Preparation of SO₂ gas in the laboratory:-

Principle:- SO₂ gas is prepared in the laboratory by the action of Cu and conc. H₂SO₄

Cu + 2H₂SO₄ → CuSO₄ + 2H₂O + CO₂

SO₂ gas is collected in the gas jar by the displacement of air.

Following reaction of SO₂ gas:

(a) Acidic properties: It is an acidic oxide. It react with water and forms sulphurous acid.

SO₂ + H₂O → H₂SO₃

(b) Bleaching properties: It bleaches the colour of moist substances. Its bleaching properties is due to reduction. It combines with water and form sulphuric acid and nascent hydrogen which bleaches the coloured substance into colourless:

SO₂ + 2H₂O → H₂SO₄ + 2[H]

Colour + 2[H] → colourless

(c) Oxidising properties: It act as an oxidising agent is some reactions. e.g.-

(i) It oxidises H₂S into sulphur

SO₂ + 2H₂S → 3S + 2H₂O

(ii) It oxidises iron powder into their oxide and sulphide.

SO₂ + 3Fe → 2FeO + FeS

(iii) It oxidises SnCl₂ in SnCl₄

SO₂ + 2SnCl₂ + 4HCl → S + 2SnCl₄ + 2H₂O

(d) Reducing properties: Aqueous solution of SO₂ is a strong reducing agent. When SO₂ reacts with water produce nacesent hydrogen, which acts as strong reducing agent.

2H₂O + SO₂ → H₂SO₄ + 2[H]

(i) It reduces halogens (Cl₂, Br₂ and I₂) in their halogen acids e.g.

SO₂ + Cl₂ + 2H₂O → 2HCl + H₂SO₄

SO₂ + Br₂ + 2H₂O → 2HBr + H₂SO₄

SO₂ + I₂ + 2H₂O → 2HI + H₂SO₄

(ii) It reduces acidic solution of potassium permanganate into manganese sulphate.

2KMnO₄ + 5SO₂ + 2H₂O → 2MnSO₄ + K₂SO₄ + 2H₂SO₄