7. Compounds of Phosphorous

COMPOUNDS OF PHOSPHORUS

Phosphine :

Preparation :

(i) Phosphine is prepared by the reaction of calcium phosphide with water or dilute HCl.

 Ca₃P₂ + 6 H₂O  ®  3Ca(OH)₂ + 2 PH₃

Ca₃P₂ + 6HCl  ®  3CaCl₂ + 2PH₃

(ii) In the laboratory, it is prepared by heating white phosphorus with concentrated NaOH solution in an inert atmosphere of CO₂.

P₄ + 3 NaOH + 3 H₂O  ®  PH₃ + 3 NaH₂PO₂   

                                                              (sodium hypophosphite)

Pure PH₃ is non inflammable but becomes inflammable owing to the presence of P₂H₄ or P₄ vapours. For removal of impurify, it is absorbed in HI to form phosphonium iodide (PH₄I) which on treating with KOH gives off phosphine.

PH₄I + KOH ®  KI + H₂O + PH₃

Properties :

(i) It is a colourless gas with a slightly garlic or rotten fish smell and is highly poisonous. It explodes in contact with traces of oxidising agents like HNO₃, Cl₂ and Br₂  vapours.

(ii) It is slightly soluble in water but soluble in CS₂ and other organic solvents. The solution of PH₃ in water decomposes in presence of light giving red phosphorus and H₂.

(iii)When absorbed in copper sulphate or mercuric chloride, the corresponding phosphides are obtained.

3CuSO₄ + 2PH₃ ® Cu₃P₂ ¯ (black) + 3H₂SO₄

3HgCl₂ + 2 PH₃ ® Hg₃P₂ ¯ (brownish black) + 6 HCl

(iv) Phosphine on heating at 150ºC burns forming H₃PO₄ 

PH₃ + 2O₂ ® H₃PO₄           

(v) Phosphine is weakly basic and like ammonia, gives phosphonium compounds with acids e.g.,

 PH₃ + HBr ® PH₄Br

Phosphonium compounds are obtained when anhydrous phosphine reacts with anhydrous halogen acids (not in aqueous solution).

Uses :

The spontaneous combustion of phosphine is made to use in Holme’s signals. Containers containing calcium carbide and calcium phosphide are pierced and thrown in the sea when the gases evolved burn and serve as a signal.

It is also used in the production of smoke screens. Calcium phosphide reacts with water producing phosphine  which burns in air to give clouds of phosphorus pentaoxide and that acts as smoke screens.