Question

Give reasons why ${{{H}}_3}{{P}}{{{O}}_3}$ undergoes disproportionation reaction but ${{{H}}_3}{{P}}{{{O}}_4}$ does not.

Answer 1

Disproportionation reaction is a type of redox reaction. In this reaction, a single substance can undergo both oxidation and reduction. Loss of electrons is termed as oxidation. Gain of electrons is termed as reduction.

Complete step by step solution:
When an atom loses its electrons or its oxidation state gets increased, it is termed as oxidation. When an atom gains electrons or its oxidation state gets reduced, it is termed as reduction.
Disproportionation reactions are the reaction in which a single atom simultaneously gets oxidized and reduced. Disproportionation reaction of ${{{H}}_3}{{P}}{{{O}}_3}$ is given below:
${{{H}}_3}{{P}}{{{O}}_3} \to {{{H}}_3}{{P}}{{{O}}_4} + {{P}}{{{H}}_3}$
${{{H}}_3}{{P}}{{{O}}_3}$ is phosphorus acid, ${{{H}}_3}{{P}}{{{O}}_4}$ is phosphoric acid and ${{P}}{{{H}}_3}$ is phosphine.
We can explain this reaction in terms of giving oxygen. Oxidation means gaining oxygen, and reduction means losing oxygen.
Here ${{{H}}_3}{{P}}{{{O}}_3}$ has undergone both oxidation and reduction.
Oxidation: ${{{H}}_3}{{P}}{{{O}}_3} \to {{{H}}_3}{{P}}{{{O}}_4}$
Here oxygen is gained.
Reduction: ${{{H}}_3}{{P}}{{{O}}_3} \to {{P}}{{{H}}_3}$
Here oxygen is lost from ${{{H}}_3}{{P}}{{{O}}_3}$.
Thus ${{{H}}_3}{{P}}{{{O}}_3}$ gets oxidized and reduced.
Disproportionation reaction is occurred in ${{{H}}_3}{{P}}{{{O}}_3}$ because ${{P}}$ is in $+ 3$ oxidation state which is not stable. Thus it converts to ${{{H}}_3}{{P}}{{{O}}_4}$ and ${{P}}{{{H}}_3}$. But in ${{{H}}_3}{{P}}{{{O}}_4}$, the oxidation state of ${{P}}$ is $+ 5$ which is stable. Thus it does not undergo a disproportionation reaction.

Additional information:
Combination reaction is a reaction in which two elements are combined to form a compound. The sum of oxidation states of reactants is equal to that of the single product. Displacement reaction is a reaction in which one element is reduced and replaces another element. A decomposition reaction is the opposite of a combination reaction. In this reaction, the molecule decomposes into its constituents. Combustion reaction is a reaction in which an organic compound reacts with oxygen and forms carbon dioxide.

Note:
A disproportionation reaction is known as a special kind of redox reaction. It is a special case of decomposition reaction. But in decomposition reaction, different elements are being oxidized and reduced. But in a disproportionation reaction, a single element is being oxidized and reduced.