Sulphuric acid is manufactured by the Contact process whichinvolves three steps :
(i) burning of sulphur or sulphide ores in air to generate SO2.
(ii) conversion of SO2 to SO3 by the reaction with oxygen in the presenceof a catalyst V2O5and
(iii) absorption of SO3 in H2SO4 to give oleum (H2S2O7).
A flow diagram for the manufacture of sulphuric acid is shown inthe figure. The SO2 produced is purified by removing dust and otherimpurities such as arsenic compounds.
The key step in the manufacture of H2SO4 is the catalytic oxidation ofSO2 with O2 to give SO3 in the presence of V2O5 (catalyst).
2SO2(g)+O2(g)

V2O5

–––––▶

2SO3(g)
ΔrH⊖=–196.6kJmol–1
Thereaction is exothermic, reversible and the forward reaction leads toa decrease in volume. Therefore, low temperature and high pressure arethe favourable conditions for maximum yield. But the temperature shouldnot be very low otherwise rate of reactionwill become slow.
In practice, the plant is operated at a pressure of 2 bar and a temperatureof 720 K. The SO3 gas from the catalytic converter is absorbed inconcentrated H2SO4 to produce oleum. Dilution of oleum with water giveH2SO4 ofthe desired concentration. In the industry, two steps are carriedout simultaneously to make the process a continuous one and also toreduce the cost.
SO3+H2SO4→

H2S2O7

(Oleum)

The sulphuric acid obtained by Contact process is 96– 98% pure.