Contact Process — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Contact Process An industrial process for manufacturing sulphuric acid in which dry, purified SO2 is catalytically oxidised to SO3 over V2O5 at about 720 K and ~1–2 atm. The SO3 is then absorbed in concentrated H2SO4 to form oleum, which is subsequently diluted with water to obtain H2SO4 of required concentration. SO2 is a colorless gas with a pungent, suffocating odour; the converter beds heat up noticeably due to the exotherm. Escaping SO3 forms dense white fumes in moist air; absorption in conc. H2SO4 gives oily oleum. Strongly exothermic and reversible: ( H -198 , kJ ) for forming 2 mol of SO3 from 2SO2 + O2. Lower temperature and higher pressure favour SO3 formation (fewer gas moles), but an optimum of ~720 K and ~1–2 atm is used to balance rate and yield. 1. Gas preparation: SO2 (from burning S or roasting sulfide ores) is cooled, dusted, dried with conc. H2SO4, and freed from catalyst poisons (e.g., As2O3) to protect V2O5. 2. Adsorption: Dry SO2 and O2 (air, slight O2 excess) adsorb onto the porous V2O5/SiO2 catalyst surface at ~720 K. 3. Surface redox (oxygen transfer): ( SO 2(g) + V 2O 5(s) SO 3(g) + V 2O 4(s) ); SO2 is oxidised to SO3 while V2O5 is reduced to V2O4. 4. Catalyst regeneration: ( V 2O 4(s) + 1 2 O 2(g) V 2O 5(s) ), completing the catalytic cycle and restoring the active oxide. 5. Desorption and heat management: Product SO3 desorbs; interstage heat exchangers and multiple catalyst beds maintain ~720 K because the reaction is exothermic and equilibrium shifts unfavourably at higher T. 6. Absorption to make acid: The hot SO3 is cooled and absorbed in conc. H2SO4 to form oleum (H2S2O7), which is later carefully diluted with water to yield H2SO4. Quoting Pt as the catalyst—modern Contact Process uses V2O5 (Pt is easily poisoned by As compounds). Thinking very high pressure is essential; actually ~1–2 atm suffices and is economical. Assuming higher temperature increases yield—oxidation to SO3 is exothermic; too high T lowers equilibrium conversion (optimum ~720 K). Saying SO3 is absorbed directly in water—this forms an acid mist; industry absorbs SO3 in conc. H2SO4 to make oleum. Forgetting catalyst poisons (moisture, dust, As2O3) must be removed before the converter.