In the contact process for industrial manufacture of sulphuric acid, some amount of sulphuric acid is used a starting material. Explain briefly. What is the catalyst used in the oxidation of `SO_2` ?

Explanation

If sulphur trioxide is directly added to water to get sulphuric acid, fumes are formed and it is highly exothermic, so the operations become difficult to handle. Therefore, first oleum is prepared by dissolving sulphur trioxide in sulphuric acid. Then oleum is treated with water to get sulphuric acid.
`H_(2) SO_(4) + SO_(3) rarr H_(2) S_(2) O_(7) ("Oleum")`
`H_(2) S_(2) O_(7) + H_(2) O rarr 2H_(2) SO_(4)`.