Explain contact process.

Explanation

In contact process, the following steps are involved :
(i) Burning of sulphur or sulphide ores in presence of air to generate `SO_2`.
(ii) Conversion of `SO_2` to `SO_3` by the reaction with oxygen in the presence of a catalyst (`V_2O_5`).
(iii) Absorption of `SO_3` in `H_2SO_4` to give oleum (`H_2S_2O_7`).
The `SO_2` produced is purified by removing dust and other impurities such as arsenic compounds. The key step is the catalytic oxidation of `SO_2` with `O_2` to give `SO_3` in the presence of `V_2O_5` (catalyst).
`2SO_(2)(g) + O_(2)(g) overset(V_(2)O_(5))to 2SO_(3)(g), Delta_(r)H^(-) = -196.6 kJ//mol^(-1)`
The reaction is exothermic, reversible and the forward reaction leads to a decrease in volume. Therefore, low temperature and high pressure are the favourable conditions for maximum yield. But the temperature should not be very low otherwise.
In practice, the process is carried out at 2 bar pressure and 720 K temperature. The `SO_3` gas from the catalytic convertor is absorbed in concentrated `H_2SO_4` of the desired concentration. In the industry, two steps are carried out simultaneously to make process continuous and cost effective.
`SO_(3) + H_(2)SO_(4) to H_(2)S_(2)O_(7)` (Oleum)
The sulphuric acid obtained by contact process is 96-98% pure.