Question

${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$on reacting with water does not form
A. tetra metaphosphoric acid.
B. phosphorous acid.
C. orthophosphoric acid.
D. pyrophosphoric acid.

Answer 1

The reaction of${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$with water is known as hydrolysis reaction. We will form hydrolysis products of ${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$ which will contain phosphorus, hydrogen and oxygen atoms. The oxidation state of the product will remain the same as in reactant.

Complete answer
The reactions with water which add water in the reactant are known as hydrolysis.
${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$ reacts differently with cold and hot water. On reacting with cold water, ${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$gives metaphosphoric acid and on reacting with hot water, ${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$gives orthophosphoric acid.
${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}\,{\text{ + }}\,{\text{2}}{{\text{H}}_{\text{2}}}{\text{O(cold)}}\, \to \,4\,{\text{HP}}{{\text{O}}_3}$
${\text{HP}}{{\text{O}}_3}$is known as metaphosphoric acid. Four molecules join to form a tetramer.
The structure of metaphosphoric acid is as follows:

${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}\,{\text{ + }}\,{\text{6}}{{\text{H}}_{\text{2}}}{\text{O(hot)}}\, \to \,4\,{{\text{H}}_3}{\text{P}}{{\text{O}}_4}$
${\text{HP}}{{\text{O}}_3}$is known as orthophosphoric acid.
The structure of orthophosphoric acid is as follows:

By the hydrolysis of ${{\text{P}}_2}{{\text{O}}_3}$, phosphorous acid forms.
${{\text{P}}_2}{{\text{O}}_3}\,{\text{ + }}\,3\,{{\text{H}}_{\text{2}}}{\text{O}}\, \to \,2\,{{\text{H}}_3}{\text{P}}{{\text{O}}_3}$
The structure of phosphorous acid is as follows:

By the heating of two molecules of orthophosphoric acid, a molecule of pyrophosphoric acid forms.
${\text{2}}\,{{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}\mathop {\, \to }\limits^\Delta \,{{\text{H}}_{\text{4}}}{{\text{P}}_{\text{2}}}{{\text{O}}_{\text{7}}}$
The structure of pyrophosphoric acid is as follows:

So, ${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$on reacting with water gives orthophosphoric acid and metaphosphoric acid.
So, ${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$on reacting with water does not form phosphorous acid and pyrophosphoric acid.

Therefore, option (B) phosphorous acid and (D) pyrophosphoric acid, are correct.

Note: Reactant and hydrolysis product both have phosphorus in the same oxidation state. Oxidation state of phosphorus in orthophosphoric acid and metaphosphoric acid is $+ 5$. The oxidation state of phosphorus in${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$ is also $+ 5$. The oxidation state of phosphorus in phosphoric acid is $+ 3$. The oxidation state of phosphorus in pyrophosphoric acid is also $+ 5$ but is produced by heating of orthophosphoric acid not be hydrolysis of ${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$.