The key step in the manufacture of sulphuric acid is oxidation of $\mathrm{SO}_2$ to $\mathrm{SO}_3$ in presence of $\mathrm{V}_2\mathrm{O}_5$ catalyst. $2\mathrm{SO}_2 + \mathrm{O}_2 \xrightarrow{\mathrm{V}_2\mathrm{O}_5} 2\mathrm{SO}_3 ,$ $\Delta H = -196.6\ \text{kJ mol}^{-1}$The reaction is exothermic and reversible. Hence, low temperature andhigh pressure are the favourable conditions for maximum yield of $\mathrm{SO}_3$ . Inpractice a pressure of 2 bar and temperature of 720 K is maintained.