Higher and foundation tiers
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.
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.
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.
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:
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.
Equations for this displacement reaction are:
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: