Question

(i) With the help of equations, give an outline for the manufacture of sulphuric acid by the contact process.
(ii) What property of sulphuric acid is shown by the reaction of concentrated sulphuric acid when heated with
(a) potassium nitrate
(b) carbon?

Answer 1

(i) Manufacture of sulphuric acid by the contact process:

• Production of SO₂: By roasting iron pyrites or by burning sulphur, SO₂ can be produced.

4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂

S + O₂ → SO₂

Or

S₈ + 8O₂ → 8SO₂

• Catalytic oxidation of SO₂: This is done in the presence of air and a catalyst (platinum or vanadium pentoxide) under high temperature and pressure to produce sulphur trioxide (SO₃). This is the key step in the manufacture of sulphuric acid.

2SO₂ + O₂ ⇌ 2SO₃ + Heat (exothermic)

• Formation of oleum (H₂S₂O₇ ): This is done by absorbing sulphur trioxide in 98% sulphuric acid.

SO₃ + H₂SO₄ → H₂S₂O₇

• Dilution of oleum : This is done to obtain sulphuric acid of the desired concentration.

H₂S₂O₇ + H₂O → 2H₂SO₄

(ii) (a) The reaction of concentrated sulphuric acid on being heated with potassium nitrate demonstrates that sulphuric acid is non-volatile in nature
${\text{KNO}}_3+{\text{H}}_{2}S{O}_4\stackrel{<200°\mathrm{C}}{\to }{\text{KHSO}}_4+{\text{HNO}}_3$

(b) The reaction of concentrated sulphuric acid on being heated with carbon demonstrates that sulphuric acid is an oxidising agent and oxidises C to CO₂.

C + 2H₂SO₄ → 2H₂O + 2SO₂ + CO₂