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

In the manufacture of sulphuric acid by the Contact Process, sulphur dioxide (SO₂) is first oxidized in the presence of a catalyst to form sulphur trioxide.

The reaction for catalytic oxidation is:

2SO₂ + O₂ → 2SO₃

Now let us answer each part carefully.

(a) Name the gas collected.
The gas collected after catalytic oxidation of sulphur dioxide is sulphur trioxide (SO₃).

(b) Which acid is formed when the gas collected is dissolved directly in water?
When sulphur trioxide dissolves in water, it forms sulphuric acid (H₂SO₄).

The reaction is:

SO₃ + H₂O → H₂SO₄

(c) Why should we not dissolve this gas directly in water?
We should not dissolve sulphur trioxide directly in water because the reaction is highly exothermic.
A large amount of heat is produced very quickly, and this causes the formation of a dense fog or mist of sulphuric acid.
This acid mist cannot be condensed easily, so the yield of sulphuric acid becomes low.

That is why, in industry, sulphur trioxide is not dissolved directly in water.
Instead, it is first absorbed in concentrated sulphuric acid to form oleum (H₂S₂O₇), and then oleum is carefully diluted with water to obtain sulphuric acid.

Reactions involved:

SO₃ + H₂SO₄ → H₂S₂O₇

H₂S₂O₇ + H₂O → 2H₂SO₄

Final Answers:
(a) Sulphur trioxide (SO₃)
(b) Sulphuric acid (H₂SO₄)
(c) It should not be dissolved directly in water because the reaction is highly exothermic and produces acid mist, which is difficult to condense.