Contact Process — Full Mechanism — Notes | NoteTube

Cue	Notes
What is the Contact process?	Industrial method for manufacturing H2SO4. 4-step sequence starting from sulfur.
Step 1	$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$ (burning sulfur in air or roasting FeS2)
Step 2 (KEY)	$2\text{SO}_2 + \text{O}_2 \underset{450°C}{\overset{\text{V}_2\text{O}_5}{\rightleftharpoons}} 2\text{SO}_3$ — REVERSIBLE, exothermic
Step 3	$\text{SO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S}_2\text{O}_7 \text{ (oleum)}$
Step 4	$\text{H}_2\text{S}_2\text{O}_7 + \text{H}_2\text{O} \rightarrow 2\text{H}_2\text{SO}_4$
Why NOT dissolve SO3 in water directly?	SO3 + H2O forms a fine acid mist (H2SO4 aerosol) that is impossible to condense and collect. Absorbed in conc. H2SO4 instead to form oleum.
Catalyst	V2O5 (vanadium pentoxide) — resistant to poisoning by As2O3 impurities. Pt was used historically but easily poisoned.
Temperature compromise	Exothermic reaction → low T favors yield (Le Chatelier) but rate too slow. 450°C balances both.
Pressure	$\Delta n$ (gas) = 2-3 = -1 → high pressure favors SO3 formation. Moderate/high pressure used industrially.
Raw material sources of SO2	(1) S + O2 → SO2 (elemental sulfur); (2) 4FeS2 + 11O2 → 2Fe2O3 + 8SO2 (iron pyrites)
Oleum formula	H2S2O7 = pyrosulfuric acid = fuming sulfuric acid

Summary: Contact process: S → SO2 → SO3 (V2O5, 450°C, reversible) → H2S2O7 (oleum) → H2SO4. SO3 NEVER dissolved directly in water.