Explain the conditions favourable for the formation of `SO_(3)` from `SO_(2)` in the contact process of `H_(2)SO_(4)`.

Explanation

Le Chatlier's principle - Application to produce `SO_(3)` :
The oxidation of `SO_(2)` to `SO_(3)` in the presence of a catalyst is a reversible reaction. The thermochemical equation for the conversion is written as
`2SO_(2(g))+O_(2(g))iff2SO_(3(g)),DeltaH=-196" kJ"`
The equati9on reveals the following points :
i. 3 volumes of the reactants convert into 2 volumes of `SO_(3)`. i.e., a decrease of volume accompanies the reaction.
ii. the reaction is an exothermic change.
iii. the catalyst may be present to increase the `SO_(3)` yields.
According to Le Chatlier's principle,
i. a decrease in volume of the system is favoured at high pressures. But in practice only about 2 bar pressure is used. The reason for not using high pressures is acid resisting towers which can withstand high pressures cannot be built.
ii. exothermic changes are favoured at low temperatures. It is not always convenient in the industry to work at low temperatures. In such situations an optimum temperature is maintained. At this temperature considerable amounts of the product are obtained. In the manufacture of `H_(2)SO_(4)`, the optimum temperature suitable for the conversion of `SO_(2)` into `SO_(3)` is experimentally found to be 720K.
iii. The rate of formation of `SO_(3)` is enhanced by the used of a catalyst. (`V_(2)O_(5)` (or) Pt - asbestos).
Favourable Conditions :
Temperature : 720K
Pressure : 2 bar
Catalyst : `V_(2)O_(5)` (or) platinized asbestos.