Phosphine preparation and properties

Phosphine preparation and properties, Group 15 P Block Elements

Phosphine (PH₃) :

It was discovered by Gengembre in 1783. It is analogous to ammonia. It is known as hydrogen phosphide.

Preparation of Phosphine

It can be prepared as follows:

→ From Phosphides : When metallic phosphide reacts with water and mineral acids then phosphine is formed.

→ From Phosphorous Acid : When phosphorus acid is heated at 478-483 K temperature then pure phosphine is prepared.

The above reaction is disproportionation reaction.

→ Laboratory Preparation : In laboratory phsophine can be prepared by heating white phosphorus with concentrated NaOH solution is an inert atmosphere of carbon dioxide or coal gas.

→ In above reaction phosphorus dihydride is also prepared with phosphine which is highly inflammable.

→ Thus, as soon as, these gas bubbles when comes in contact with air, catches fire spontaneously forming rings of smoke known as vortex rings.

→ It is interesting to note that pure phosphine itself is non-inflammable. Actually the combustion is due to the presence of highly inflammable P, H, which burns and forms vortex ring.

→ From Phosphonium lodide : When phosphonium iodide is heated with sodium hydroxide solution, then phosphine is prepared.

Structure of Phosphine

→ The structure of phosphine is pyramidal. Phosphorus atom in phosphine is sp hybridised. In PH,,three bonds are formed by the overlaping of sp -hybrid orbitals with 18-orbital of hydrogen, one lone pair of electrons is also present. The H-P-H bond angle is 936 and P-H bond length is 142 pm

Properties of Phosphine

Physical and chemical properties of phosphine are as follows:

Physical Properties : Some important physical properties are :

• It is a colourless gas with rotten fish or garlic smell.
• It is heavier than air.
• It is highly poisonous gas.
• It is slightly soluble in water.
• Its solution in water is neutral to litmus paper.
• It liquifies at 185.5 K and solidifies at 139.5 K.

Chemical Properties : Some important chemical properties of phosphine are as follows :

→ Decomposition : In presence of sunlight or on heating upto 713 K, it decomposes into red phosphorus and hydrogen.

→ Basic Nature : Phosphine is neutral to litmus but it behaves as weak base. It is weaker base than ammonia. It reacts with halogen acids and form phosphonium salts.

→ Combustibility : Phosphine when heated to 423 K in air or oxygen phosphorus pentaoxide is formed.

2PH₃ + 4O₂ → P₂O₅ + 3H₂O

→ Phosphorus pentaoxide when reacts with water forms meta phosphoric acid.

P₂O₅ + H₂O → 2HPO₃

→ Reaction with Metallic Salts : Phosphine when reacts with metallic salts of copper, silver and mercury etc., then it forms phosphide.

→ Formation of addition compounds : Phosphine forms addition compounds with anhydrous AlCl₃ and SnCl₄.

AlCl₃ + 2PH₃ → AlCl₃ . 2PH₃
SnCl₄ + 2PH₃ → SnCl₄ . 2PH₃

→ It also forms addition compounds with cuprous chloride solution in HCl.

→ Reaction with N₃O : The mixture of PH, and N O or PH, and NO explodes in presence of electric spark.

→ Reaction with Nitric Acid : Phosphine when comes in contact with nitric acid then it begins to burn.

→ Reaction with Chlorine : Phosphine reacts with chlorine and it forms phosphorus pentachloride.

Uses of Phosphine

• The spontaneous combustion of phosphine is technically used in Holme’s signals.
• To produce these signals the containers containing calcium carbide and calcium phosphide are pierced and thrown in the sea.
• When the gases evolved, they burn and serve as a signal. It is also used in smoke screens.
• It is used for making metallic phosphide.

Chemistry Notes