Question

Which of the following is a disproportionation reaction?
A.${ Cu }_{ 2 }O\quad +\quad { 2H }^{ + }\quad \longrightarrow \quad Cu\quad +\quad { Cu }^{ 2+ }\quad +\quad { H }_{ 2 }O$
B.$2Cr{ O }_{ 4 }^{ 2- }\quad +\quad { 2H }^{ + }\quad \longrightarrow \quad { Cr }_{ 2 }{ O }_{ 7 }^{ 2- }\quad +\quad { H }_{ 2 }O$
C.$CaC{ O }_{ 3 }\quad +\quad { 2H }^{ + }\quad \longrightarrow \quad { Ca }^{ 2+ }\quad +\quad { H }_{ 2 }O\quad +\quad C{ O }_{ 2 }$
D.${ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }\quad +\quad { 2OH }^{ - }\quad \longrightarrow \quad 2Cr{ O }_{ 4 }^{ 2- }\quad +\quad { H }_{ 2 }O$

Answer 1

Hint: Disproportionation reaction is a reaction in which a reactant undergoes reduction as well as oxidation. It is a special type of a redox reaction.

Complete step by step answer: Disproportionation reaction can be defined as a redox reaction in which the same element undergoes oxidation and reduction both simultaneously. In this, a compound of intermediate oxidation state converts to 2 compounds, one which is of higher oxidation state than the previous one and one of lower oxidation state than the previous one.
For example, when chlorine reacts with water, it forms hydrochloric acid and hypochlorous acid. The oxidation of chlorine in ${Cl}_{2}$ was 0 and after the reaction, its oxidation state in hydrochloric acid is -1 and in hypochlorous acid is +1. From this we can see that chlorine gets both reduced and oxidized in the same reaction.

Now let us move on to the question given to us.
${ Cu }_{ 2 }O\quad +\quad { 2H }^{ + }\quad \longrightarrow \quad Cu\quad +\quad { Cu }^{ 2+ }\quad +\quad { H }_{ 2 }O$
In this reaction, the initial oxidation state of copper is +1 and after the reaction, its oxidation state changes to 0 in copper atoms and to +2 in copper. Therefore, we can see that the same element is getting reduced as well as oxidized. Hence, it is a disproportionation reaction.
$2Cr{ O }_{ 4 }^{ 2- }\quad +\quad { 2H }^{ + }\quad \longrightarrow \quad { Cr }_{ 2 }{ O }_{ 7 }^{ 2- }\quad +\quad { H }_{ 2 }O$
In this reaction, the initial oxidation state of chromium is +6 and after the reaction it is also +6. Thus there is no change in the oxidation state. Hence, it is not a disproportionation reaction.
$CaC{ O }_{ 3 }\quad +\quad { 2H }^{ + }\quad \longrightarrow \quad { Ca }^{ 2+ }\quad +\quad { H }_{ 2 }O\quad +\quad C{ O }_{ 2 }$
In this reaction, the initial oxidation state of calcium is +2 and after the reaction it is also +2. Thus there is no change in the oxidation state. Hence, it is not a disproportionation reaction.
${ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }\quad +\quad { 2OH }^{ - }\quad \longrightarrow \quad 2Cr{ O }_{ 4 }^{ 2- }\quad +\quad { H }_{ 2 }O$
In this reaction, the initial oxidation state of chromium is +6 and after the reaction it is also +6. Thus there is no change in the oxidation state. Hence, it is not a disproportionation reaction.

Therefore, only the option (A) shows a disproportionation reaction. Hence, the correct answer is option (A).

Note: Don't confuse between redox reaction and disproportionation reaction. Generally in redox chemical reactions one entity is oxidized and another is reduced. In a disproportionation reaction one entity undergoes both oxidation as well as reduction and two different products are formed.