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 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.
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.