A process for making sulphuric acid from sulphur dioxide (SO2), which is made by burning sulphur or by roasting sulphide ores. A mixture of sulphur dioxide and air is passed over a hot catalyst 2SO2+O2 → 2SO3The reaction is exothermic and the conditions are controlled to keep the temperature at an optimum 450°C. Formerly, platinum catalysts were used but vanadium-vanadium oxide catalysts are now mainly employed (although less efficient, they are less susceptible to poisoning). The sulphur trioxide is dissolved in sulphuric acid H2SO4+SO3 → H2S2O7and the oleum is then diluted.
2SO2+O2 → 2SO3
H2SO4+SO3 → H2S2O7