of identifying Br – and I – in the laboratory through the test popularly known as ‘Layer Test’. It may not be out of place to mention here that bromine likewise can displace iodide ion in solution: – – Br (l) + 2I – (aq) → 2Br – (aq) + I (s) ( . ) The halogen displacement reactions have a direct industrial application. The recovery of halogens from their halides requires an oxidation process, which is represented by: 2X – → X + 2e – ( .
) here X denotes a halogen element. Whereas chemical means are available to oxidise Cl – , Br – and I – , as fluorine is the strongest oxidising agent; there is no way to convert F – ions to F by chemical means. The only way to achieve F from F – is to oxidise electrolytically, the details of which you will study at a later stage. .
Disproportionation reactions Disproportionation reactions are a special type of redox reactions. In a disproportionation reaction an element in one oxidation state is simultaneously oxidised and reduced. One of the reacting substances in a disproportionation reaction always contains an element that can exist in at least three oxidation states. The element in the form of reacting substance is in the intermediate oxidation state; and both higher and lower oxidation states of that element are formed in the reaction.
The decomposition of hydrogen peroxide is a familiar example of the reaction, where oxygen experiences disproportionation. + – + – 2H O (aq) → 2H O(l) + O (g) ( . ) Here the oxygen of peroxide, which is present in – state, is converted to zero oxidation state in O and decreases to – oxidation state in H O. Phosphorous, sulphur and chlorine undergo disproportionation in the alkaline medium as shown below : – + P (s) + 3OH – (aq)+ 3H O(l) → PH (g) + 3H PO – (aq) ( .