement reactions header

Higher and foundation tiers

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 of the halogens decreases as you descend group 7 of the periodic table.

Displacement reactions can be carried out using aqueous solutions of the halogens, that is solutions of the halogens dissolved in water. Examples of these displacement reactions are explained in the video opposite. Here examples of halogen displacement reactions using chlorine water, bromine water and iodine solutions are shown along with colour changes which are used to indicate that a halogen displacement reaction has happened. The remainder of this page explains how to use an orgnaic solvent such as cyclohexane or hexane in halogen displacement reactions.

Bottle of chlorine water which is prepared by bubbling chlorine gas through water.

To make the first three halogens (F, Cl, Br) easier and safer handle; they are toxic gases and liquids they are often dissolved in water; for example chlorine dissolves in water to form a very pale green solution called chlorine water, similarly bromine dissolves to form a brown solution called bromine water. Iodine however is barely soluble in water; so to prepare iodine solution solid iodine is dissolved in a solution of potassium iodide and gently heated until the solid iodine dissolves to form a dark brown solution, which is simply referred to as iodine solution. Fluorine is simply too reactive and it oxidises the water to form a weak acid called hydrofluoric acid and a mixture of oxygen and ozone gases.

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 immiscible. 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 using an organic solvent

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 is immisicible with water, that is like oil it floats on top of the water.

Chlorine water displacing iodine from a solution of sodium iodide in a halogen displacement reaction.

Sodium iodide dissolves in water to form a colourless solution containing sodium ions (Na+) and chloride ions (CI - ).
When chlorine water is added to the test-tube containing the sodium iodide solution and cyclohexane and then 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 displacement reaction that has taken place:

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 ions in the sodium iodide 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 ions. The iodide ions are now kicked out of the solution and now have a choice of places to go to; either into the cyclohexane layer or the aqueous (water layer). Iodine is more soluble in the cyclohexane layer rather than the aqueous (water) layer and so most of the iodine will dissolve in the cyclohexane turning it a vivid purple colour. Some iodine will also be dissolved in the aqueous layer turning it a light brown colour.

The halogens chlorine, bromine and iodine are not particularly soluble in water and tend to form solutions with rather washed out colours, while an advantage of cyclohexane is that the halogens form vivid coloured solutions when dissolved in it. Iodine forms a purple solution, chlorine is green/yellow and bromine is red-brown. So the purple colour of the cyclohexane above as mentioned is due to the iodine dissolved in it.

Displacing bromine from a solution

In the example shown in the image below we again have 2 halogens on the reactants side of the equation; these are bromine; in the form of bromide ions from the sodium bromide solution and chlorine from chlorine water. Since chlorine is more reactive than bromine it will displace the bromide ion from the sodium bromide solution, the bromide will be kicked out of solution and most of it will dissolve in the cyclohexane layer, turning it red/brown. Some bromine will also dissolve in the aqueous sodium chloride layer turning it a pale yellow colour.

Halogen displacement reaction, chlorine displacing bromide ions from a solution of sodium bromide to form  bromine and a sodium chloride solution

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.

Care is needed sometimes when considering halogen displacement reactions as you can get caught out quite easily! 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 halogens present so in this case the 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.

Check your understanding - Quick Quiz on halogen displacement reactions.