Question

What happens when:
(i) $PC{l}_5$ is heated?
(ii) $H_{3}P{O}_3$ is heated?

Also write the reactions involved.

Answer 1

(i) When phosphorus pentachloride $\left(PC{l}_5\right)$ is heated, it decomposes into $PC{l}_3$ and $C{l}_2$. This reaction is reversible.

$PC{l}_5\iff PC{l}_3+C{l}_2$

(ii) Orthophosphorous acid $\left(H_{3}P{O}_3\right)$ on heating gives $H_{3}P{O}_4$ and $P{H}_3$. In this reaction, the oxidation number of $P$ in $H_{3}P{O}_3$ is +3. After reaction, the oxidation number of $P$ increases to +5 in $H_{3}P{O}_4$ and decreases to -3 in $P{H}_3$. Therefore, this is called a disproportionation reaction.

$H_{3}P{O}_3\to 3H_{3}P{O}_4+P{H}_3$