Question

Explain why high pressure is required in the manufacture of sulphur trioxide by contact process. State the law or principle used.

Answer 1

High pressure is required in the manufacture of sulphur trioxide by contact process. Le Chatelier's principle is used.
$2S{O}_2+O_2\rightleftharpoons 2S{O}_3$
The number of moles of gaseous reactants $=2+1=3$.
The number of moles of gaseous products $=2$.

Change in the number of moles of gaseous species $\Delta n=2-3=-1<0$.

The reaction proceeds with decrease in the number of moles of gaseous species. When high pressure is applied, the equilibrium shifts in the forward direction (in the direction containing fewer number of moles of gaseous species). This nullifies the effect of increased pressure and decreases the yield of the reaction.