Question

In the preparation of red phosphorus from white phosphorus:
(A) $Mn{{O}_{2}}$ is used as a catalyst
(B) The white phosphorus is treated in an electric furnace
(C) A little iodine is used as a catalyst
(D) The gas ${{P}_{4}}$ is released

Answer 1

Both white and red phosphorus are the allotropic forms of phosphorus. The white phosphorus only exists as ${{P}_{4}}$ unit and the red phosphorus the ${{P}_{4}}$ units are joined together. The white phosphorus can be converted into red phosphorus by using a catalyst and heating in an inert atmosphere.

Complete step by step solution:
White, red, and black are the three allotropic forms of phosphorus and these are interconvertible.
White phosphorus is the simplest of all having a ${{P}_{4}}$ unit. White phosphorus is obtained if we heat the phosphate rock with coke or sand in an electric furnace. White phosphorus is highly poisonous and the person working with white phosphorus develops a disease called Phossy jaw.
It is a soft, white waxy solid that can be cut with a knife.
It can be converted into red phosphorus in which many ${{P}_{4}}$ units are joined together. When white phosphorus is heated at 573 K in an inert atmosphere i.e., out of contact with air like $C{{O}_{2}}$ gas in the presence of little iodine as a catalyst, red phosphorus is formed. The reaction is given below:
$\underset{white\text{ }phosphorus}{\mathop {{P}_{4}}}\,\xrightarrow[C{{O}_{2}},\text{ }{{I}_{2}}]{573K}\underset{red\text{ }phosphorus}{\mathop {{P}_{4}}}\,$
Red phosphorus is not poisonous and is an iron grey lustrous hard crystalline odorless solid.

Therefore, the correct answer is an option (C)- A little iodine is used as a catalyst.

Note: White phosphorus is not soluble in water but it is soluble in organic solvent whereas the red phosphorus is not soluble in either water or organic solvent. White phosphorus has a garlic odour but red phosphorus is odourless.