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Group 15: Phosphorus - Properties And Uses Of Phosphine

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Group 15: Phosphorus - Properties And Uses Of Phosphine - Lesson Summary

Properties of phosphine:
  • Phosphine is a colourless and extremely poisonous gas that smells like garlic or rotten fish.
  • It is slightly soluble in water and the aqueous solution is neutral. It is more soluble in carbon disulphide and other organic solvents.
  • The melting and boiling points of phosphine are much less than that of ammonia due to the absence of hydrogen bonding. It condenses to a colourless liquid at -870c and freezes at -1320c.
  • Phosphine acts as a Lewis base. It is a weaker base than ammonia because its central atom phosphorus is larger in size and less electro-negative.
  • Phosphine combines with hydrogen iodide, thereby acting as a Lewis base by donating its lone pair to form phosphonium iodide salt.
  • Pure phosphine is non-combustible at normal conditions, but catches fire when heated to about 1500 to form phosphoric acid.
                          150°C
PH 3      +  2O 2      →         H 3PO 4
Pure                                 Phosphoric
Phosphine                         Acid

When phosphine is heated to about 440 0c in an inert atmosphere, or when it is subjected to an electric spark, it decomposes into phosphorus and hydrogen.

                  440°C
4PH 3           →                       P 4                   +         6H 2
            Inner Atmosphere      Red Phosphorus        Hydrogen


            Electic spark
4PH 3           →                       P 4                   +         6H 2
                                            Red Phosphorus        Hydrogen

An aqueous solution of phosphine decomposes in the presence of light into red phosphorus and hydrogen.

                 Light
4PH 3 (aq)          →                       P 4                   +         6H 2
Phosphine                             Red Phosphorus        Hydrogen

It burns with an explosion when it comes in contact with even small amounts of oxidising agents.
It burns with an explosion when it comes in contact with even small amounts of oxidising agents such as nitric acid and chlorine gas.

2PH 3          + 16HNO 3      →  P 2O 5       +     16NO 2   + 11H 2O
Phosphine      Nitric Acid     Phosphorus       Nitrogen
                                                    Pentoxide         Dioxide

PH 3          +       4Cl 2      →  PCl 5         +     3HCl         
Phosphine      Chlorine     Phosphorus          Hydrochloric
                                                 Pentachloride         Acid

When passed through aqueous metallic salt solutions such as copper sulphate, silver nitrate and mercuric chloride, phosphine gas precipitates the corresponding metal phosphides.

3CuSO 4     +    2PH 3           → Cu 3P 2 ↓        +       3H 2SO 4
Copper         Phosphine         Copper                 Sulphuric
Sulphate                               Phosphide                  Acid

3AgNO 3     +    PH 3           → Ag 3P ↓        +       3HNO 3
Silver         Phosphine         Silver                     Nitric
   Nitrate                               Phosphide                  Acid

3HgCl 2     +    2PH 3           → Hg 3P 2 ↓        +       6HCl
Mercuric         Phosphine         Mercuric                 Hydrochloric
      Chloride                                   Phosphide                  Acid

Uses of phosphine:
It finds use in the manufacture of Holme’s signals and smoke screens.

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