Sulphuric Acid

Sulfuric acid is produced from sulfur, oxygen and water by contact process.

 1. Contact Process:

⇒ Sulfur is burned to produce sulfur dioxide.

S (s) + O2 (g) → SO2 (g)

⇒ This is then oxidized to sulfur trioxide using oxygen in the presence of a vanadium oxide catalyst. This reaction is reversible and the formation of the sulfur trioxide is exothermic. So low temperature and high pressure are the favourable conditions.

2SO2 (g) + O2 (g) ⇌ 2 SO3 (g) (in presence of V2O5)Contact Process(In Grillo process, the catalyst used is platinum impregnated magnesium sulphate).

⇒ The sulfur trioxide is absorbed into 97–98% H2SO4 to form oleum (H2S2O7), also known as fuming sulfuric acid. The oleum is then diluted with water to form concentrated sulfuric acid.

H2SO4 (l) + SO3 (g) → H2S2O7 (l)

H2S2O7 (l) + H2O (l) → 2 H2SO4 (l)

Note: Directly dissolving SO3 in water is not practical due to the highly exothermic nature of the reaction between sulfur trioxide and water. The reaction forms a corrosive aerosol that is very difficult to separate, instead of a liquid.

SO3 (g) + H2O (l) → H2SO4 (l)

2. Sulfuric acid can be produced in the laboratory by burning sulfur in air and dissolving the gas produced in ahydrogen peroxidesolution.

SO2 + H2O2 → H2SO4


⇒ Colourless, dense, oily liquid.

⇒ Specific gravity: 1.84

⇒ Freezing point = 283K

⇒ Dissolves in water evolving a large amount of heat.

⇒ Salts: Normal sulphates (Na or Cu sulphate)

Acid sulphates (NaHSO4)

⇒ Strong dehydrating agent.

⇒ Removes water from organic compounds.

⇒ Strong oxidizing agent – oxidizes both metals and non-metals.

Cu + 2H₂SO₄ (conc) → CuSO₄ + SO₂ + 2H₂O

C + 2H₂SO₄ (conc) → CO₂ + 2 SO₂ + 2H₂O


⇒ In manufacture of fertilisers.

⇒ Used in petrol refining.

⇒ Manufacture of pigments, paints etc.

⇒ It is widely used in the manufacture of chemicals, e.g., in making HCl, HNO3.

Sulphurous Acid:

Properties of Sulphurous Acid:

⇒ Sulphurous Acid, H2SO3, is a weak dibasic acid, known in the form of its salts (e.g. sodium sulphite).

⇒ Sulphurous acid is unstable and has never been isolated as a pure compound.

Preparation of Sulphurous Acid:

⇒ Sulphurous acid is formed when sulphur dioxide is dissolved in water.

SO2 + H2O → H2SO3

⇒ Sulphurous acid is unstable and has never been isolated as a pure compound. It may be better represented by the following reactions.

SO₂ + H₂O → HSO¯₃ + H⁺

HSO¯₃ → H⁺ + SO₃¯²

⇒ This is known as the ionisation of Sulphurous acid.

⇒ Both the bisulfite ion, HSO3 (ion), and the sulphate ion, SO3 (ion) exist, for salts of both are well known. Examples of the above are:

2NaOH + SO2 → Na2SO3 + H2O

Na2SO3 + H2O + SO2 → 2NaHSO3

Reactions of Sulphurous acid:

The solution when heated in a sealed tube at 150°C deposits sulphur.

3H2SO3 → 2H2SO4 + H2O + S

Sulphurous acid can be oxidised by the use of strong oxidising agents.

Oxidising of Sulphurous acid by Oxygen:

2H₂SO₃ + O₂ + 4H₂O → 4H₂O + 4H⁺ + 2SO₄¯²

Sulphurous acid solution is slowly oxidised by atmospheric oxygen to sulphuric acid.

Oxidising of Sulphurous acid by Permanganate ions:

When Sulphurous acid is added to permanganate ion which is coloured purple, SO₂ will decolourise the MnO₄ (ion) when it is reduced to the colourless Mn (ion).

2 MnO₄¯ + 5H₂SO₃ + 4H₂O → 2Mn⁺² + 4H₃O⁺ + 5SO₄¯² + 3H₂O

Uses of Sulphurous acid:

⇒ Sulphurous acid is a strong reducing agent.

⇒ The solution has bleaching properties.