Disproportionation reactions are those in which an element is simultaneously oxidized and reduced.

One example is the decomposition of hydrogen peroxide:

2H2O2 -----> 2H2O + O2

In hydrogen peroxide, oxygen has an oxidation number of -1. In water it is -2, while in elemental oxygen it is 0. Thus, of the four Os with a charge of -1, two have increased to 0, and two have decreased to -2.

Another example is the reaction between halogens and alkalis. For example, chlorine with sodium hydroxide:

Cl2 + 2NaOH -----> NaCl + NaClO + H2O.

Chlorine started with an oxidation number of 0; in NaCl (common salt) it has an oxidation number of -1, while in NaClO (sodium chlorate (I)) its oxidation number is +1.

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