The main step in the production of \(\mathrm{H}_2 \mathrm{SO}_4\) is :
\(\mathrm{SO}_2(g)+\mathrm{O}_2(g) \rightleftharpoons 2 \mathrm{SO}_3(g) ; \Delta_{\mathrm{f}} \mathrm{H}^{\circ}=-196.6 \mathrm{kJmol}^{-1}\)
The reaction is exothermic, reversible and forward reaction proceeds with decrease in volume. According to Le-Chateliers' principle, to maximise the yield of \(\mathrm{SO}_3\) and hence \(\mathrm{H}_2 \mathrm{SO}_4\), a low \(\mathrm{T}\), a high pressure and \(\mathrm{V}_2 \mathrm{O}_5\) is used as a catalyst.