Question

Describe the manufacturing of H₂SO₄ by the contact process.

Write chemical reactions in the manufacture of sulfuric acid by contact process.

Answer 1

Sulfuric acid is manufactured by the contact process, which involves the following three steps.

1) Roasting in air:

Sulfur or sulfide ore (iron pyrites) on burning or roasting in air produces sulfur dioxide.

i) \[\ce{\underset{\text{Sulfur}}{S_{(s)}} + O_{2(g)} ->[Δ] \underset{\text{Sulfur dioxide}}{SO_{2(g)}}}\]

ii) \[\ce{\underset{\text{Iron sulfide}}{4FeS_{2(s)}} + 11O_{2(g)} ->[Δ]  2Fe2O_{3(s)} +\underset{\text{Sulfur dioxide}}{8SO_{2(g)}}}\]

2) Catalytic oxidation of sulfur dioxide:

i) Sulfur dioxide is oxidised catalytically with oxygen to sulfur trioxide, in the presence of V₂O₅ catalyst.

\[\ce{\underset{\text{Sulfur dioxide}}{2SO_{2(g)}} +  O_{2(g)}->[V2O5] \underset{\text{Sulfur trioxide}}{2SO_{3(g)}}}\]

ii) The reaction is exothermic and reversible and the forward reaction leads to decrease in volume. Therefore, low temperature (720K) and high pressure (2 bar) are favourable conditions for maximum yield of SO₃.

3) Absorption, followed by dilution of sulfur trioxide gas:

i) Sulfur trioxide gas (from the catalytic converter) is absorbed in concentrated H₂SO₄ to produce oleum.

\[\ce{\underset{\text{Sulfur trioxide}}{SO3} + H2SO4 -> \underset{\text{Oleum}}{H2S2O7}}\]

ii) Dilution of oleum with water gives sulfuric acid of desired concentration.

\[\ce{\underset{\text{Oleum}}{H2S2O7} + H2O -> \underset{\text{Sulfuric acid}}{2H2SO4}}\]

iii) The sulfuric acid obtained by contact process is 96-98 % pure.