Question

Discuss how sulphuric acid can be obtained by the contact process.

Answer 1

The industrial production of sulphuric acid involves three main stages:

Stage 1: Production of Sulphur Dioxide (SO₂):

Sulphur is burned in dry air to produce sulphur dioxide gas.

\[\ce{S + O2 -> SO2}\]

Stage 2: Catalytic Oxidation (SO₂ to SO₃):

Sulphur dioxide reacts with oxygen to form sulphur trioxide.

\[\ce{2SO2 + O2 <=>[V2O5][450°C] 2SO3 + Heat}\]

Favourable Conditions:

• Catalyst: Vanadium pentoxide (V₂O₅) is used because it is cheap and does not get poisoned by impurities.
• Temperature: An optimum temperature of 400°C to 450°C is maintained.
• Pressure: A high pressure of 1 to 2 atmospheres (atm) is used to increase the yield.
• Excess Oxygen: Extra air is supplied to push the reaction forward.

Stage 3: Conversion of SO₃ to Sulphuric Acid

SO₃ is not dissolved directly in water because it forms a dangerous acid mist.

1. Oleum Formation: SO₃ gas is absorbed in concentrated sulphuric acid to form Oleum.

\[\ce{H2SO4 + SO3 -> H2S2O7}\]

2. Dilution: Oleum is diluted with water to get pure concentrated sulphuric acid.

\[\ce{H2S2O7 + H2O -> 2H2SO4}\]