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.