Question

Which among the following is not a disproportionation reaction?
A. $3{{H}_{3}}P{{O}_{3}}\to 2{{H}_{3}}P{{O}_{2}}+P{{H}_{3}}$
B. $2HCHO+O{{H}^{-}}\to HCO{{O}^{-}}+C{{H}_{3}}OH$
C. $4P+3NaOH+3{{H}_{2}}O\to P{{H}_{3}}+3Na{{H}_{2}}P{{O}_{2}}$
D. $~N{{H}_{4}}N{{O}_{3}}\to {{N}_{2}}O+2{{H}_{2}}O$

Answer 1

In a reaction if the same element undergoes both oxidation and reduction reactions then the chemical reaction is called disproportionation reaction. That particular element will show two different oxidation states due to reduction and oxidation reaction.

Complete answer:
- In the question it is asked to find which chemical reaction is not a disproportionation reaction among the given options.
- Coming to the given options, option A.
$3{{H}_{3}}P{{O}_{3}}\to 2{{H}_{3}}P{{O}_{2}}+P{{H}_{3}}$
- In the above chemical reaction the oxidation state of phosphorus in ${{H}_{3}}P{{O}_{3}}$ is +3 in the reactant. In ${{H}_{3}}P{{O}_{2}}$ the oxidation state of phosphorus is +1 and in $P{{H}_{3}}$ -3. So, it is a disproportionation reaction because the phosphorus element has two different oxidation states in the formed products. It is an incorrect option.
- Coming to option B.
$2HCHO+O{{H}^{-}}\to HCO{{O}^{-}}+C{{H}_{3}}OH$
- The oxidation state of carbon in HCHO is zero. The oxidation state of carbon in $HCO{{O}^{-}}$ is +2 and the oxidation state of carbon in $C{{H}_{3}}OH$ is +2. So, it is a disproportionation reaction. Therefore it is an incorrect option.
- coming to option C,
$4P+3NaOH+3{{H}_{2}}O\to P{{H}_{3}}+3Na{{H}_{2}}P{{O}_{2}}$
- The oxidation state of phosphorus is zero in the reactant side. The oxidation state of phosphorus in $P{{H}_{3}}$ is -3 and the oxidation state of phosphorus in $Na{{H}_{2}}P{{O}_{2}}$ is +1. So, it is a disproportionation reaction. Therefore it is an incorrect option.
- Coming to option D,
$~N{{H}_{4}}N{{O}_{3}}\to {{N}_{2}}O+2{{H}_{2}}O$
- The oxidation state of nitrogen in $N{{H}_{4}}N{{O}_{3}}$ is -3 and +1 in reactant. Coming to products the oxidation state of nitrogen in ${{N}_{2}}O$ is +2. It is not a disproportionation reaction because in this chemical reaction only reduction occurs not oxidation.
- Therefore the chemical reaction in option D is not a disproportionation reaction.
So, the correct option is D.

Note:
The element should undergo both oxidation (increases in oxidation state) and reduction (decrease in oxidation state) in a chemical reaction then only it is called a disproportionation chemical reaction.