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​C1 O) Group 7 Displacement Reactions
​From the previous section about group 7 elements, we learnt that the reactivity of halogens decreases as we go down the group 7 column; fluorine is the most reactive and astatine is the least reactive. When we react a more reactive halogen with the salt of a less reactive halogen, the more reactive halogen will displace the less reactive halogen; this is a displacement reaction.

For example, let suppose that we react chlorine gas (Cl2) with a potassium bromide solution (KBr). We undertake this reaction by bubbling the chlorine gas through a test tube filled with potassium bromide solution. The setup of the experiment is shown below.


​Chlorine is above bromine in the group 7 column, which means that chlorine is more reactive than bromine. Therefore, chlorine will displace the bromine in potassium bromide – the chlorine will force the bromine out of potassium bromide resulting in potassium chloride and bromine. The chemical equation is shown below.

If we then reacted the potassium chloride with fluorine gas, we would get a similar result. Fluorine is more reactive than chlorine as it is higher up the group 7 column. This means that the fluorine will displace the chlorine in potassium chloride resulting in potassium fluoride and chlorine. The chemical equation for this reaction is shown below.

If we then pumped the gas of a less reactive halogen into a solution of potassium fluoride, no reaction would take place. This is because the less reactive halogen will not be able to displace the more reactive halogen. For example, if we put potassium fluoride and iodine together, no reaction would take place because iodine is less reactive than fluorine.
Redox Reactions
All halogen displacement reactions are redox reactions as electrons are being gained and lost. We can use OIL RIG to help us remember what oxidation and reduction means; Oxidation Is Lost, Reduction Is Gain.
I am now going to look at which elements gain and lose electrons for the first reaction that we looked at, which was potassium bromide reacting with chlorine to produce potassium chloride and bromine.

Before the Reaction

Chlorine before the reaction is covalently bonded – the 2 chlorine atoms in Cl2 share a pair of electrons.

 

Potassium bromide is held together by ionic bonds. The potassium has given 1 electron to the bromine, which results in both of the atoms having full outer shells. As bromine received an electron, it will have a charge of 1- (it is Br-; it is a halide).

 

After the Reaction

During the reaction, the chlorine took the electron from bromine as chlorine is more reactive. This resulted in potassium chloride and bromine. As the chlorine has gained an electron, it has been reduced (reduction is gain). As bromine has lost an electron, it has been oxidised (oxidisation is loss).

 

After the reaction, the chlorine is now ionically bonded to potassium; it is now Cl-. And, the bromine is now covalently bonded to another bromine atom; it is now Br2.

 

The ionic equation for this reaction is shown below.

The key outcomes for a displacement reaction with respect to redox reactions are:
  • The displacing halogen is reduced (it gains electrons) – chlorine for the above example
  • The displaced halogen is oxidised (it loses electrons) – bromine for the above example
An Experiment Showing the Reactivity of Halogens
We are able to undertake the following experiment to observe the reactivity of halogens. In this reaction, we use a variety of different halide salts and halogen solutions to see which ones react, and which ones do not react.
 
The experiment starts by placing a small amount of a halide salt solution in a test tube. We then add a few drops of a halogen solution to the test tube and shake gently. There are two different outcomes that we can observe:
  • If we observe a colour change, a reaction has taken place; the halogen in the halogen solution has displaced the halogen in the halide salt solution. This tells us that the halogen in the halogen solution is a more reactive halogen than the halogen in the halide salt solution.
  • If we do not observe a colour change, no reaction has taken place. This tells us that the halogen in the halide salt solution is more reactive than the halogen in the halogen solution.
 
We repeat the process using many different halide salt solutions (such as potassium chloride, potassium bromide and potassium iodine) and halogen solutions (chlorine, bromide and iodide). We then record our results, and we will obtain the table that is shown below.
​

​From the above table, we can see that chlorine is the most reactive halogen because it will displace both bromine from potassium bromide and iodine from potassium iodide. The next most reactive halogen is bromine as bromine is able to displace iodine in potassium iodide (but it cannot displace chlorine in potassium chloride). The least reactive halogen is iodine as it is unable to displace chlorine or bromine.
 
The reactiveness for the halogens that we tested from most reactive to least reactive is chorine, bromine and then iodine. This proves that as we go down the group 7 column, the halogens get less reactive.