Chemical Technology
Chemical technology is a memory-linked domain: each question pairs a product with its feedstock, catalyst, operating window and the reason for each. GATE CH draws mostly on chlor-alkali, sulphuric and nitric acid, ammonia and urea, methanol, petroleum refining and the main petrochemicals.
Core rules
- Sulphuric acid (contact process). Burn sulphur to SO₂, oxidise over V₂O₅ at 400–450 °C in stages with intercooling, because the reaction is exothermic and equilibrium-limited; absorb SO₃ in concentrated H₂SO₄ or oleum, never in water, to avoid acid mist.
- Ammonia (Haber–Bosch). Stoichiometric H₂:N₂ of 3:1 over promoted iron at roughly 400–500 °C and 150–300 atm; CO₂ is removed and residual oxides methanated before the converter, and a purge keeps inerts (argon, methane) from accumulating in the recycle loop.
- Urea. 2NH₃ + CO₂ → ammonium carbamate, then dehydration to urea; unconverted carbamate is recycled.
- Nitric acid. Catalytic oxidation of ammonia on platinum–rhodium gauze, then absorption of NO₂ in water.
- Petroleum. Crude distillation, then FCC, hydrocracking and catalytic reforming (naphtha to aromatics and high-octane gasoline); steam cracking of naphtha gives ethylene and propylene.
- Chlor-alkali. Brine electrolysis gives Cl₂ at the anode, H₂ and NaOH at the cathode.
Worked example
Ethylene oxide synthesis over silver: 100 mol C₂H₄ fed, 80 mol reacted, of which 64 mol go to C₂H₄O and 16 mol burn to CO₂ and water. Selectivity = 64/80 = 80%, while yield on feed = 64/100 = 64%. Reporting 80% as the yield is the classic error - selectivity counts only what reacted.
Traps
- Quoting the wrong catalyst (V₂O₅ belongs to SO₂ oxidation, not ammonia synthesis).
- Absorbing SO₃ in water instead of concentrated acid.
- Forgetting catalyst poisons and the need for gas purification upstream.
- Mixing selectivity with yield, or giving conversion on the wrong reactant.