(i) Phosphorus trioxide P2O3 or P4O6 It is formed by burning phosphorus in limited supply of air. P4 + 3O2 → P4O6 The pentoxide, formed in small amount, is removed by passing through glass wool. The vapours of trioxide pass through the glass wool and are condensed in a reveiver cooled by a freezing mixture. Structure of Phosphine Phosphine is a covalent molecule. It has pyramidal structure like ammonia.
The bond angle H-P-H is 93o. Uses: (i) For making Holme’s signals. (ii) For making smoke screens. (iii) For making metallic phosphides. Vapours of phosphorus at low pressure react with N2O at 600oC to form P2O3. o C P4 + 6N2O 600 → 2P2O3 + 6N2
Properties (a) It is a waxy solid having garlic odour. (b) It is poisonous in nature. (c) It is soluble in benzene or chloroform. (d) When heated above 210oC, it forms red phosphorus and another oxide, P4O8.
4P4O6 → 3P4O8 + 4RePd Phosphorus tetroxide
When heated at about 440oC, it dissociates to phosphorus dioxide. 2P2O3 → 3PO2 + P (e) In contact with air, it is oxidised to phosphorus pentoxide. P4O6 +2O2 → P4O10 (f) It burns in chlorine forming oxy-chlorides. 2POCl3 P4O6 + 4Cl2 → Phosphorus + Oxy− chloride
2PO Cl
2 Metaphosphorus Oxy− chloride
(g) In cold water it dissolves slowly forming phosphorus acid. P4O6 + 6H2O (cold) → 4H3PO3 With hot water, a violent reaction occurs forming orthophosphoric acid and phosphine. P4O6 + 6H2O (hot) → 3H3PO4 + PH3 The above reaction is actually the conversion of phosphorus acid into orthophosphoric acid and phosphine. 4H3PO3 → 3H3PO4 + PH3 This oxide is known as acid anhydride of phosphorus acid.
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2. Oxides of Phosporus Phosphorus forms three important oxides. These exist in dimeric forms. (i) Phosphorus trioxide, P2O3 or P4O6 (ii) Phosphorus tetroxide, P2O4 or P4O8 (iii) Phosphorus pentoxide, P2O5 or P4O10