Phosphine

Phosphine (PH₃)

Phosphine is the most important hydride of phosphorous

Preparation:

Phosphine is prepared by action of sodium hydroxide with white phosphorous in an inert atmosphere of carbon dioxide or hydrogen.

Phosphine is freed from phosphine dihydride(P₂H₄) by passing through a freezing mixture. The dihydride condenses while phosphine does not.

Phosphine can also prepared by the hydrolysis of metallic phosphides with water or dilute mineral acids.

Phosphine is prepared in pure form by heating phosphorous acid.

A pure sample of phosphine is prepared by heating phosphonium iodide with caustic soda solution.

Physical properties:

It is colourless, poisonous gas with rotten fish smell. It is slightly soluble in water and is neutral to litmus test. It condenses to a colourless liquid at 188 K and freezes to a solid at 139.5 K .

Chemical properties:

Thermal stability: Phosphine decomposes into its elements when heated in absence of air at 317 K or when electric current is passed through it.

4PH₃ → ^317K →P₄ + 6H₂

Combustion: When phosphine is heated with air or oxygen it burns to give meta phosphoric acid.

Basic nature: Phosphine is weakly basic and forms phosphonium salts with halogen acids.

PH₃ + HI → PH₄I

It reacts with halogens to give phosphorus penta halides.

PH₃ + 4Cl₂ → PCl₅ + 3HCl

Reducing property : Phosphine precipitates some metal from their salt solutions.

3AgNO₃ + PH₃ → Ag₃P + 3HNO₃

It forms coordination compounds with lewis acids such as boron trichloride.

Structure:

In phosphine, phosphorus shows sp3 hybridisation.

Three orbitals are occupied by bond pair and fourth corner is occupied by lone pair of electrons. Hence, bond angle is reduced to 94°. Phosphine has a pyramidal shape.

Uses of phosphine:

Phosphine is used for producing smoke screen as it gives large smoke. In a ship, a pierced container with a mixture of calcium carbide and calcium phosphide, liberates phosphine and acetylene when thrown into sea. The liberated phosphine catches fire and ignites acetylene. These burning gases serves as a signal to the approaching ships. This is known as Holmes signal.