ICSE Class 8 Chemistry Chapter 4 Carbon and Its Compounds

Carbon And Its Compounds

In This Chapter You Will Learn

Introduction and occurrence

A crystalline form of carbon

Amorphous form of carbon

Types of fuels and characteristics of good fuels

Allotropy and allotropes of carbon

Diamond

Graphite

Fuels: as source of energy

Fossil fuels: coal, petroleum and its products, natural gas

Combustion

Flame: parts of a candle flame

Fire and fire extinguishers

Carbon monoxide: a poisonous gas and a reducing agent

Symbol Of Carbon

Symbol of carbon: C, Atomic number: 6, Mass number: 12, Valency: 4

Introduction

Carbon is the very basis of life, since matter constituting all living things, whether plants or animals, contains carbon. Therefore it is not surprising that it is one of the most widely distributed elements on the earth. In fact it is the third most important element, i.e. after oxygen and hydrogen, for the existence of life on the earth.

The name carbon is derived from the Latin word carbo (meaning coal).

Orbital structure of carbon has four electrons in its valence shell, i.e. the valency of carbon is 4.

Section 4.1 - Occurrence

Carbon occurs in the earth's crust in the free as well as in the combined state.

In the free state, it occurs as coal, diamond and graphite.

In the combined state, carbon occurs in:

(i) the atmosphere (as carbon dioxide gas).

(ii) natural water as dissolved carbon dioxide

(iii) natural gas and petroleum.

(iv) food nutrients like starch, sugar, fats, proteins, vitamins, etc.

(v) carbonates and bicarbonates such as chalk, limestone, marble (CaCO3), calamine (ZnCO3), washing soda (Na2CO3-10H2O) and baking soda (NaHCO3).

(vi) Clothing materials like cotton, silk, terylene, rayon etc.

Carbon is unique non-metal having widely differing forms and properties. Mankind has known carbon in the form of charcoal and soot ever since the discovery of fire.

Although carbon constitutes only 0-03% of the earth's crust, it forms an enormously large number of compounds. The number is so large that an entire branch of chemistry, called organic chemistry, is devoted to the study of carbon and its compounds.

In fact, for the convenience of study, all compounds are classified into two classes:

(i) organic compounds

(ii) inorganic compounds.

All organic compounds essentially contain carbon as a constituent.

A few examples of useful organic compounds are starch, wax, vinegar, alcohol, dyes, detergents, soaps, plastics, clothing materials like nylon, silk, wool and cotton, as well as paper, polythene, perfumes, disinfectants and medicines.

CaCO3 is the chemical formula for all the three substances, chalk, marble and limestone.

In addition, petroleum is a mixture of carbon compounds found in a liquid state. Also, vegetable oils are the liquid forms of carbon compounds.

Oxides of carbon (CO2, CO), carbonates and bicarbonates belong to inorganic compounds.

Activity 1

To show the presence of carbon in sugar.

Take some sugar in a test tube. Heat it for some time. You will observe that sugar first melts, then turns brown, and finally gets charred and turns black. This black substance is carbon. This experiment proves that sugar contains carbon.

Some Useful Carbon Products

1. PVC: The full form is polyvinyl chloride. It is used for making pipes.

2. Bakelite: It is tough, resistant plastic that can be moulded into different shapes. It is used to make cameras, telephones, plugs etc.

3. Polyethene: This is another important compound of carbon with various uses. All of you must be familiar with this word polyethene, often used to make carry bags.

4. Carbon fibre: Organic textile fibres are heated to make silky threads of pure carbon. These fibres are combined with other materials such as plastic to make very strong and light composite materials, which are useful for objects where tightness and strength are important, from tennis ball to small aeroplanes.

Do you know that these carbon fibres are much thinner than human hair, but are eight times stronger than steel?

Section 4.2 - Allotropy And Allotropes Of Carbon

Allotropy is defined as the phenomenon due to which an element exists in two or more forms in the same physical state with identical chemical properties but with different physical properties. Such forms of an element are known as its allotropic forms or allotropes.

Some elements exhibiting allotropy are carbon, phosphorus and sulphur.

The chart given on this page outlines the different allotropes of carbon. The allotropes of carbon are divided into two types:

(i) crystalline allotropes

(ii) amorphous or non-crystalline allotropes

These two broad types are sub-divided into a number of specific allotropes.

All these forms of carbon differ in their physical properties, but when burnt in the presence of oxygen, they all produce carbon dioxide, with the release of heat.

