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Disproportionation reactions

Clorine water is made by dissolving chlorine gas in water. 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:
chlorine(g) + water(l) hydrochloric acid(aq) + chloric (I) acid (aq)
Cl2(g) + H2O(l) HCl(aq) + HClO (aq)
The products of this reaction are two acids. Hydrochloric acid which 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 to show all the ions produced when the chlorine goes into solution:
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 in one of the products namely the hydrochloric acid the oxidation number of chlorine 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.

Equation showing disproportionation reaction when chlorine dissolves in water.

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 though it is higher in the water in swimming pools. Care needs to be taken to carefully control the amount of chlorine added to water 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 cholera, 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, the solution 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 reaction happens with bromine though the position of equilibrium lies much more to the left.

Br2(aq) + H2O(l) HBr(aq) + HBrO (aq)
while 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:

2Cl2(g) + 2H2O(l) 4H+(aq) + 4Cl-(aq) + O2(g)

Chlorine levels in swimming pools must be regularly checked to ensure they are not too high or too low 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 it forms 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 Chatelier's 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 state 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 disproportionation 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:
Cl2(g) + 6NaOH(aq) → 5NaCl(aq) + NaClO (aq) + 3H2O(l)

Key points

Practice questions

Check your understanding - Questions disproportionation reactions.