Sulphuric acid: (H2SO4)
Sulphuric acid can be manufactured by lead chamber process, cascade process or contact process. Here we discuss the 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.
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
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
· 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..
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
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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