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)

 (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 a hydrogen peroxide solution.

SO₂ + H₂O₂ → H₂SO₄

Properties:

⇒ 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

Uses:

⇒ 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.