Question

In the preparation of sulphuric acid by contact process, SO3 is not absorbed directly in water to form sulphuric acid.Why ?

Answer 1

Dear Student,

The reaction between sulfur trioxide and water is very highly exothermic.
$S{O}_3\left(g\right)+H_{2}O\left(l\right)\to H_{2}S{O}_4\left(l\right)$

The heat generated is so much that it will vaporize the aqueous solution of Sulfuric acid which is being formed.
So, on an industrial scale, the cost of controlling this heat will be enormous which will make H₂SO₄ very costly.
Also, the vaporised H₂SO₄ is a highly corrosive aerosol that is very difficult to separate.

But, dissolving the SO₃ in H₂SO₄, oleum is formed:
$S{O}_3\left(g\right)+H_{2}S{O}_4\left(l\right)\to H_2{S}_2{O}_7\left(l\right)$

This reaction does not generate much heat which makes separation easy and cost of the acid negligible.

Regards