Question
Describe the Contact process for the manufacture of sulphuric acid. Give the reaction at each step, the conditions used, and explain the role of V₂O₅ catalyst. Why is concentrated H₂SO₄ used in the absorption step instead of water?
(NCERT Class 12, p-Block Elements)
Solution — Step by Step
Sulphur or sulphide ores are burned in air:
or from pyrite:
The SO₂ gas is purified (dust removed, dried) before the next step.
Conditions: Temperature 400-450°C, pressure 1-2 atm, catalyst V₂O₅ (vanadium pentoxide).
This is the rate-determining step and the reason the process is called the “Contact” process — SO₂ and O₂ come into contact with the solid catalyst surface.
SO₃ is not dissolved directly in water. Instead, it is absorbed in concentrated H₂SO₄ to form oleum (fuming sulphuric acid):
Oleum is then carefully diluted with water:
If SO₃ is added directly to water, the reaction is so exothermic that it produces a fine mist of H₂SO₄ droplets. This acid mist is very difficult to condense and causes severe pollution. Using concentrated H₂SO₄ as the absorbing medium gives a controlled, liquid-phase reaction without mist formation.
Why This Works
The key step (SO₂ → SO₃) is an equilibrium reaction. Le Chatelier’s principle guides the choice of conditions:
- Low temperature favours product (exothermic reaction), but too low = too slow. 450°C is the compromise — fast enough with V₂O₅ catalyst while giving ~97% conversion.
- High pressure favours product (3 moles gas → 2 moles gas), but the yield at 1-2 atm is already good enough. Higher pressures would increase equipment costs without significant benefit.
- Excess O₂ shifts equilibrium right — typically a 1:1 ratio of SO₂:air is used (giving excess O₂).
V₂O₅ works by temporarily oxidizing and reducing: SO₂ reduces V₂O₅ to V₂O₄, then O₂ re-oxidizes V₂O₄ back to V₂O₅. This redox cycle lowers the activation energy without being consumed.
Alternative Method
For a quick-recall framework, remember the three C’s of the Contact process: Combustion (make SO₂) → Conversion (SO₂ to SO₃ with catalyst) → Capture (absorb SO₃ in H₂SO₄).
CBSE and NEET frequently ask: “Why is SO₃ not dissolved directly in water?” This is a 2-mark question. The answer centres on acid mist formation. They also ask for the catalyst name (V₂O₅) and the temperature (450°C). These are direct-recall marks — don’t lose them.
Common Mistake
Students write that “high pressure is used in the Contact process” — many textbooks mention 2 atm, which is barely above atmospheric. This is NOT a high-pressure process (unlike the Haber process which uses 200 atm). The equilibrium is already favourable at low pressures because the conversion at 450°C with V₂O₅ is ~97%. Writing “high pressure” in a board exam can cost you marks.