Phosphine preparation and properties, Group 15 P Block Elements

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.

Phosphine preparation and properties, Group 15 P Block Elements 1

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

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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.

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

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

Phosphine preparation and properties, Group 15 P Block Elements

→ 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.

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Phosphine preparation and properties, Group 15 P Block Elements 6

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

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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

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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.

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→ 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.

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→ 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

Phosphine preparation and properties, Group 15 P Block Elements

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

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→ 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.

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

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

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

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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

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