Question

Conversion of (a) sulphur and (b) iron pyrites to sulphur dioxide in the first step of the contact process is;
 $ S+{{O}_{2}}\to S{{O}_{2}} $ and $ Fe{{S}_{2}}+xF{{e}^{3+}}+8{{H}_{2}}O\to 15F{{e}^{2+}}+ySO_{4}^{2-}+16{{H}^{+}} $
Find the value of x and y:
(A) $ x=15,y=1 $
(B) $ x=14,y=2 $
(C) $ x=7,y=2 $
(D) $ x=14,y=1 $

Answer 1

Hint : Sulphur and iron pyrites in order to get is industrially manufactured by the Contact process. It is a colorless, dense, syrupy liquid. It is also known as oil of vitriol. The sulphuric acid obtained by the Contact process is free from arsenic impurities and is $ 96-98% $ pure.

Complete Step By Step Answer:
Manufacture of sulphuric acid by the Contact process involves the following three steps: Burning of sulphur or roasting of iron pyrites $ (Fe{{S}_{2}}) $ to give sulphur dioxide $ (S{{O}_{2}}) $ Chemical reactions involved:
 $ S+{{O}_{2}}\to S{{O}_{2}} $
 $ Fe{{S}_{2}}+xF{{e}^{3+}}+8{{H}_{2}}O\to 15F{{e}^{2+}}+ySO_{4}^{2-}+16{{H}^{+}} $
Oxidation of $ S{{O}_{2}} $ by air to sulphur trioxide $ S{{O}_{2}} $ in the presence of a catalyst vanadium pentoxide $ ({{V}_{2}}{{O}_{5}}) $ . Low temperature: Since the reaction involved is an exothermic reaction, low temperature will push the reaction forward resulting in higher yield of $ S{{O}_{2}} $ . However, the rate of reaction also depends on temperature, so the temperature has to be maintained at an optimum value of $ 720K $ so that the reaction rate is not slowed down.
Here a chemical reaction is said to be balanced only when the charges and the number of atoms in the reactants and products are the same. As there are two Sulphur atoms in the reactants side, there should be two Sulphur atoms at the products side also. So, $ y=2 $ to make equal sulphur atoms on both sides. As there are total $ 15\text{ }Fe\left( II \right) $ ions on the side of products, there should be the same number of $ Fe $ on the reactants side. As there is already one $ Fe $ present in $ Fe{{S}_{2}}. $
 $ S+{{O}_{2}}\to S{{O}_{2}} $
 $ Fe{{S}_{2}}+14F{{e}^{3+}}+8{{H}_{2}}O\to 15F{{e}^{2+}}+2SO_{4}^{2-}+16{{H}^{+}} $
Here in order to balance we need $ 14 $ more iron compounds.
Hence $ x=14, $ and $ y=2. $
Therefore, the correct answer is option B.

Note :
The plant used in the Contact process for manufacturing of has four main parts, namely, sulphur burner where oxidation of sulphur ores is carried out, purifying unit where any dust or arsenic impurities are removed, catalytic converter and absorption tower. Sulphuric acid is used for the manufacture of a number of compounds like fertilizers (e.g. superphosphate, ammonium sulphate, etc.), dyes, paints and nitrocellulose products.