Ostwald Process — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Ostwald Process Industrial manufacture of nitric acid by catalytic oxidation of ammonia to nitric oxide over Pt–Rh gauze at high temperature, followed by oxidation to NO2 and absorption in water to give HNO3 with NO recycling. The net stoichiometry is NH3 + 2 O2 → HNO3 + H2O; the first step is highly exothermic and selectivity-sensitive. Pt–Rh gauze glows red-hot due to the exothermic burn of NH3; initially formed NO is colorless, which turns into brown NO2 fumes on exposure to air. Strong heat evolution occurs; absorption columns yield a colorless, fuming nitric acid solution (typically ~50–68% w/w after single absorption). Highly exothermic. For the primary step 4 NH3 + 5 O2 → 4 NO + 6 H2O, ΔH° ≈ −9.06 × 10 2 kJ (per 4 mol NH3). Subsequent NO → NO2 oxidation and absorption are also exothermic; overall synthesis is exothermic and favored at lower temperatures, but high temperature is required for fast kinetics and acceptable NO selectivity. 1. Catalytic oxidation of ammonia on hot Pt–Rh gauze (ammonia burner): 4 , NH 3(g) + 5 , O 2(g) [ 1 - 9 , atm ] Pt-Rh gauze , 800 - 900 , C 4 , NO(g) + 6 , H 2O(g) . 2. Side reactions (minimized by ~10% NH3 in air, proper temperature, and rapid quenching) that lower NO yield: 4 , NH 3 + 3 , O 2 2 , N 2 + 6 , H 2O ; 4 , NH 3 + 4 , O 2 2 , N 2O + 6 , H 2O . 3. As gases cool, rapid gas-phase oxidation of NO: 2 , NO(g) + O 2(g) 2 , NO 2(g) (exothermic). 4. Absorption in water (packed/plate tower): 3 , NO 2(g) + H 2O(l) 2 , HNO 3(aq) + NO(g) ; higher pressure (8–12 bar) enhances absorption. 5. Re-oxidation and recycle: the NO evolved is re-oxidized (2 , NO + O 2 2 , NO 2 ) and returned to the absorber, increasing overall conversion to HNO3. 6. Net stoichiometry (overall Ostwald process): NH 3(g) + 2 , O 2(g) HNO 3(aq) + H 2O(l) . Confusing the Ostwald catalyst (Pt–Rh gauze) with V2O5 (used in the Contact process for H2SO4). Mixing conditions with the Haber process (Fe–Mo–K2O at 200 atm, ~700 K) — Ostwald uses hot Pt–Rh gauze at ~800–900 °C and moderate pressure. Thinking NO2 is the first product; the primary oxidation gives NO, which then oxidizes to NO2. Forgetting the absorption step regenerates NO: 3 NO2 + H2O → 2 HNO3 + NO (NO is re-oxidized and recycled). Missetting the stoichiometry of the burner reaction (correct coefficients: 4 NH3 : 5 O2 → 4 NO + 6 H2O).