Q.(i) Explain the principle of manufacturing sulphuric acid by contact process.

Contact process makes H2SO4 by catalytic oxidation of SO2 to SO3 over V2O5, then absorption; conc. H2SO4 dehydrates oxalic acid to CO + CO2 + H2O.

Part (i) — Principle of manufacture of sulphuric acid by the Contact Process:

The contact process makes H2SO4 in three main steps.

1. Production of SO2: sulphur (or iron pyrites, FeS2) is burnt in air to give sulphur dioxide.

   S + O2 --> SO2

   4 FeS2 + 11 O2 --> 2 Fe2O3 + 8 SO2

2. Catalytic oxidation of SO2 to SO3 (the key/heart of the process): the purified, dried SO2 mixed with excess air is passed over a catalyst (vanadium pentoxide, V2O5) at about 720 K and about 2 bar. This reversible, exothermic reaction is the 'contact' step:

   2 SO2 + O2 <=(V2O5)=> 2 SO3 (exothermic)

   By Le Chatelier's principle, moderate temperature, higher pressure and excess air favour a high yield of SO3.

3. Absorption of SO3 to form oleum: SO3 is NOT absorbed directly in water (that gives a dense acid mist). Instead it is absorbed in concentrated sulphuric acid to form oleum (pyrosulphuric acid):

   SO3 + H2SO4 --> H2S2O7 (oleum)

4. The oleum is then diluted carefully with a calculated amount of water to obtain sulphuric acid of the desired concentration:

   H2S2O7 + H2O --> 2 H2SO4

Part (ii) — Reaction of conc. sulphuric acid with oxalic acid: …