Question

Discuss important properties of phosphine.

Answer 1

Physical Properties:

1. It is sparingly soluble in water.
2. It has the ability to freeze to a white solid (m.p. 139.5 K) and condense to a colourless liquid (b.p. 188 K).
3. The colourless gas phosphorus has a rotten fish odour. In nature, it is extremely poisonous.

Chemical properties:

1. Combustion: It burns readily in the presence of oxygen to form phosphorus pentoxide.

\[\ce{4PH3 + 8O2 -> P4O10 + 6H2O}\]

2. Decomposition: It breaks down to release hydrogen and red phosphorus when heated to 317 K without air.

\[\ce{4PH3 -> P4 + 6H2}\]

3. Basic nature: In contrast to ammonia, phosphorus is a far weaker base. Litmus has no effect on its aqueous solution. However, it produces phosphonium salts when it combines with halogen acids.

For Example,

\[\ce{PH3 + HI -> PH4I}\]

4. Reaction with chlorine: Phosphorus trichloride is produced when phosphorus spontaneously burns in chlorine.

\[\ce{PH3 + 3Cl2 -> PCl3 + 3HCl}\]

5. Reaction with copper and silver salts: It precipitates the matching phosphides and ultimately reduces them to the metals when it passes through copper or silver salt solutions.

\[\ce{3CuSO4 + 2PH3 -> \underset{Copper phosphide}{Cu3P2} + 3H2SO4}\]

\[\ce{PH3 + 6AgNO3 + 3H2O -> \underset{(black ppt)}{6Ag} + H3PO3 + 6HNO3}\]