Question

The function of $Fe{(OH)_3}$ in the contact process is:
A. To detect colloidal impurities
B. To remove moisture
C. To remove dust particles
D. To remove arsenic impurities

Answer 1

Contact process is a manufacturing process of the most important chemical used by industries worldwide. Sulphuric acid is one of the oxoacids of sulphur.

Complete step by step answer:
Let us know all about the contact process. It involves three steps mainly that are burning of sulphur or its oxides to generate the poisonous gas sulphur dioxide. Then the sulphur oxide thus produced is converted into sulphur trioxide by the reaction with oxygen in the presence of the catalyst ${V_2}{O_5}$ after the gas is cooled, the sulfur dioxide enters the washing tower where it is sprayed by water to remove any soluble impurities. In the drying tower, sulfuric acid is sprayed on the gas to remove the moisture from it. Finally, arsenic oxide is a poison for catalyst and is removed when the gas is exposed to ferric hydroxide.
The reaction is as follows:
$A{s_2}{O_3} + 2Fe{(OH)_3} \to 2FeAs{O_3} + 3{H_2}O$
Hence, the function of $Fe{(OH)_3}$ in the contact process is to remove arsenic impurities.
In the manufacture of sulfuric acid vanadium pentoxide is used as a catalyst, arsenic oxide acts a catalytic poison which decreases the rate of reaction and $Fe{(OH)_3}$ is a positive sol removes arsenic impurity which is a negative sol.
Absorption of sulphur trioxide in sulphuric acid takes place to give oleum. The sulphuric acid produced through the contact process is 96-98% pure in nature. Hence to remove arsenic impurities $Fe{(OH)_3}$ is added in the process, initially arsenic is added to remove impurities from sulphur dioxide.

Therefore, the correct option is D.

Note:
Sulphuric acid is a colourless, dense liquid that dissolves in water with an evolution of a lot of heat. Hence care must be taken while preparing its solution from the concentrated sulphuric acid. It must be added slowly into water with continuous stirring.