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Friday, April 14, 2017

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)
Contact Process
 (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, SO will decolourise the MnO (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.

1 comment:

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