Reaction of phosphorus with chlorine
(1) When white phosphorus (P4) is reacted with excess of dry chlorine formation of phosphorus pentachloride (PCl5) takes place:
P4+10 Cl2→4 PCl5
PCl5 is a yellowish white powder.
(2) When dry chlorine is passed over heated white phosphorus (P4), formation of phosphorus trichloride (PCl3) takes place:
P4+6 Cl2→4 PCl3
PCl3 is a colorless oily liquid.
Thus halide A is PCl5 and halide B is PCl3.
Hydrolysis reaction of halides of phosphorus
(1) Reaction of phosphorus pentachloride PCl5 with water H2O.
In moist air, PCl5 hydrolyses to POCl3 and finally gets converted to phosphoric acid (H3PO4).
PCl5+H2O→POCl3+2 HCl
POCl3+3H2O→H3PO4+3 HCl
Thus on hydrolysis of PCl5 formation of H3PO4 takes place.
(2) Reaction of phosphorus trichloride (PCl3) with water H2O. In moist air, PCl3 hydrolyses and gets converted to phosphorous acid (H3PO3).
PCl3+3 H2O→H3PO3+3 HCl