Explain the contact process for the manufacture of sulphuric acid.

(N/A) The contact process involves the following key steps:
$(i)$ Burning of sulphur or sulphide ores in the presence of air to generate $SO_{2}$.
$(ii)$ Purification of $SO_{2}$ gas by removing dust and impurities like arsenic compounds.
$(iii)$ Catalytic oxidation of $SO_{2}$ to $SO_{3}$ using oxygen in the presence of a $V_{2}O_{5}$ catalyst:
$2 SO_{2(g)} + O_{2(g)} \rightarrow 2 SO_{3(g)}$,$\Delta_{r}H^{\ominus} = -196.6 \ kJ/mol$.
This reaction is exothermic,reversible,and involves a decrease in volume. Thus,low temperature and high pressure are favourable for maximum yield. In practice,the process is carried out at $2 \ bar$ pressure and $720 \ K$ temperature.
$(iv)$ Absorption of $SO_{3}$ in concentrated $H_{2}SO_{4}$ to produce oleum $(H_{2}S_{2}O_{7})$:
$SO_{3} + H_{2}SO_{4} \rightarrow H_{2}S_{2}O_{7}$ (Oleum).
$(v)$ Dilution of oleum with water to obtain sulphuric acid of the desired concentration ($96-98 \%$ pure).