Q.Describe the manufacture of H2SO4 by contact process?

Step 1: Burning sulphur to $SO_2$.

(Alternatively, $SO_2$ can be obtained by roasting sulphide ores, e.g. $4FeS_2 + 11O_2 \rightarrow 2Fe_2O_3 + 8SO_2$.) The $SO_2$ gas is purified by removing dust and arsenic impurities.

Step 2: Catalytic oxidation of $SO_2$ to $SO_3$.

This is the key catalytic step of the Contact process, carried out over a vanadium pentoxide ($V_2O_5$) catalyst at about 720 K and 2 atm pressure — conditions chosen as a compromise for good yield and acceptable rate (the forward reaction is exothermic and mole-decreasing, so lower temperature/higher pressure favour yield, but a moderately high temperature and catalyst are needed for a workable rate).

Step 3: Absorption of $SO_3$ to form oleum.

$SO_3$ is not absorbed directly in water (this would produce a dense fog of $H_2SO_4$ that is hard to condense); instead it is absorbed in concentrated $H_2SO_4$ to give oleum (pyrosulphuric acid): …

Present the Contact process as its four sequential steps — burning S to SO2, catalytic oxidation to SO3 (V2O5, 720 K, 2 atm), absorption in conc. H2SO4 to for …

• A very common mistake is to say SO₃ is absorbed directly in water — it must be absorbed in concentrated H₂SO₄ first (to avoid an unmanageable acid mist), forming oleum, which is only then diluted. …