Question

Manufacture of sulphuric acid by contact process involves dissolution of $S{O}_3$ in $H_{2}S{O}_4$ and not in water. Justify.

Answer 1

The contact process is the current method of producing sulfuric acid in the high concentrations needed for industrial processes. Platinum used to be the catalyst for this reaction; however, as it is susceptible to reacting with arsenic impurities in the sulfur feedstock, vanadium(V) oxide (V2O5) is now preferred.

The sulfur trioxide is cooled and passed through two towers. In the first tower it is washed with oleum (fuming sulfuric acid, 100% sulfuric acid with sulfur trioxide dissolved in it).

$S+O_2\underset{}{\overset{\Delta }{\to }}{SO}_2{SO}_2+\frac{1}{2}{O}_2⟶{SO}_3{SO}_3+H_2{SO}_4⟶H_2{S}_2{O}_7$

${SO}_3$ when dissolved in $H_2{SO}_4$ forms foggy droplets that cannot be condensed. Hence, Manufacture of sulphuric acid by contact process involves dissolution of ${SO}_3$ in $H_2{SO}_4$ and not in water.