On reaction with $C l_{2}$ , phosphorus forms two types of halides ‘A’ and ‘B’. Halide A is yellowish-white powder but halide ‘B’ is colorless oily liquid. Identify A and B and write the formulae of their hydrolysis products.

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Solution

Reaction of phosphorus with chlorine

(1) When white phosphorus $(P_{4})$ is reacted with excess of dry chlorine formation of phosphorus pentachloride $(P C l_{5})$ takes place:

$P_{4} + 10 C l_{2} \to 4 P C l_{5}$

$P C l_{5}$ is a yellowish white powder.

(2) When dry chlorine is passed over heated white phosphorus $(P_{4})$ , formation of phosphorus trichloride $(P C l_{3})$ takes place:

$P_{4} + 6 C l_{2} \to 4 P C l_{3}$

$P C l_{3}$ is a colorless oily liquid.

Thus halide A is $P C l_{5}$ and halide B is $P C l_{3}$ .

Hydrolysis reaction of halides of phosphorus

(1) Reaction of phosphorus pentachloride $P C l_{5}$ with water $H_{2} O$ .
In moist air, $P C l_{5}$ hydrolyses to $P O C l_{3}$ and finally gets converted to phosphoric acid $(H_{3} P O_{4})$ .

$P C l_{5} + H_{2} O$ $\to P O C l_{3} + 2 H C l$
$P O C l_{3} + 3 H_{2} O$ $\to H_{3} P O_{4} + 3 H C l$

Thus on hydrolysis of $P C l_{5}$ formation of $H_{3} P O_{4}$ takes place.

(2) Reaction of phosphorus trichloride $(P C l_{3})$ with water $H_{2} O$ . In moist air, $P C l_{3}$ hydrolyses and gets converted to phosphorous acid $(H_{3} P O_{3})$ .

$P C l_{3} + 3 H_{2} O \to H_{3} P O_{3} + 3 H C l$

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