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Preparation, Properties, Structure, Uses, Test - Sulphuric acid | 12th Chemistry : UNIT 3 : p-Block Elements-II

Chapter: 12th Chemistry : UNIT 3 : p-Block Elements-II

Sulphuric acid

Pure sulphuric acid is a colourless, viscous liquid (Density: 1.84 g/mL at 298 K).

Sulphuric acid: (H2SO4)


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 → H2S2O7H2O 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

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