Phosphorus

Occurrence
Phosphorus is a very reactive element and hence it does not occur free in nature. It occurs mainly in the form of phosphate minerals in the crust of the earth. Some important minerals of phosphorus are

• Phosphorite, Ca₃(PO₄)₂
• Fluorapatite, Ca₅(PO₄)₃F or 3Ca₃(PO₄)₂.CaF₂
• Hydroxyapatite, Ca₅(PO₄)₃OH or 3Ca₃(PO₄)_2.Ca(OH)₂
• Chlorapatite, Ca₅(PO₄)₃CI or 3Ca₃(PO₄)_2.CaCl₂

Preparation
Phosphorus is isolated by heating Ca₃(PO₄)₂with coke and silica in an electric furnace at 1770 K. The reactions taking place may be represented as under
2Ca₃(PO₄)₂ + 6SiO₂6CaSiO₃ + P₄O₁₀
P₄O₁₀ + 10C P₄ + 10CO
The vapours of phosphorus thus obtained upon condensation give white phosphorus which exists as P₄ molecules.

OXIDES AND OXYACIDS OF PHOSPHORUS
Oxides
Two important oxides of phosphorus are
Phosphorus trioxide - P₄O₆,also called phosphorus oxide or phosphorus (III) oxide
Phosphorus pentoxide -P₄O₁₀,also called as phosphoric oxide or phosphorus (V) oxide

P₄O₆ Preparation
Prepared by burning white phosphorus in limited supply of air
P₄ + 3O₂ → P₄O₆
Properties

• On heating in air, it forms phosphorus (V) oxide
  P₄O₆ + 2O₂→P₄O₁₀
• Reacts with water as follows :
  P₄O₆ + 6H₂O(cold)→ 4H₃PO₃
          Phosphorus acid
  P₄O₆ + 6H₂O(hot)→ 3H³PO₄ + PH₃
          (orthophosphoric acid)

P₄O₁₀ Preparation
Prepared by burning white phosphorus in an excess of air or oxygen
P₄ + 5O₂→ P₄O₁₀

Properties

• Because of its great affinity for water, it acts as a dehydrating agent.
  2HNO₃ + P₄O₁₀ → 2N₅O₅ + 4HPO₃
  2H₂SO₄ + P₄O₁₀ → 2SO₃ + 4HPO₃
• Reacts with water as follows :
  P₄O₁₀ + 2H₂O(cold) → 4HPO₃
  P₄O₁₀ + 6H₂O(hot) → 4H₃PO₄

Structure of P₄O₆ and P₄O₁₀

Oxyacids
Phosphorus forms two series of oxyacides:

• The phosphoric series of acids, in which the oxidation state of P is +5 or +4 and have oxidizing properties.
• The phosphorous series of acids, which contain P in the oxidation state +1  or +3 and are reducing agents.

Phosphorus forms numerous oxy-acids, all of which contain tetrahedral co-ordinated phosphorus containing at least one P = 0 unit and one P − OH group. The condensed systems are formed by P − O − P links or by direct P − P links.

Preparation

• Phosphorus acid (H₃PO₃)
  Prepared by  the hydrolysis of phosphorus trichloride or phosphorus trioxide.
  PCl₃ + 3H₂O → H₃PO₃ + 3HCl
  P₄O₆ + 6H₂O → 4H₃PO₃


• Orthophosphoric acid (H₃PO₄)
  (a)Prepared by burning phosphorus followed by the hydrolysis of the P₄O₁₀ so formed
  PO₄ + 5O₂ → P₄O₁₀→ 4H₃PO₄
  (b)By heating phosphate rock with sulphuric acid
  Ca₃(PO₄)₂ + 3H₂SO₄→ 3CaSO₄ + 2H₃PO₄
  On heating, it forms pyrophosphoric acid and then metaphosphoric acid on further heating.
  2H₃PO₄H₄P₂O₇ + H₂O
            Pyrophosphoric acid
  H₃PO₄HPO₃ + H₂O
              Meta phosphoric acid


• Metaphosphoric acid (HPO₃):
  Prepared by heating orthophosphoric acid to 575K
  H₃PO₄HPO₃ + H₂O
• Pyrophosphoric acid (H₄P₂O₇):
  Prepared by heating orthophosphoric acid to about 525 K.
  2H₃PO₄H₄P₂O₇ + H₂O.

Structure of Oxyacids of Phosphorus

Hypophosphoric acid, (H4P2O6)	Phosphorus acid, H3PO3