The formation of SO3 takes place according to the following reaction, 2SO2  +  O2  ⇌   2SO3 ;   ∆H=45.2  kcal The formation of SO3 is favoured by

In this reaction ∆H is negative so reaction moves forward by decrease in temperature while value of ∆n=2-3=-1, i.e., negative so the reaction moves forward by increase in pressure.