Which of the following is a disproportionation reaction? (a) Cu O 2H Cu Cu H O → (b) 2CrO 2H Cr O H O → (c) CaCO 2H Ca H O CO → (d) Cr O 2O H 2CrO H O → Ans (a) Sol. A chemical reaction in which the same element in a compound undergoes the oxidation, as well as reduction reaction, is known as the disproportionation reaction. This is a reaction in which the same element exhibits a higher oxidation state and lower oxidation state. In + 2+ Cu O+2H Cu+Cu +H O → reaction, Copper Cu O shows 1 an oxidation state. Cu O undergoes the reduction reaction to produce elemental copper Cu . The reduction reaction is as shown below, + - Reduction : Cu +e Cu → Similarly, the same + Cu in Cu O loses its electron to exhibit 2 oxidation state. That is Cu O undergoes an oxidation reaction. The reaction is as shown below, + 2+ - Oxidation : Cu Cu + e → Here, Cu O or + Cu undergoes oxidation as well as reduction reaction. So it is a disproportionation reaction. Thus, (a) is the correct option.