Notes On Group 15: Phosphorus - Preparation Of Oxoacids - CBSE Class 12 Chemistry
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.
                                                                 Δ
2P4                +    3Ba(OH)2   +     6H2O     →     3Ba(H2PO2)2       +  2PH3
White                   Barium                Water              Barium                   Phosphine
phosphorus              hydroxide                                hypophosphite

3Ba(H2PO2)2    +   H2SO4(dil)         →  BaSO4↓       +     2H3PO2
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.

      P4O      +      6H2O       →       4H3PO3
Phosphorous           water                 Orthophosphorous
    trioxide                                                       acid

                               or

       PCl3       +        3H2O       →        H3PO3            +        3HCl
Phosphorus            Water              Orthophosphorous       Hydrochloric acid
    trichloride

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

       5H3PO3               +   PCl3              →    3H4P2O5               +   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.


P4O10             +    6H2O           Δ→      4H3PO4
Phosphorous           water                        Orthophosphoric acid
       pentoxide

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


2H3PO4                  220°C→    H4P2O7                   +    H2O
Phosphoric Acid                   Pyrophosphoric acid            water

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

           H4P2O7          Red Hot→   2HPO3             +      H2O
pyrophosphoric acid                  Metaphosphoric acid       water

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

H3PO3            +  Br2       Δ→  HPO3             +   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.

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.
                                                                 Δ
2P4                +    3Ba(OH)2   +     6H2O     →     3Ba(H2PO2)2       +  2PH3
White                   Barium                Water              Barium                   Phosphine
phosphorus              hydroxide                                hypophosphite

3Ba(H2PO2)2    +   H2SO4(dil)         →  BaSO4↓       +     2H3PO2
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.

      P4O      +      6H2O       →       4H3PO3
Phosphorous           water                 Orthophosphorous
    trioxide                                                       acid

                               or

       PCl3       +        3H2O       →        H3PO3            +        3HCl
Phosphorus            Water              Orthophosphorous       Hydrochloric acid
    trichloride

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

       5H3PO3               +   PCl3              →    3H4P2O5               +   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.


P4O10             +    6H2O           Δ→      4H3PO4
Phosphorous           water                        Orthophosphoric acid
       pentoxide

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


2H3PO4                  220°C→    H4P2O7                   +    H2O
Phosphoric Acid                   Pyrophosphoric acid            water

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

           H4P2O7          Red Hot→   2HPO3             +      H2O
pyrophosphoric acid                  Metaphosphoric acid       water

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

H3PO3            +  Br2       Δ→  HPO3             +   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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