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?


`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.