↪ The contact process derives its name from the fact that the oxidation of SO₂ to SO₃ takes place only when the reactants come in contact with the surface of a catalyst.
↪ It is the most modern method.

Principle

↪ Sulphur dioxide is obtained by burning sulphur or iron pyrites.

S + O₂ → SO₂
4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂

↪ The contact process for the manufacture of sulphuric acid is based on the catalytic oxidation of SO₂ to SO₃ by atmospheric oxygen.
↪ SO₂ and air are passed over a catalyst (V₂O₅) heated to 450-500°C.
↪ About 98% of the possible yield of sulphur trioxide is obtained.

2SO_2(g) + O_2(g) ⇌ 2SO_3(g) + 45.0 kcal

↪ The mechanism of oxidation of SO₂ to SO₃ by using V₂O₅ catalyst consists of two steps.

Step 1: Oxidation of SO₂ into SO₃ by V⁵⁺

2SO₂ + 4V⁵⁺ + 10O^2- → 2SO₃ + 4V^4- + 8O^2- 

Step 2: Regeneration of catalyst  by oxidation of V⁴⁺ to V⁵⁺ by Oxygen (O₂).

4V⁴⁺ + 8O^2- + O² → 4V⁵⁺ + 10O^2-

↪ The sulphur trioxide can't be satisfactorily absorbed by water.
↪ A mist of fine drops of dilute sulphuric acid fill the factory if direct absorption is tried.

↪ Therefore, SO₃ produced is absorbed in conc.
↪ H₂SO₄ and the fuming liquid called Oleum or pyro sulphuric acid, liquid is formed and it is diluted water to get sulphuric acid of required concentration.

H₂SO₄ + SO_3(g) → H₂S₂O_7(l) {Oleum}

H₂S₂O_7(l) + H₂O_(l) → 2H₂SO_4(aq.)

↪ Thus, the manufacture of H₂SO₄ is nothing but getting SO₃ in excess from air by the oxidation of SO₂.

Suitable Conditions for maximum yield 

↪ The reaction 2SO₂ + O₂ ⇌ 2SO₃ + 45 kcal is reversible, exothermic and proceeds with decrease in volume.

⁕ Low temperature

↪ Low temperature will favor the forward reaction because of its exothermic nature.

⁕ High Pressure

↪ High pressure favors the forward reaction.
↪ A pressure of 2-3 atms is sufficient to make steady flow of gases.

⁕ Excess of Oxygen

↪ A slight excess of oxygen in the reacting mixture helps to carry the reaction in the direction of the formation of sulphur trioxide.

⁕ Use of Catalyst (V₂O₅)

↪ At the low temperature maintained, the velocity of reaction is very slow and in order to accelerate the formation of SO₃, the use of a catalyst is extremely necessary.

⁕ Purity of gases

↪ As the catalyst is open to poisoning, the mixture of SO2 and air of oxygen should be pure and dry and free from all types of impurities, particularly the dust.

⁕ Flowsheet diagram of Manufacture of Sulphuric Acid by Contact Process