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?

Sulphur dioxide is a colourless gas which is present in the exhaust gases coming out of automobiles and factories. It is a poisonous gas having molecular formula SO₂. It can cause inflammation of the lungs when inhaled in the body. It is the main source of air pollution.

Sulphur dioxide is highly soluble in water. It dissolves in rainwater to form sulphuric acid and hence causes acid rain which is responsible for damaging historical buildings, bridges, metal works and plants.

Preparation of Sulphur Dioxide

In the laboratory, sulphur dioxide is prepared by heating copper turnings with concentrated sulphuric acid.

Cu    +    2H₂SO₄     →    CuSO₄     +     SO₂     +     2H₂O

(Copper + Sulphuric acid → Copper sulphate + Sulphur dioxide + Water)

Physical Properties of Sulphur Dioxide

1. It is a colourless and poisonous gas having a pungent smell.
2. It is highly soluble in water.
3. It is heavier than air.

Chemical Properties of Sulphur Dioxide

1. Sulphur Dioxide is Acidic in Nature

Sulphur dioxide is acidic in nature because solution of sulphur dioxide in water turns blue litmus red.

2. Reaction with Lime Water

Sulphur dioxide turns lime water (calcium hydroxide) cloudy due to formation of calcium sulphite.

SO₂     +     Ca(OH)₂     →     CaSO₃     +     H₂O

(Sulphur dioxide + Calcium hydroxide  → Calcium sulphite + Water)

3. Reaction with Sodium Hydroxide

Sulphur dioxide produce sodium sulphite when passed through sodium hydroxide solution.

2NaOH     +     SO₂     →     Na₂SO₃     +     H₂O

(Sodium hydroxide + Sulphur dioxide → Sodium sulphite + Water)

4. Reaction with Oxygen

When sulphur dioxide is heated to 450°C with oxygen in the presence of vanadium oxide, sulphur trioxide is formed.

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

5. Reaction with Water

Sulphur dioxide dissolves in water to form sulphuric acid.

2SO₂     +     2H₂ O     →     H₂SO₄     +     2[H]
(Sulphur dioxide + Water → Sulphuric acid + Hydrogen)

Uses of Sulphur Dioxide

1. It is used in the manufacturing of sulphuric acid.
2. It prevents the growth of bacteria. So, it is used as a preservative for the preservation of dry fruits, jams and lemon and orange squashes.
3. It is used for bleaching wood pulp, straw, wool and silk.
4. It is also used as a disinfectant.

Test your understanding and answer these questions:

1. What are chemical properties of sulphur dioxide?
2. How sulphur dioxide is prepared?

Allotropy may be defined as the existence of an element in two or more different forms. These different forms of an element are called allotropes. Sulphur also exhibits allotropy and exists in the form of two allotropes:

1. Rhombic sulphur
2. Monoclinic sulphur.

Rhombic Sulphur

Monoclinic Sulphur

 

These two allotropes of sulphur have same chemical properties but different physical properties. The allotropes have different physical properties because they have different crystal shapes. The shape of crystals of rhombic sulphur is octahedral shape, whereas monoclinic sulphur has needle shaped crystals.

The difference in shapes of two allotropes of sulphur is due to the difference in the packing arrangement of S₈ molecules. In rhombic sulphur the S₈ molecules fit tightly into each other, whereas in monoclinic sulphur, the S₈ molecules are piled up on top of each other.

 

Rhombic Sulphur

 

Monoclinic sulphur

It should be noted that rhombic sulphur is stable below 96°C whereas monoclinic sulphur is stable above 96°C.

Test your understanding and answer these questions:

1. Define allotropy.
2. Give two allotropes of sulphur.
3. Explain allotropy in sulphur.
4. What do you understand by rhombic and monoclinic sulphur?

Sulphur is a non metal having atomic number 16. So, it electronic configuration is 2.8,6. It is represented by symbol S. it is placed in group 16 of the periodic table.

