Sulphuric Acid (H₂SO₄)

It is the most important oxoacid of sulphur and is known from ancient times. It is called 'oil of vitriol' because in early days it has been prepared from ferrous sulphate crystals (green vitriol) and has an oily appearance. It occurs in certain minerals and springs. Because of its large applications in industries it is also known as the "King of chemicals".

Manufacture of sulphuric acid

Sulphuric acid is manufactured by contact process.

The process involves the following steps:

1. Production of sulphur dioxide

It is carried out by burning powdered sulphur or roasting of sulphur rich ores.

2. Oxidation of sulphur dioxide

This step is the key step in the manufacture of sulphuric acid. By applying Le-Chatelier's principle, the following conditions can be worked out for the better yield of sulphur trioxide.

Temperature

As the reaction is exothermic, a low temperature favors the forward reaction. An optimum temperature is maintained between 673-723 K.

Pressure

A high pressure favors the process. Thus, the optimum pressure of 1.5 - 2 atm. is maintained.

Catalyst

At such a low temperature the reaction is slow. Therefore, a catalyst is used.

The catalyst employed is platinized asbestos or divanadium pentoxide (V₂O₅).

d) Purity of gases

To prevent the poisoning of catalyst, the gases must be free from the impurities of As₂O₃ dust particles and moisture.

e) Excess of oxygen

To have a better yield of SO_3, O₂ is used in excess.

3. Conversion of SO₃ into H₂SO₄ - SO₃ is absorbed in conc. H₂SO₄ to get oleum.

Oleum may then be diluted with calculated quantity of water to get H₂SO₄ of required concentration.

The flow diagram of the plant used is as shown below.

fig 8.7 - Flow diagram for the manufacture of sulphuric acid

The plant consists of:

1. Sulphur burner -

Here, sulphur dioxide is produced.

2. Purification unit

- This unit is an assembly of various parts like,

a) Dust precipitator, which removes dust from the gases.

b) Scrubber, which removes soluble impurities.

c) Drying tower, which removes moisture from the gases.

d) Arsenic purifier, which removes impurities of As₂O₃ etc.

The gases coming out of the purification unit are passed through the testing box to make sure that they are completely free from the impurities. If impurities are still present, these are recirculated again through the purification unit. The pure gases coming from the testing box are preheated to 673 to 723 K in a preheater.

3. Catalyst converter

The hot gases are then passed through the catalyst chamber where SO₂ is oxidised to SO_3.

4. Absorption tower

SO₃ from contact tower is introduced at the base of the absorption tower from the top of which conc. H₂SO₄ is showered. As SO₃ gas moves up it is dissolved in sulphuric acid forming oleum, which may be drawn out from the base of the tower.

Oleum is then diluted with water to get H₂SO₄ of the desired concentration.

SO₃ is not absorbed in water directly to form sulphuric acid because it results in the formation of mist or acid particles in air which does not condense easily. Hence, the operation becomes difficult to handle.

fig 8.8 - Flowsheet for the manufacture of sulphuric acid by contact

process

Structure

Sulphuric acid is a covalent molecule having hexavalent S atom. It has a tetrahedral structure.

Tetrahedral structure of sulphuric acid

Each S = O bonds is 142 pm and S - O single bond is 157 pm.

Physical properties

1. Pure H₂SO₄ is colorless but commercial acid is yellow in color due to presence of impurities.

2. Highly concentrated H₂SO₄ (98%) has a specific gravity of 1.84 and high boiling point of 621K. The high boiling point shows that H₂SO₄ has associated structure due to hydrogen bonding as shown in figure:

(b)

3. Cone. H₂SO₄ has a great affinity for water. Its dissolution in water is highly exothermic in nature. When water is added to acid spurting takes place because of lot of heat produced during the process. Therefore, for dilution of the acid, always acid is to be added to H₂O slowly, and not water to acid.

4. On coming in contact with the skin it produces severe burns, therefore, it must be carefully handled.

Uses of H

₂SO₄

Sulphuric acid is used in large number of industries. It is called the "King of chemicals". Its important uses are:

i) As an acid in laboratories and industries.

ii) In storage batteries and lead accumulators.

iii) In chemical industries for the preparation of hydrochloric acid, nitric acid, sulphates, ether, dyes and paints.

iv) As a dehydrating agent.

v) In fertilizers, for the preparation of ammonium phosphate, ammonium sulphate, superphosphate of lime, etc.

vi) As a laboratory reagent in mixture analysis and drying of gases.

vii) In metallurgy, for electrolytic refining of metals.

viii) For pickling i.e., cleaning of metal surfaces before electroplating.