Q.iv. Describe the manufacturing of H$_2$SO$_4$ by contact process.

Step 1: Burning/roasting to SO2. Sulfur, or a sulfide ore such as iron pyrites, is burnt or roasted in air to produce sulfur dioxide: S(s) + O2(g), on heating, gives SO2(g); or 4FeS2(s) + 11O2(g), on heating, gives 2Fe2O3(s) + 8SO2(g).

Step 2: Catalytic oxidation to SO3. The sulfur dioxide is oxidised catalytically with oxygen to sulfur trioxide, in the presence of a V2O5 catalyst: 2SO2(g) + O2, with V2O5, gives 2SO3(g). Since this reaction is exothermic, reversible, and reduces gas volume on the forward direction, a comparatively low temperature (around 720 K) combined with high pressure (around 2 bar) gives the best yield of SO3, by Le Chatelier's principle -- too high a temperature would favour the reverse (SO3-decomposing) reaction instead, even though a higher temperature would otherwise speed up the reaction rate. …

Working through all three named steps of the Contact process from section 7.10.2.a, including the specific temperature/pressure condition …

• Omitting the intermediate oleum (H2S2O7) step and describing SO3 as dissolving directly in water to give H2SO4; industrially, SO3 is absorbed into existing concentrated H2SO4 first (forming oleum), which is only then diluted …