Diamond/graphite/coal/coke + Oxygen - Carbon dioxide + Heat

C + O2 - CO2 + Heat

What Are Crystals?

A crystal is a solid whose particles (atoms, molecules or ions) are arranged in a definite pattern and outwardly expressed by a geometrical form (with plane faces).

For example: Sodium chloride and sugar are crystalline in nature, i.e. their molecules have a definite cubic shape.

Section 4.3 - Crystalline Allotropes Of Carbon

Section 4.3.1 - Graphite

Graphite is a crystalline form of carbon. Natural graphite is extensively found as the mineral plumbago in both Sri Lanka and Siberia. In India, graphite is found in Jammu and Kashmir, Bihar, Orissa, Rajasthan and West Bengal.

Preparation of graphite: Pure graphite is prepared artificially by heating powdered coke mixed with a little sand and Iron (III) oxide in an electric furnace to a temperature of about 3000-C.

SiO2 + 3C - 2CO + SiC

(Sand) (ferricoxide) (Silicon carbide)

SiC - Si + C (graphite)

Structure of graphite: In a graphite molecule each carbon atom is linked with three neighbouring carbon atoms, thus forming a hexagonal arrangement of atoms. These hexagonal groupings of carbon atoms are arranged as layers or sheets piled one on top of the other. The layers are held together by weak forces such that they can slide over one another. This is why graphite is soft and slippery and can be used as a lubricant in machines and in pencil leads. Also, in a graphite molecule, one valence electron of each carbon atom remains free, thus making graphite a good conductor of electricity.

Carbon - Carbon binding force is strong in the hexagon itself so the melting point and boiling point are high.

Properties of graphite:

1. Graphite is a greyish black, opaque substance, with a metallic lustre.

2. Its density is 2-39 g/cm3.

3. It is stable to heat and has a very high melting point of 3700-C.

4. It is soft and greasy to touch.

5. It marks a black stain on paper.

6. It is a good conductor of heat and electricity.

Activity 2

To show that graphite is a good conductor of electricity

Take a graphite rod. Connect it to a battery, a bulb and a switch with the help of connecting wires. Now close the circuit with the help of the switch. What do you observe? The bulb starts glowing. This is because graphite conducts electricity.

Uses of Graphite

1. With petroleum jelly to form graphite grease (a lubricant).

2. For making the electrodes of electric furnaces.

3. For making crucibles for melting metals due to its high melting point.

4. For making carbon brushes for electric motors.

5. For making pencil leads because it can mark black on paper (pencil leads are made by mixing graphite with variable quantities of clay).

6. It is used in nuclear reactors as moderator to slow down the speed of neutrons.

Graphite is used in many other ways too.

Section 4.3.2 - Diamond

Diamond is perhaps the purest form of carbon. It occurs in all shapes and sizes. Diamonds are found in South Africa, Brazil, Namibia, Russia, Australia, U.S.A. and India. In India, diamonds are found at Goleconda in Karnataka and at Panna in M.P.

Formation of natural diamonds: Natural diamonds are formed by the action of high pressure and temperature on the carbon present in the earth at depths of 150 km or so. They are mostly brought to the surface by volcanic eruptions. Diamond bearing rocks are called kimberlite rocks, after the Kimberley Mines in South Africa.

Preparation of artificial diamonds: Synthetic or artificial diamonds are made from graphite. Graphite is subjected to very high temperature (about 3000-C) and pressure. The diamonds produced under such conditions are rather small.

Value of diamonds: The value of a diamond as a gem depends upon:

(i) its weight

(ii) the impurities present in it.

The weight of a diamond is expressed in terms of carats [1 carat = 0-2 g].

Some famous natural diamonds are:

(i) KOHINOOR

(ii) PITT DIAMOND - mined in India

(iii) CULLINAN (the biggest diamond ever found, mined in South Africa).

Colourless, transparent diamonds are the costliest because they have negligible impurity. The value of a diamond decreases with an increase in the impurities present in it.

It is the presence of small traces of metallic oxide and salt that imparts distinct colours to diamonds. These coloured diamonds are called gems. Diamond gems of the darker colours, i.e. red, pink and blue, are extremely rare and therefore very valuable. Diamonds can also be grey, yellow, brown, green and orange, and even black. Black diamonds have copper oxide present in them as impurity. They are not used as gems, but they have important industrial uses.

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