Question

Give reasons $H_{3}P{O}_3$ undergoes disproportionation reaction but $H_{3}P{O}_4$ does not.

Answer 1

The oxidation number of P in $H_{3}P{O}_3$ and $H_{3}P{O}_4$ is $+3$ and $+5$ respectively.
$H_{3}P{O}_4$ can only undergo reduction as maximum possible oxidation state for P is $+5$.
$H_{3}P{O}_3$ can undergo oxidation as well as reduction.
Hence, $H_{3}P{O}_3$ undergoes disproportionation reaction but $H_{3}P{O}_4$ does not.
Note:
Disproportionation reaction of $H_{3}P{O}_3$.
$4H_{3}P{O}_3\stackrel{\Delta }{\to }3H_{3}P{O}_4+P{H}_3$