Q. Show reversible and exothermic reaction?

The fact that SO₂, can be oxidised to SO₃, by air in the presence of platinum catalyst is the basis of the contact process which is used the world over for the manufacture of H₂SO₄.SO₃ is obtained in a manner similar to the chamber process. But before oxidation it requires thorough purification in order to avoid the poisoning of the catalyst, particularly by the arsenic impurities. The purified SO₂ is then oxidised directly according to the following reaction:

SO₂+ 1/2O₂----------------> SO₃.DH⁰= -98 kj mol^-1

It is a reversible and exothermic reaction proceeding with a reduction in the volume.  According to Le Chatelier principle, the yield of SO, would increase under the following conditions:

i) High pressure

ii) High concentration of 0₂,

iii) Low temperature

iv)Continuous removal of SO₃, from the reaction