Sulphuric acid

The chemical formula of sulphuric acid is H₂SO₄. It is a very important industrial chemical. It is also called king of chemicals because a large number of substances are manufactured from it.

Manufacture of Sulphuric Acid

Sulphuric acid is manufactured by Contact process. This process involves the following four steps:

1. Formation of Sulphur Dioxide

In first step sulphur is burnt in air to produce sulphur dioxide gas.

S     +     O₂     →     SO₂
(Sulphur + Oxygen → Sulphur dioxide)

2. Formation of Sulphur Trioxide

Sulphur dioxide is then mixed with more air and heated to 450°C by passing over vanadium oxide catalyst, to form sulphur trioxide.

            2SO₂     +     O₂           2SO₃     +     Heat
(Sulphur dioxide + Oxygen → Sulphur trioxide + Heat)

3. Formation of Oleum

Sulphur trioxide is then dissolved in concentrated sulphuric acid to form a thick liquid called oleum (H₂S₂O₇).
H₂SO₄       +       SO₃     →     H₂S₂O₇
(Sulphuric acid + Sulphur trioxide → Oleum)

4. Formation of Sulphuric Acid

Oleum is then diluted with water to get sulphric acid.

H₂S₂O₇       +       H₂O    →     H₂SO₄
(Oleum + Water → Sulphuric acid)

Physical Properties of Sulphuric Acid

Sulphuric acid is used in the form of concentrated sulphuric acid and dilute sulphuric acid. So, we will study their physical and chemical properties separately.

1. Concentrated Sulphuric Acid

It is a colourless oily liquid which contains about 98% sulphuric acid and 2% water.

Chemical Properties of Concentrated Sulphuric Acid

1) As a Dehydrating Agent

Concentrated sulphuric acid can dehydrate i.e. can remove water from many substances such as sugar, paper, wood, copper sulphate etc.

2) Dehydration of Sugar

When concentrated sulphuric acid is added in sugar, it removes 11 molecules of water from sugar molecule and converts it into a black mass of carbon called sugar charcoal.

C₁₂H₂₂O₁₁             12C      +      11H₂O
(Sugar → Carbon + Water)

Dehydration of Copper Sulphate Crystals

Copper sulphate is a blue coloured compound containing 5 molecules of water of crystallization. Its molecular formula is CuSO₄.5H₂O. When concentrated sulphuric acid is added in blue crystals of hydrated copper sulphate, it removes 5 molecules of water of crystallization from crystals of copper sulphate to form anhydrous copper sulphate which is white in colour.

CuSO₄.5H₂O       CuSO₄      +      5H₂O
(Copper sulphate(Blue crystals) → Copper sulphate(White powder) + Water)

As an Oxidising Agent

Concentrated sulphuric acid acts as an oxidising agent and oxidises carbon and sulphur to carbon dioxide and sulphur dioxide respectively.

C    +    2H₂SO₄   →    2H₂O   +     2SO₂     +      CO₂
(Carbon + Sulphuric acid → Water + Sulphur dioxide + Carbon dioxide)

S    +    2H₂SO₄   →    2H₂O   +     2SO₂
(Sulphur + Sulphuric acid → Water + Sulphur dioxide)

2. Dilute Sulphuric Acid

It contains about 10% sulphuric acid and 90% water. It is always obtained by diluting concentrated sulphuric acid with water. It should be noted that while preparing dilute sulphuric acid concentrated sulphuric acid is added into water. Water should never be added into concentrated sulphuric acid. This is because when water is added to acid a large amount of heat is produced which can throw out the drops of acid and can burn you. However, if acid is added to water, the heat evolved is absorbed by a large amount of water.

Chemical Properties of Dilute Sulphuric Acid

1) Reaction with Metals

Dilute sulphuric acid reacts with reactive metals such as zinc and iron to form hydrogen gas and metal sulphates.

Zn    +    H₂SO₄   →    ZnSO₄     +      H₂
(Zinc + Sulphuric Acid → Zinc sulphate + Hydrogen)

Fe    +    H₂SO₄   →    FeSO₄     +      H₂
(Iron + Sulphuric Acid → Iron sulphate + Hydrogen)

Reaction with Carbonates

Dilute sulphuric acid reacts with metal carbonates to liberate carbon dioxide.

Na₂CO₃    +     H₂SO₄   →    Na₂SO₄   +     CO₂    +     H₂0
(Sodium carbonate + Sulphuric acid → Sodium sulphate + Carbon dioxide + Water)

Reaction with Alkalis

Dilute sulphuric acid reacts with alkalis such as sodium hydroxide to form normal salts called sulphates and acid salts called hydrogensulphates.

2NaOH       +         H₂SO₄   →    Na₂SO₄     +      2H₂O
(Sodium hydroxide(Normal salt) + Sulphuric Acid → Sodium sulphate + Water)

NaOH       +         H₂SO₄   →    NaHSO₄     +      2H₂O
(Sodium hydroxide(Acid salt) + Sulphuric Acid → Sodium hydrogen sulphate + Water)

Uses of Sulphuric Acid

1. It is used in the manufacture of fertilizers.
2. It is used as a reagent in laboratories.
3. It is used for manufacturing paints, pigments and dyes.
4. It is used for manufacturing soaps and detergents.
5. It is used for manufacturing plastics and synthetic fibres.
6. It is used as a dehydrating agent.

Test your understanding and answer these questions:

1. What is oleum?
2. Explain the process of manufacture of sulphuric acid.
3. What are chemical properties of sulphuric acid?
4. What are uses of sulphuric acid?
5. Why sulphuric acid is called king of chemicals?