## Disproportionation reactions

As an example to explain what a disproportionation reaction is consider the reaction that occurs when chlorine gas is dissolved in water. An equation for this equilibrium reaction is shown below:
##### Cl2(g) + H2O(l) ⇌ HCl(aq) + HClO (aq)
The products of this reaction are two acids. Hydrochloric acid is a strong acid and chloric (I) acid which is a weak acid (note in chloric(I) acid the I refers to the oxidation state/number of the chlorine). So we could rewrite the above equation as:
##### Cl2(g) + H2O(l) ⇌ 2H+(aq) + Cl-(aq) + ClO-(aq)
In this equation chlorine has been both oxidised and reduced. The equation below shows the oxidation number of chlorine before and after it reacts. To begin with being an element means the oxidation state of chlorine is 0, however one of the products, namely hydrochloric acid the oxidation number of chlorine here is -1, this means it has gained 1 electron and been reduced. In the other product, the chloric(I) acid, the oxidation number of the chlorine is +1, this means it has lost an electron and been oxidised. Reactions like this where one substance is both oxidised and reduced are called disproportionation reactions.

This is the reaction which takes place when chlorine is added to drinking water to kill any bacteria present. The chloric(I) acid is an oxidising agent and also a bleach, it is responsible for killing any harmful organisms present in water to ensure it is safe to drink. The concentration of chlorine in drinking water is around 0.7mg/dm3 and it is higher in water in swimming pools. Care needs to be taken to carefully control the amount of chlorine added to wate simply because chlorine is very toxic even in small amounts. There is the additional problem that it can react with organic compounds in the water and form organochloro compounds which are also very toxic. However if too little chlorine is added it may not necessarily kill all the potentially harmful organisms which maybe present such as chloera, typhus and E.coli bacterium.

You may have used chlorine water in your chemistry lessons, this is simply made by bubbling chlorine gas through water. It is a pale green coloured solution. If universal indicator is added to a chlorine water, solution it initially turns red due to the presence of the hydrochloric acid, which of course is a strong acid. However the indicator quickly turns colourless due to the bleaching action of the chloric(I) acid present.

A similar reactions happens with bromine though the position of equilibrium lies much more to the left.

##### Br2(aq) + H2O(l) ⇌ HBr(aq) + HBrO (aq)
and iodine is for all practical purposes insoluble in water.

If you plan to use chlorine water in the lab it has to be freshly prepared. The reason for this is because if a bottle of chlorine water is left exposed to sunlight its pale green colour fades and oxygen gas is released according to the equation below:

##### Cl2(g) + H2O(l) → 4H+(aq) + 4Cl-(aq) + O2(g)

This unwanted reaction can create problems, for example chlorine is added to drinking water to kill unwanted pathogens. Some of these pathogens can be a particular problem in swimming pools which is why chlorine is added to water used in public baths. However sunlight can cause the chlorine to leave the water reducing the amounts present. Care has to be taken to ensure that the levels of chlorine are maintained at the correct level.

### Bleach manufacture and disproportionation reactions

We saw above that chlorine (Cl2(g)) dissolves in cold water to form a mixture of 2 acids, hydrochloric and chloric (I) acid.

##### Cl2(g) + 2H2O(l) ⇌ HCl(aq) + HClO (aq)
This is an equilibrium reaction, so what would happen if we added cold sodium hydroxide (a strong alkali) to this equilibrium mixture? Adding hydroxide ions will reduce the number of H+ ions in the equilibrium mixture, so according to Le Chateliers principle the equilibrium should shift to the left to give:
##### Cl2(g) + 2NaOH(aq) → NaCl(aq) + NaClO (aq) + H2O(l)
This is also a disproportionation reaction, initially the oxidation stat of the chlorine is 0 however on the products side of the equation the oxidation state of chlorine is -1 in NaCl and +1 in NaClO [sodium chlorate(I)]. The mixture of sodium chloride and sodium chlorate in solution is sold as bleach. The chlorate ion (ClO3-) is responsible for the disinfecting and bleaching properties associated with household bleaches.

If the solution containing chlorate (I) ions is heated then a further disporportonation reaction occurs and chlorate(V) (ClO3-) ions are formed
##### 3NaOCl(aq) → 2NaCl(aq) + NaClO3(aq)
Here the chlorine has a +1 oxidation state in the chlorate(I) ion (ClO-) while it is reduced to Cl- in NaCl and oxidised to chlorine with a +5 oxidation state in the chlorate(V) ion (ClO3-).

If hot alkali sodium hydroxide is used instead of cold alkali the a similar disproportionation reaction occurs but the halogen is oxidised directly to the chlorate(V) ion (ClO3-) missing out the chlorate(I) stage completely, an equation for this reaction is shown below:

### Key points

• A disporportionation reaction is one where a substance is both oxidised and reduced.
• Chlorine gas dissolves in water to form a mixture of 2 acids, a weak acid called chloric(I) acid and the strong hydrochloric acid.
• Bleach is mixture of sodium chloride and sodium chlorate.