Question

During the manufacture of sulphuric acid, a chemist collected the gas formed after catalytic oxidation of sulphur dioxide.
(a) Name the gas collected.
(b) Which acid is formed when the gas collected by the chemist is dissolved directly in water?
(c) Why should we not dissolve this gas directly in water?

Hint:

Hydrocarbon had only reacted with one mole of hydrogen, the starting material would have been Ethene (C_2H_4)!

Answer 1

(a) Sulphur trioxide (SO_3): The catalytic oxidation of SO_2 in the Contact Process yields SO_3.
(b) Sulphuric acid (H_2SO_4): Dissolving SO_3 in water directly forms H_2SO_4.
(c) Reason for not dissolving directly: The reaction between SO_3 and water is highly exothermic. It produces a dense fog or mist of tiny sulphuric acid droplets that is very difficult to condense and can damage the apparatus. To avoid this, SO_3 is first dissolved in concentrated sulphuric acid to form Oleum (H_2S_2O_7), which is then safely diluted.