oxoacids of sulfur and the contact process

If you had to name the single most-produced industrial chemical, the answer is sulfuric acid, H2SO4 — a country's sulfuric acid output is a rough gauge of its industrial activity. It is the most important of sulfur's oxoacids, and the road to it runs through sulfur's oxides and a clever industrial sequence called the contact process. This corner of chemistry quietly underpins fertilisers, batteries, metals, and a great deal of modern life.

Start with sulfur's oxides. Sulfur dioxide, SO2, is the choking gas of burning sulfur and volcanoes; dissolved in water it gives weakly acidic sulfurous acid, H2SO3. Sulfur trioxide, SO3, is the higher oxide; dissolved in water it gives the strong, vital sulfuric acid, H2SO4. The trouble is turning SO2 into SO3 — that step is slow and, being exothermic, fights heating. The contact process solves it: burn sulfur to SO2, then pass SO2 and O2 over a vanadium pentoxide (V2O5) catalyst at a carefully chosen moderate temperature and modest pressure to make SO3. Pouring SO3 straight into water makes a dangerous acid mist, so it is instead absorbed into existing concentrated H2SO4 to give oleum, which is then safely diluted to sulfuric acid. The sulfate ion, SO4 2-, is a stable tetrahedron with the central sulfur surrounded by four oxygens.

Sulfuric acid is an industrial workhorse with three personalities: a strong acid (giving up two protons), a powerful dehydrating agent (it strips water out of sugar, leaving a column of black carbon), and, when hot and concentrated, an oxidising agent. It makes phosphate fertilisers, refines petroleum, pickles steel, and fills lead-acid car batteries. A useful honesty point: the contact process is yet another Le Chatelier balancing act, like Haber-Bosch — high yield wants low temperature, acceptable speed wants higher, and the catalyst plus a chosen middle temperature buys both, which is why so many great industrial syntheses look alike under the hood.