Question

What happens when ${{\text{H}}_{3}}\text{P}{{\text{O}}_{3}}$ is heated?

Answer 1

${{\text{H}}_{3}}\text{P}{{\text{O}}_{3}}$ is known as orthophosphorus acid. It undergoes a disproportionation reaction on heating according to which the oxidation number of phosphorus present in it has different oxidation states. The products formed are orthophosphoric acid and phosphine.

Complete Step By Step Solution
The oxidation state of phosphorus in ${{\text{H}}_{3}}\text{P}{{\text{O}}_{3}}$ is $+3$ . Oxidation state determines the no. of electrons an atom can lose or gain to form chemical bonds with other atoms. Let oxidation state for phosphorus be ‘x’. For hydrogen we have oxidation no.1 and for oxygen we have oxidation number $-2$ . So, oxidation state of P in ${{\text {H}} {3}}\text {P} {{\text{O}}_{3}}$ is-
$3\times 1+x+3\times (-2) = 0$
$3+x-6=0$
$\Rightarrow x=+3$ (It is equated equal to zero because overall charge of ${{\text{H}}_{3}}\text{P}{{\text{O}}_{3}}$ is zero)
The products formed on heating ${{\text {H}} {3}}\text {P} {{\text{O}}_{3}}$ are orthophosphoric acid and phosphine. The chemical reaction is given as-
$4{{H}_{3}{PO}_{3}}\to{3{{H}_{3}{PO}_{4}}}+{{PH}_{3}}$
This heating reaction is a Disproportionation reaction because the oxidation state of phosphorus varies in the reactant and the products. As seen above oxidation state of phosphorus in reactant i.e. in orthophosphorus acid is $+3$ . Now checking its oxidation state in orthophosphoric acid-
$3\times 1+x+4\times (-2) = 0$
$3+x-8=0$
$\Rightarrow x=+5$
Oxidation no. for hydrogen and oxygen are the same as previously used but the no. of oxygen atoms varies for this acid. Overall charge is still zero so all the oxidation states multiplied with their respective no. of atoms are equated equal to zero. So, the oxidation state of phosphorus in orthophosphoric acid is $+5$ .
Now checking in phosphine
We get,
$x+3\times 1=0$
$x=-3$
So, the oxidation states of phosphorus are $+3$ , $+5$ and $-3$ in orthophosphorus acid, orthophosphoric acid and phosphine respectively.

Note
Orthophosphoric acid is used in sugar and textile industries. It is a weak acid. Beverages like cola include phosphoric acid which can promote risk factor for kidneys. It reacts exothermically with bases. Reaction with cyanide compounds leads to the release of gaseous hydrogen cyanide. It is also present in tea, coffee and cigarettes which lead to the dissolving of calcium present in teeth.