Group VII: Halogens
The electronegativity of halogens decrease down the group. Fluorine is very reactive and is the most electronegative element.
A further physical property is the halogens boiling points which increase down group seven. This is because the atoms get bigger and so the Van der Waals forces get smaller.
And now a chemical property. The oxidising power of the halogens decreases from Fluorine to Iodine. This property can be demonstrated by displacement reactions.
This means that if halogen ions are dissolved in water. If a halogen higher up the group is added; this higher halogen will replace it. For example, when chlorine is addded to Br-(aq), the following reaction takes place.
2Br-(aq) + Cl2 ® 2Cl-(aq) + Br2
A halide is simply the ion of a halogen. Their ability to reduce increases down the group from F to I.
This ability is shown in the reactions of NaX (where X is a halogen) with sulphuric acid or as chemists like to call it: H2SO4. The table below shows the results of the reactions of NaX with sulphuric acid.
Rotten eggs smell
A black solid a purple fumes
What this shows is that more products are formed with the sodium halides further down the group and therefore the reducing ability of these halides is greater at the bottom of the group.
Now how to test for the presence of the different halides. A classic test is to add silver nitrate, and it produces a precipitate of different colours depending on the halide. However, as you can see below the difference in colours is not very distinct. Therefore we add another substance: ammonia.
A few drops of dilute ammonia are added first to the precipitate to see how much it dissolves. Chlorine will dissolve quite well. Bromine will dissolve sparingly and Iodine not at all. So then we add concentrated ammonia and the precipitate in both the chloride and bromide solutions will dissolve and the iodided precipitate turns white. The diagram below outlines this testing procedure.
The ionic equation for this reaction is as follows (using Br as the halide).
Cl-(aq) + Ag+(aq) ® AgCl(s)
Halogens will react with metals to form salts. For example, if chlorine gas is passed over a heated iron wire, you will see a brown solid upon cooling.
iron + chlorine ® iron (III) chloride
2Fe(s) + 3Cl2 (g) ® 2FeCl2 (s)
The most reactive halogen is fluorine, and they become less reactive as you go down. Due to this, a more reactive halogen will displace a less reactive one. For example, when chlorine gas is bubbled though potassium bromide solution...
2KBr(aq) + Cl2 (g) ® 2KCl (aq) + Br2 (aq)
Chlorine and Chlorides
Chlorine has a very interesting reaction with water, it is a reversible reaction and also shows something called disproportionation [dis-pro-por-shon-ay-shon].
Disproportionation means a reaction where one species (in this case chlorine) is simultaneously oxidised and reduced. If you have a look at chlorine in HCl it has an oxidation number of -1 and in hydrogen chlorate. This reaction has been used in water treatment to destroy bacteria, however this practice has been replaced by granular calcium hypochlorite Ca(ClO)2 which is less dangerous.
Another reaction we are going to look at is chlorine with cold, dilute sodium hydroxide (NaOH). This causes the following reaction to take place.
Cl2 + 2OH- ® Cl- + ClO- + H2O
This reaction is very important because it is used commercially to produce bleach!