In this video we are going to look at the relative reactivity of a group of non-metals
called the halogens. The halogens can be found on the periodic table in group 7. This
means that they are all non-metals. The word halogen comes from the Greek words 'hal' meaning
salt and 'gen' meaning to produce. Halogens react with metals to form metal salts, for example,
the halogen chlorine forms the common table salt sodium chloride. You need to be able to recall
relative reactivities for the periodic table. A good way to work out the relative reactivities
of different elements is by using displacement reactions. A displacement reaction is a type
of chemical reaction where part of one reactant displaces or replaces another reactant. You may
have already met displacement reactions when you looked at the reactivity series of metals.
In that situation, a more reactive metal will displace a less reactive metal from a solution of
their salt. Sometimes displacement reactions can be used to extract metals from other compounds.
A displacement reaction can be summed up using the general equation
In this practical we are going to use a displacement reaction to investigate the reactivity of the halogens.
We will carry out this experiment using a dimple tray. It is important
that you label the rows and columns so you don't forget what you put in each dimple.
You could use, for example, a piece of paper underneath your dimple tray
or use a non-permanent marker to label the rows and columns.
Before we begin, let's think about health and safety. Chlorine gas and liquid bromine are both toxic and dangerous to the environment.
To minimise this risk we will be using chlorine water, bromine water and iodine solution.
These are aqueous solutions of the halogens. Also, don't forget to put your eye protection on.
Chlorine water and iodine solution are both classified as having a low risk and bromine water
as harmful. To minimise this risk I will only be adding a few drops of each solution. Once used,
the dropper bottles will then be stored in the fume cupboard. I'm going to start the
experiment now. I will start with chlorine water. I will add the chlorine water to the first row.
Next, I'll add bromine water to the second row.
And finally, I will add iodine solution to the third row.
It is important to make careful observations and note down any changes in a table.
Remember that the colour of chlorine water is described as colourless and not clear.
This is because a clear solution is simply one that does not contain any particles
in the solid or gaseous state. So the colour of bromine water can be described as clear pale yellow.
I'm going to start by adding distilled water to the first column.
Next, I'm going to add potassium chloride to the second column. Again, carefully observe and note down any changes.
Here we can conclude that no displacement reactions have taken place
when potassium chloride is added to chlorine bromine and iodine water
as the colours have not changed. I will now add potassium bromide to the third column.
Carefully observe and note down any changes.
Here we observe that when potassium bromide is added to chlorine water a reaction has taken
place. The resulting pale yellow colour indicates that bromine has been produced
which means that a displacement reaction has taken place. Observe very carefully because
sometimes the change is very slight here you can see a very slight yellow colour.
the animation shows you what is happening on the sub-microscopic level
and the equation shows how we can easily represent the reaction
Finally, I will add potassium iodide to the fourth column.
Carefully observe and note down any changes.
Here we can see that there has been a colour change here and here.
In each case the iodide ion has been displaced by the chlorine and by the bromine that was added
in this set of experiments. There is evidence that three reactions have taken
place. These reactions can also be described as redox reactions as well as displacement reactions.
A redox reaction is a reaction where both oxidation and reduction occur at the same time.
A redox reaction is easier to spot if we have it written out in an ionic equation.
By looking at the ionic equation we can see that each chlorine atom in the chlorine molecule has
gained an electron to become a chloride ion. Therefore, chlorine has been reduced. Each iodide
ion has lost an electron to form two iodine atoms which combine together to form an iodine molecule.
Therefore, iodine has been oxidised. If we look at our completed results table we can see that
chlorine has displaced bromine and iodine from their compounds while bromine has displaced
iodine from its compound. However, iodine has not displaced either of the other halides
therefore the order of reactivity must be chlorine then bromine then iodine. So, the reactivity
decreases as we go down the group. Check out the additional resources in this video to explore
displacement reactions further. Thank you for watching and have fun with your experiments.
Inspiring your teaching and learning.