Sulphuric acid

Sulphuric acid: (H₂SO₄)

Preparation:

Sulphuric acid can be manufactured by lead chamber process, cascade process or contact process. Here we discuss the contact process.

Manufacture of sulphuric acid by contact process:

The contact process involves the following steps.

i. Initially sulphur dioxide is produced by burning sulphur or iron pyrites in oxygen/air.

S + O₂ → SO₂

4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂

ii. Sulphur dioxide formed is oxidised to sulphur trioxide by air in the presence of a catalyst such as V₂O₅ or platinised asbestos.

iii. The sulphur trioxide is absorbed in concentrated sulphuric acid and produces oleum (H₂S₂O₇). The oleum is converted into sulphuric acid by diluting it with water.

SO₃ + H₂SO₄ → H₂S₂O₇ →^H2O→ 2H₂SO₄

To maximise the yield the plant is operated at 2 bar pressure and 720 K. The sulphuric acid obtained in this process is over 96 % pure.

Physical properties:

Pure sulphuric acid is a colourless, viscous liquid (Density: 1.84 g/mL at 298 K). High boiling point and viscosity of sulphuric acid is due to the association of molecules together through hydrogen bonding.

The acid freezes at 283.4 K and boils at 590 K. It is highly soluble in water and has strong affinity towards water and hence it can be used as a dehydrating agent. When dissolved in water, it forms mono (H₂SO₄.H₂O) and dihydrates (H₂SO₄.2H₂O) and the reaction is exothermic.

The dehydrating property can also be illustrated by its reaction with organic compounds such as sugar, oxalic acid and formic acid.

C₁₂H₂₂O₁₁ + H₂SO₄ → 12C + H₂SO₄ .11H₂O

HCOOH + H₂SO₄ → CO + H₂SO₄ .H₂O

(COOH)₂ + H₂SO₄ → CO + CO₂ + H₂SO₄ .H₂O

Chemical Properties:

Sulphuric acid is highly reactive. It can act as strong acid and an oxidising agent. Decomposition: Sulphuric acid is stable, however, it decomposes at high temperatures to sulphur trioxide.

H₂SO₄ → H₂O + SO₃

Acidic nature: It is a strong dibasic acid. Hence it forms two types of salts namely sulphates and bisulphates.

H₂SO₄ + NaOH → NaHSO₄ + H₂O

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

H₂SO₄ + 2NH₃ →  (NH₄ )₂SO₄

Oxidising property: Sulphuric acid is an oxidising agent as it produces nascent oxygen as shown below.

H ₂ SO₄ → H₂O + SO₂ + (O) nascent oxygen

Sulphuric acid oxidises elements such as carbon, sulphur and phosphorus. It also oxides bromide and iodide to bromine and iodine respectively.

C + 2H₂SO₄ → 2SO₂ + 2H₂O + CO₂

S + 2H₂SO₄ → 3SO₂ + 2H₂O

P₄ + 10H₂SO₄ → 4H₃PO₄ + 10SO₂ + 4H₂O

H₂S + H₂SO₄ → SO₂ + 2H₂O + S

H₂SO₄ + 2HI → 2SO₂ + 2H₂O + I₂

H₂SO₄ + 2HBr → 2SO₂ + 2H₂O + Br₂

Reaction with metals: Sulphuric acid reacts with metals and gives different product depending on the reactants and reacting condition.

Dilute sulphuric acid reacts with metals like tin, aluminium, zinc to give corresponding sulphates.

Zn + H₂SO₄ → ZnSO₄ + H₂ ↑

2Al + 3H₂SO₄ → Al₂ (SO₄ )₃ + 3H₂ ↑

Hot concentrated sulphuric acid reacts with copper and lead to give the respective sulphates as shown below.

Cu + 2H₂SO₄ → CuSO₄ + 2H₂O + SO₂ ↑

Pb + 2H₂SO₄ → PbSO₄ + 2H₂O + SO₂ ↑

Sulphuric acid doesn’t react with noble metals like gold, silver and platinum.

Reaction with salts: It reacts with different metal salts to give metal sulphates and bisulphates.

KCl + H₂SO₄ → KHSO₄ + HCl

KNO₃ + H₂SO₄ → KHSO₄ + HNO₃

Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂

2NaBr + 3H₂SO₄ → 2NaHSO₄ + 2H₂O + Br₂ + SO₂

Reaction with organic compounds: It reacts organic compounds such as benzene to give sulphonic acids.

C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O

Uses of sulphuric acid:

·           Sulphuric acid is used in the manufacture of fertilisers, ammonium sulphate and super phosphates and other chemicals such as hydrochloric acid, nitric acid etc...

·           It is used as a drying agent and also used in the preparation of pigments, explosives etc..

Test for sulphate/sulphuric acid:

Dilute solution of sulphuric acid/aqueous solution of sulphates gives white precipitate (barium sulphate) with barium chloride solution. It can also be detected using lead acetate solution. Here a white precipitate of lead sulphate is obtained.

BaCl₂ + H₂SO₄ → BaSO₄ ↓ + 2HCl

(CH₃COO)₂Pb + H₂SO₄ → PbSO₄ ↓ + 2CH3COOH

Structure of oxoacids of sulphur:

Sulphur forms many oxoacids. The most important one is sulphuric acid. Some acids like sulphurous and dithionic acids are known in the form of their salts only since the free acids are unstable and cannot be isolated.

Various oxo acids of sulphur with their structures are given below