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Group 15: Phosphorus - Preparation Of Oxoacids

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Group 15: Phosphorus - Preparation Of Oxoacids - Lesson Summary

Phosphorus forms a number of oxoacids, such as hypophosphorus acid, orthophosphorus acid, pyrophosphorous acid, hypophosphoricacid, orthophosphoricacid, pyro phosphoric acid and metaphosphoricacid.

The oxoacids in which phosphorus exhibits lower oxidation state of +1or +3constitute the “phosphorus acid series.” While the ones in which phosphorus exhibits higher oxidation states of +4 or +5 constitute the “phosphoric acid series”.

Phosphorus acid series:

Preparation of hypophosphorus acid or phosphinicacid:
  • It is prepared by the alkaline hydrolysis of white phosphorus.
  • When white phosphorus is heated with an alkali solution like barium hydroxide, it forms  hypophosphite.  It is further treated with dilute sulphuric acid to recover the acid.
2P 4                +    3Ba(OH) 2   +     6H 2O     →     3Ba(H 2PO 2) 2       +  2PH 3
White                   Barium                Water              Barium                   Phosphine
phosphorus              hydroxide                                hypophosphite

3Ba(H 2PO 2) 2    +   H 2SO 4(dil)         →  BaSO 4↓       +     2H 3PO 2
Barium                    Sulphuric              Barium                 Hypophosphorous
 hypophosphite               acid                 sulphate                          acid

Preparation of orthophosphorousacid or phosphonic acid or phosphorous acid:
  • The hydrolysis of phosphorus trioxide or phosphorus trichloride forms phosphorous acid.

      P 4O       +      6H 2O       →       4H 3PO 3
Phosphorous           water                 Orthophosphorous
    trioxide                                                       acid


       PCl 3       +        3H 2O       →        H 3PO 3            +        3HCl
Phosphorus            Water              Orthophosphorous       Hydrochloric acid

Preparation of pyrophosphorousacid:
  • Orthophosphorousacid, when treated with phosphorus trichloride, gives pyrophosphorousacid.

       5H 3PO 3               +   PCl 3              →    3H4P 2O 5               +   3HCl
Orthophosphorous       Phosphorus             Pyrophosphorous        Hydrogen
              acid                          trichloride                          acid                 chloride

Phosphoric acid series:

Preparation of Hypophosphoric acid:
  • It is obtained by the alkaline hydrolysis and oxidation of red phosphorus

Preparation of Orthophosphoric acid or phosphoric acid:
  • It is prepared by dissolving phosphorus pent oxide in water and then boiling the solution.

P 4O 10             +    6H 2O           Δ→      4H 3PO 4
Phosphorous           water                        Orthophosphoric acid

Preparation of pyrophosphoric acid:
  • When phosphoric acid is heated gently to about 2200c, it loses water molecules to form pyro phosphoric acid.

2H 3PO 4                  220°C→    H 4P 2O 7                   +    H 2O
Phosphoric Acid                   Pyrophosphoric acid            water

Preparation of metaphosphoricacid acid:
  • Pyrophosphoric acid or Orthophosphoric acid, when heated to red hot, forms metaphosphoricacid acid.

           H 4P 2O 7          Red Hot→   2HPO 3             +      H 2O
pyrophosphoric acid                  Metaphosphoric acid       water

Crystalline phosphorus acid, on being heated with bromine in a sealed tube, gives metaphosphoricacid acid.

H 3PO 3            +  Br 2       Δ→  HPO 3             +   2HBr
Phosphorous      Bromine      Metaphosphoric       Hydrobromic acid
           acid                                     acid

Metaphosphoricacid acid exists as a polymer.
Cyclotrimetaphosphoric acid and polymetaphosphoricacid are some polymeric forms of metaphosphoricacid acid.


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