Chemistry Allotropic Forms of Phosphorous and Phosphine

### Topics Covered :

● Allotropes of Phosphorous
● White Phosphorous
● Red Phosphorous
● Black Phosphorous
● Preparation of Phosphine
● Properties and Uses of Phosphine

### Phosphorus — Allotropic Forms :

Phosphorus is found in many allotropic forms, the important ones being white, red and black.

### White phosphorus :

● It is a translucent white waxy solid.

● It is poisonous, insoluble in water but soluble in carbon disulphide

● It glows in dark (chemiluminescence).

● It dissolves in boiling color{red}(NaOH) solution in an inert atmosphere giving color{red}(PH_3).

color{red}(P_4+3NaOH +3H_2O → PH_3 + undersettext{(sodium hypophosphite)}(3NaH_2PO_2))

● White phosphorus is less stable and therefore, more reactive than the other solid phases under normal conditions because of angular
strain in the color{red}(P_4) molecule where the angles are only 60^0.

● It readily catches fire in air to give dense white fumes of color{red}(P_4O_(10)).

color{red}(P_4+5O_2 → P_4O_(10))

● It consists of discrete tetrahedral color{red}(P_4) molecule as shown in Fig. 7.2.

### Red phosphorus :

● Red phosphorus is obtained by heating white phosphorus at 573K in an inert atmosphere for several days.

● When red phosphorus is heated under high pressure, a series of phases of black phosphorus are formed.

● Red phosphorus possesses iron grey lustre.

● It is odourless, non-poisonous and insoluble in water as well as in carbon disulphide.

● Chemically, red phosphorus is much less reactive than white phosphorus.

● It does not glow in the dark.

● It is polymeric, consisting of chains of color{red}(P_4) tetrahedra linked together in the manner as shown in Fig. 7.3.

### Black phosphorus :

=> Black phosphorus has two forms color{red}(α)-black phosphorus and color{red}(β)-black phosphorus.

color{green}(α-text(Black Phosphorus ))

●It is formed when red phosphorus is heated in a sealed tube at 803K.

● It can be sublimed in air and has opaque monoclinic or rhombohedral crystals.

● It does not oxidise in air.

color{green}(β-text(Black Phosphorus ))

● It is prepared by heating white phosphorus at 473 K under high pressure.

● It does not burn in air upto 673 K.

### Phosphine :

Preparation, properties and uses of phosphine is as follow :

### Preparation :

=> Phosphine is prepared by the reaction of calcium phosphide with water or dilute color{red}(HCl).

color{red}(Ca_3P_2 +6H_2O → 3 Ca(OH)_2+2PH_3)

color{red}(Ca_3P_2+6HCl → 3Ca Cl_2+2PH_3)

=> In the laboratory, it is prepared by heating white phosphorus with concentrated color{red}(NaOH) solution in an inert atmosphere of color{red}(CO_2).

color{red}(P_4+3NaOH +3H_2O → PH_3 undersettext{(sodium hypophosphite)} (3 NaH_2PO_2))

● When pure, it is non inflammable but becomes inflammable owing to the presence of color{red}(P_2H_4) or color{red}(P_4) vapours.

● To purify it from the impurities, it is absorbed in color{red}(HI) to form phosphonium iodide color{red}(PH_4I) which on treating with color{red}(KOH) gives off phosphine.

color{red}(PH_4I +KOH → KI +H_2O +PH_3)

### Properties :

● It is a colourless gas with rotten fish smell and is highly poisonous.

● It explodes in contact with traces of oxidising agents like color{red}(HNO_3, Cl_2) and color{red}(Br_2) vapours.

● It is slightly soluble in water. The solution of PH_3 in water decomposes in presence of light giving red phosphorus and color{red}(H_2).

● When absorbed in copper sulphate or mercuric chloride solution, the corresponding phosphides are obtained.

color{red}(3CuSO_4 +2PH_3 → Cu_3P_2+3H_2SO_4)

color{red}(3HgCl_2+2PH_3 → Hg_3P_2 +6HCl)

● Phosphine is weakly basic and like ammonia, gives phosphonium compounds with acids e.g.,

color{red}(PH_3+HBr → PH_4Br)

### Uses :

● The spontaneous combustion of phosphine is technically used in Holme’s signals. Containers containing calcium carbide and calcium phosphide are pierced and thrown in the sea when the gases evolved burn and serve as a signal.

● It is also used in smoke screens.
Q 3070391216

In what way can it be proved that PH_3 is basic in nature?

Solution:

PH_3 reacts with acids like HI to form PH_4I which shows that it is basic in nature.
 PH_3 + HI → PH_4 I
Due to lone pair on phosphorus atom, PH_3 is acting as a Lewis base in the above reaction.