Question
Describe the contact process for manufacturing sulphuric acid. What are the conditions at each step, why is V₂O₅ used as catalyst, and why is SO₃ not dissolved directly in water?
(NEET 2024 asked about the catalyst; CBSE 12 boards ask the complete flowchart)
Solution — Step by Step
Sulphur (or sulphide ores like pyrite FeS₂) is burned in air:
Or from pyrite:
The SO₂ gas is then purified (dust removal, washing) before the next step.
This is the heart of the contact process:
Conditions: V₂O₅ catalyst, temperature 450°C, pressure 2 atm, excess of O₂.
Why these conditions? The reaction is exothermic and reversible. By Le Chatelier’s principle, low temperature favours the forward reaction — but too low makes it impractically slow. 450°C is the optimum where the catalyst works efficiently and yield is about 97%.
SO₃ is absorbed in concentrated H₂SO₄ (not water!) to form oleum (fuming sulphuric acid):
Why not water? Because SO₃ + H₂O is highly exothermic and creates a fine mist of H₂SO₄ that is impossible to condense — you lose the product as fog.
Oleum is diluted with water to get concentrated H₂SO₄ of the desired strength:
flowchart TD
A["S or FeS₂ + O₂<br/>Burn in air"] -->|"SO₂ produced"| B["Purification<br/>Remove dust, As₂O₃"]
B --> C["Catalytic converter<br/>V₂O₅, 450°C, 2 atm"]
C -->|"SO₃ formed (97% yield)"| D["Absorption tower<br/>SO₃ + conc. H₂SO₄"]
D -->|"Oleum H₂S₂O₇"| E["Dilution with water"]
E --> F["H₂SO₄ product"]
style C fill:#ffeb3b,stroke:#333Why This Works
The contact process is a beautiful example of applying chemical equilibrium principles to industry. The exothermic nature of Step 2 means we want low temperature — but the catalyst needs at least 450°C to function. V₂O₅ is chosen because it provides a good conversion rate at moderate temperature and is cheaper than platinum (which was used earlier).
Excess O₂ shifts equilibrium right (Le Chatelier). High pressure also favours the forward reaction (fewer moles of gas on the product side: 3 moles → 2 moles). But only moderate pressure (2 atm) is used because the cost of high-pressure equipment is not justified by the small extra yield.
Alternative Method
For MCQs, remember the conditions using the mnemonic: “V for Victory at 450 and 2” — V₂O₅ catalyst, 450°C, 2 atm. The absorption step’s trick question (“why not water?”) appears in almost every board exam — always mention the acid mist problem.
Common Mistake
Students write that SO₃ is directly dissolved in water to form H₂SO₄. This is the most predictable mistake in this topic. SO₃ + H₂O is never done directly because the reaction is so exothermic that H₂SO₄ forms as an uncontrollable mist. The correct industrial approach is absorption in conc. H₂SO₄ first (forming oleum), then controlled dilution. Examiners specifically set this as a true/false trap.