Occurrence of Sulphur

Sulphur occurs both in free state and combined state in nature. In free state sulphur is found in large amount in the form of sulphur beds under the earth. In combined state sulphur occurs in the form of sulphides of many metals such as iron, copper, silver, zinc, lead, and mercury. For example,

1. Iron pyrite (FeS₂)
2. Copper pyrite (CuFeS₂)
3. Silver glance (Ag₂S)
4. Zinc blende (ZnS)
5. Galena (PbS)
6. Cinnabar (HgS)

Structure of Sulphur Molecule(S₈)

One molecule of sulphur contains 8 atoms of sulphur. These 8 atoms of sulphur combine with each other to give a puckered ring structure to sulphur molecule.

Puckered ring structure of sulphur molecule

Extraction of Sulphur (Frasch Process)

Sulphur occurs in free state under the surface of earth. It is extracted by Frasch process, which is based on the fact that sulphur has low melting point of 115°C. In this process, a hole is bored in the earth up to sulphur bed. Three concentric pipes A, B, C of different diameter are placed in the hole. Superheated water is forced down through the outer pipe A in to the sulphur bed. The super heated water melts the sulphur. Now, through innermost pipe C hot and compressed air is forced down. The force of hot compressed air brings the molten sulphur at the surface of earth through middle pipe B.

 

Physical Properties of Sulphur

1. It is a yellow crystalline solid, having no taste and odour.
2. It is a poor conductor of electricity and heat, thus it acts as an insulator.
3. It has a melting point of 115°C.
4. It is insoluble in water but freely soluble in carbon disulphide solution.
5. The vapours of sulphur are poisonous for bacteria and fungi but not for human beings.
6. It exhibits allotropy. The important allotropes of sulphur are rhombic sulphur and monoclinic sulphur.

Chemical Properties of Sulphur

1. Burning of Sulphur

On strong heating sulphur burns in air with a blue flame to form sulphur dioxide.

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

2. Reaction with Metals

Sulphur reacts with many metals to form metal sulphides. These are exothermic reactions in which heat is evolved. For example,

Fe     +     S     →     FeS     +     Heat
(Iron + Sulphur → Iron sulphide + Heat)

Zn     +     S     →     ZnS     +     Heat
(Zinc + Sulphur → Zinc sulphide + Heat)

Cu     +     S     →     CuS     +     Heat
(Copper + Sulphur → Copper sulphide + Heat)

3. Reaction of Sulphur with Non-metals

Sulphur reacts with non-metals to form covalent sulphides. For example,

1) Reaction with Hydrogen:- Sulphur reacts with hydrogen to form hydrogen sulphide.

S     +     H₂     →     H₂S
(Sulphur + Hydrogen → Hydrogen sulphide)

2) Reaction with Carbon:- Upon heating sulphur reacts with carbon to form carbon disulphide

C     +     2S     →     CS₂
(Carbon + Sulphur → Carbon disulphide)

4. Reaction with Acids

Sulphur is oxidised to sulphur dioxide by hot concentrated sulphuric acid.

Sulphur is oxidised to sulphuric acid by concentrated nitric acid.

S     +     2H₂SO₄     →     3SO₂     +     2H₂O
(Sulphur + Sulphuric acid  → Sulphur dioxide + Water)

S    +    6HNO₃    →    H₂SO₄     +     6NO₂     +     2H₂O
(Sulphur + Nitric acid → Sulphuric acid + Nitrogen dioxide + Water)

Effect of Heat on Sulphur

Following are the effect of heat on a sulphur molecule:

1. When sulphur is heated to its melting point (115°C) it melts and forms a pale yellow liquid which flows freely like water.
2. On further heating to about 160°C, sulphur becomes viscous and dark in colour.
3. On further heating to about 440°C, the molten sulphur becomes light in colour and less viscous.
4. At 444°C molten sulphur begins to boil producing yellow brown vapours.

Uses of Sulphur

1. The major use of sulphur is in the manufacturing of sulphuric acid.
2. It is used in the manufacture of carbon disulphide, gun powder, matches and in fireworks.
3. It is used in vulcanization of natural ruber.
4. It is used as an antiseptic in making skin ointments.
5. It is used as a disinfectant, fungicide and insecticide for killing bacteria, fungi and insects.
6. It is also used in the manufacture of sodium thiosulphate which is used in photography.

Test your understanding and answer these questions:

1. Name ores of sulphur?
2. Explain Frasch process of manufacture of sulphur.
3. What are uses of sulphur?
4. What are properties of sulphur?
5. Explain structure of sulphur molecule.