9.1 Manufacture of Sulphuric Acid

The manufacture of sulphuric acid is called Contact Process.

The raw materials required in contact process are sulphur, air and water.

The contact process involves THREE stages of chemical reactions as below:

Stage I : Production sulphur dioxide, SO₂

Sulfur is burnt in dry air in the furnace to produce sulphur dioxide.

S + O₂ → SO₂

Stage II : Conversion of SO₂ to SO₃

Pure SO₂ reacts with oxygen in catalytic converter to produce sulphur trioxide at a temperature between 450°C to 550°C.

2SO₂ + O₂ → 2SO₃

This stage requires catalyst vanadium(V) oxide.

Stage III : Production of sulphuric acid

In the absorber, sulphur trioxide is dissolved in concentrated sulphuric acid to produce oleum, H₂S₂O₇.

H₂SO₄ + SO₃ → H₂S₂O₇

Oleum is the diluted in water to produce two molecule of sulphuric acid.

H₂S₂O₇ + H₂O → 2H₂SO₄

SO₃ gas is not directly dissolved in water to produce H₂SO₄ because the reaction releases large amount of heat and is able to vaporize sulphuric acid and produces acid mist.

Among the uses of sulphuric acids are:

i. to manufacture fertilizers

ii. to manufacture detergent

iii. to manufacture paint pigment

iv. to remove metal oxide from metal surface

v. to manufacture pesticide

vi. as the electrolyte in lead-acid accumulators

The burning of products manufactured from sulphuric acid will produce SO₂. The SO₂ dissolved in atmospheric water vapour to produce acid and subsequently causes the acid rain.

SO₂ + H₂O → H₂SO₃

6.2 Electrolysis Of Molten Compounds

A small quantity of solid lead bromide is taken in a silica crucible and two graphite electrodes are inserted. A battery consisting of two dry cells is connected to the electrodes through a switch and an ammeter.

When the crucible containing lead bromide is heated the solid lead (II) bromide melts. Now, on pressing the switch, electricity flows through the system and a red brown gas (bromine) evolves at the anode and metallic lead deposits at the cathode.

Thus, electrolysis of lead bromide using graphite electrodes produces lead metal at the cathode and bromine gas at the anode.

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6.1 Electrolysis And Electrode Processes

The chemical reactions, which take place at the surface of electrodes are called electrode processes.

According to the theory of ionization, electrolytes are present as ions in solution. These ions are directed towards the respective electrodes by the electricity supplied.

The electrolytes can be electrolyzed only in the dissolved or molten state.

Electrolysis is a process whereby a compound is decomposed into its constituent elements when an electric current passes through an electrolyte.

A simple electrolytic cell consists of a battery, an electrolyte and two electrodes.

Electrodes are conductors in the form of wires, rods or plates which pass electricity through the electrolyte during electrolysis.

Electrodes that do not take part in chemical reactions during electrolysis are known as inert electrodes. Examples are graphite (carbon) and platinum electrodes.

Electrodes that take part in chemical reactions during electrolysis are known as active electrodes. Examples are metal electrodes such as copper and silver.

The electrode which is connected to the positive terminal of a battery is called the anode while the electrode which is connected to the negative terminal of a battery is called the cathode.

Electrolytes are substances that can conduct electricity either in molten state or in aqueous solution, and undergo chemical changes.

Electrolytes can conduct electricity because the presence of free moving ions.

Examples of electrolytes are

• acid solutions
• alkali solutions
• salt solutions
• molten ionic compounds

Ionic compounds do not conduct electricity in the solid state because the ions are held in a lattice and do not move freely. However, when they are melted or dissolved in water, they can conduct electricity. This is because the ions are free to move in the molten state or aqueous solutions.

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4.3 Group 1 Elements, The Alkali Metals

4.31 Physical Properties

All the Group 1 elements are silvery-coloured metals.

They are soft, and can be easily cut with a knife to expose a shiny surface.

They have low melting and boiling temperatures.

They have low densities - Li, Na and K are less dense than water.

4.32 Reactivity

These elements are highly reactive metals.

All elements in Group 1 have one valence electron each.

The reactivity increases on descending the Group from lithium to cesium.

4.33 Reaction with Oxygen

The alkali metals react vigorously with oxygen and form ionic solid oxides of composition M₂O.

4M + O₂ → 2M₂O

This equation applies to any of these metals and water - simply replace M with any element of Group 1.

Lithium and potassium react with oxygen almost the same manner as sodium. The difference is that lithium is less reactive while potassium is more reactive than sodium.

4Li + O₂ → 2Li₂O

4K + O₂ → 2K₂O

Reaction of sodium with oxygen

4.33 Reaction with Water

All of these metals react vigorously or even explosively with cold water.

In each case, a solution of the metal hydroxide is produced together with hydrogen gas.

2M + 2H₂O → 2MOH + H₂

This equation applies to any of these metals and water - simply replace M with any element of Group 1.

Let us look at a reaction between sodium and water.

Lithium and potassium react with water almost the same manner as sodium. The difference is that lithium is less reactive while potassium is more reactive than sodium.

2Li + 2H₂O → 2LiOH + H₂

2K + 2H₂O → 2KOH + H₂

Reaction of lithium with water


Reaction of sodium with water


Reaction of potassium with water