Contact Process: S → SO2 → SO3 (V2O5, 450°C, reversible) → H2S2O7 oleum (SO3 + H2SO4) → H2SO4; SO3 is NEVER dissolved in water (acid mist problem).

Xenon Fluorides: XeF2 = linear (sp3d, 3 LP equatorial); XeF4 = square planar (sp3d2, 2 LP trans axial); XeF6 = distorted octahedral (sp3d3, 1 LP); XeF6 + 3H2O → XeO3 (explosive) + 6HF.

HX Acid Trends: Acidic strength HF < HCl < HBr < HI (bond strength determines, not electronegativity); thermal stability HF > HCl > HBr > HI; HF is weak acid (H-F bond = 568 kJ/mol).

Oxoacid Strength: HOCl < HClO2 < HClO3 < HClO4 (more oxygen = stronger acid); Fluorine has NO oxoacids (no d-orbitals, no positive oxidation states).

Interhalogen Shapes: AB = linear; AB3 = T-shaped; AB5 = square pyramidal; AB7 = pentagonal bipyramidal; more electronegative halogen is always the outer (terminal) atom.