Chemistry of Sulphurous Acid

Formula and Structure

Sulphurous acid has the molecular formula H₂SO₃. It contains one sulfur atom in the +4 oxidation state. The sulfur atom is bonded to three oxygen atoms, with one double bond (S=O) and two single bonds (S–OH), forming a pyramidal‑like structure around sulfur.

Preparation

Sulphurous acid is formed by dissolving sulfur dioxide gas in water:

SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq)

Physical Properties

• Molar mass: Approximately 82.07 g/mol.
• Acidity: It is a weak, diprotic acid.
  • H₂SO₃ ⇌ H⁺ + HSO₃⁻ (pK_a1 ≈ 1.8)
  • HSO₃⁻ ⇌ H⁺ + SO₃²⁻ (pK_a2 ≈ 7.0)
• Stability: Highly unstable; exists only in solution and decomposes upon heating.

Chemical Reactions

1. Redox (Reducing Agent)

H₂SO₃ is a strong reducing agent, commonly used to remove residual chlorine in water treatment:

H₂SO₃ + Cl₂ + H₂O → H₂SO₄ + 2HCl

2. Reaction with Bases

Neutralizes bases to form sulfites and bisulfites:

• Partial: H₂SO₃ + NaOH → NaHSO₃ + H₂O
• Full: H₂SO₃ + 2NaOH → Na₂SO₃ + 2H₂O

3. Reaction with carbonates and bicarbonates

Like other acids, H₂SO₃ liberates CO₂ from carbonates and bicarbonates.

• With sodium carbonate:
  Na₂CO₃ + H₂SO₃ → Na₂SO₃ + CO₂↑ + H₂O
• With sodium bicarbonate:
  NaHCO₃ + H₂SO₃ → NaHSO₃ + CO₂↑ + H₂O

Environmental Impact

Sulphurous acid is a key component of acid rain. When sulfur dioxide is released from burning fossil fuels, it reacts with atmospheric moisture, lowering the pH of rainwater and damaging ecosystems and stone structures.