Explain in detail the manufacture of sulphuric acid by contact process.

Explanation

Manufacture of `H_(2)SO_(4)` by contact process
Manufacturing of `H_(2)SO_(4)` involves three main steps.
Step-1
`SO_(2)` production : The required `SO_(2)` for this process is obtained by burning S(or) Iron pyrites in oxygen.
`S+O_(2)toSO_(2)`
`4FeS_(2)+15O_(2)to2Fe_(2)O_(3)+8SO_(3)`
Step-2
`SO_(3)` formation : `SO_(2)` is oxidised in presence of catalyst with atmosphric air to form `SO_(3)`
`2SO_(2(g))+O_(2(g))underset(Delta)overset("catalyst")iff2SO_(3(g))`
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 equation 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 in 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 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 use 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.

Step-3
Formation of `H_(2)SO_(4):SO_(3)` formed in the above step absorbed in `98% H_(2)SO_(4)` to get oleum `(H_(2)S_(2)O_(7))`. This oleum is diluted to get desired concentration of `H_(2)SO_(4)`.
`SO_(3)+H_(2)SO_(4)toH_(2)S_(2)O_(7)`
`H_(2)S_(2)O_(7)+H_(2)O to2H_(2)SO_(4)`