SSS 1: CARBON AND ITS COMPOUNDS (I)

Carbon is a non-metal and occurs naturally in both free and combined states.

In the free state, carbon occurs in different allotropic forms, including:

1. Diamond

2. Graphite

3. Amorphous carbon

Carbon also occurs in the combined state in many substances, including:

1. Carbon(IV) oxide (CO₂) in the atmosphere and dissolved in water.

2. Metallic trioxocarbonates(IV), such as:

Calcium trioxocarbonate(IV) (limestone), CaCO₃, Magnesium trioxocarbonate(IV) (dolomite), MgCO₃

3. Petroleum

4. Natural gas

5. Coal

6. Wood and other living matter

Carbon is an essential constituent of all living organisms. It is present in carbohydrates, proteins, fats and many other organic substances.

The branch of chemistry that deals mainly with carbon compounds is known as organic chemistry.

                                              Allotropes of Carbon

Allotrope of carbon is the ability of carbon to exist in various forms in the same physical state.

Allotropy

Allotropy is the ability of an element to exist in two or more different forms in the same physical state.

Carbon has several allotropes, which can be broadly classified into:

1. Crystalline Allotropes

The crystalline allotropes of carbon include:

(a) Diamond

(b) Graphite

2. Amorphous Forms

Examples of amorphous forms of carbon include:

(i) Coal

(ii) Coke

(iii) Charcoal

(iv) Lampblack

(v) Sugar charcoal

(vi) Animal charcoal

                                                          Diamond

Diamond occurs naturally as a colourless, lustrous solid and can be transformed into brilliant gemstones. It is the hardest naturally occurring crystalline allotrope of carbon. It is found in several parts of the world, including Africa, Brazil and Australia.

Diamond consists of carbon atoms arranged in a three-dimensional giant covalent structure. Each carbon atom is strongly bonded to four other carbon atoms.

 Properties of Diamond

1. Diamond is the hardest naturally occurring substance on Earth.

2. It has a high melting point.

3. It is very dense.

4. It is resistant to high temperatures.

5. It is resistant to chemical attack.

6. Diamond is a non-conductor of electricity because it has no free valence electrons. All the valence electrons are involved in forming strong covalent bonds within the crystal.

7. Diamond crystals are octahedral in shape.

8. It contains strong covalent bonds between the carbon atoms.

9. Diamond has a high refractive index and high dispersive power.

                   Uses of Diamond

1. Diamonds are used industrially in drilling equipment for mining.

2. They are used as abrasives for sharpening very hard tools.

3. They are used for cutting glass and metals.

4. They are used as pivot supports in precision instruments.

5. They are used as dies for drawing wires.

6. They are used as jewellery.

                                                         Graphite

Graphite occurs naturally as plumbago, an opaque black solid. It is mined mainly in Austria, China and other countries.

The carbon atoms in graphite are arranged in flat layers. The layers are arranged parallel to one another, with one layer above another, forming a crystal lattice.

Graphite can also be produced industrially through the Acheson process.

 Properties of Graphite 

1. Graphite is soft. It flakes easily because of its layered crystalline structure.

2. It has a high melting point.

3. It is less dense than diamond.

4. It is a good conductor of electricity because it has mobile electrons in its crystal lattice. Mobile electrons exist because only three of the four valence electrons of each carbon atom in the graphite crystal are involved in bond formation.

5. It is relatively inert chemically, but it can be oxidised to a six-carbon-atom organic compound under suitable conditions.

 Uses of Graphite

1. Graphite is used as a dry lubricant because it is non-volatile and non-sticky.

2. It is used as an electrode in electroplating and dry cells.

3. A mixture of graphite and clay is used as the lead in pencils.

4. It is used as a black pigment in paint.

5. It is used as a neutron moderator in atomic piles.

6. It is used for lining crucibles used for making high-grade steel and other alloys.

                                                     Amorphous carbon 

Amorphous carbon includes coal, coke, charcoal, lampblack, sugar charcoal, etc.

Amorphous forms of carbon consist of minute crystals of graphite bound together by impurities, with the exception of coal, which is mined from natural deposits. The other amorphous forms can be prepared in various ways.

 Coal

Coal is formed naturally through a process called carbonisation. During the process of carbonisation, the vegetable materials are converted in stages into:

Peat → Lignite (brown coal) → Bituminous (soft) coal → Anthracite (hard coal).

Anthracite is about 95% pure carbon. The impurities present may include nitrogen, sulphur and phosphorus.

 Uses of Coal

Coal is used mainly as a fuel to generate power for steam engines, factories and electric power plants.

It is also used for making various chemicals.

Coke

Coke is produced from the process called destructive distillation of coal.

Uses of Coke

1. Coke is used mainly as a fuel.

2. It is used as an industrial reducing agent.

3. It is used in the extraction of metals, especially iron from their ores.

4. It is used in the production of gaseous fuels, such as water gas and producer gas.

5. It is used for the manufacture of graphite, calcium carbide, silicon carbide and carbon disulphide.

 Carbon black (Soot)

Carbon black is a finely divided carbon produced by burning carbonaceous materials in a limited supply of air. It is obtained from coal gas, natural gas or fuel oils. Lamp-black is obtained from vegetable oils or lamp oils.

 Uses of Carbon Black

Carbon black or soot is used in the manufacture of:

(I) Rubber tyres

(ii) Black shoe polish

(iii) Printer's ink

(IV) Typewriting ribbons

(v) Carbon paper, etc.

 Charcoal 

Charcoal can be made by heating wood, nut shells, bones, sugar and blood.

The different types of charcoal are:

(i) Wood Charcoal

Wood charcoal is prepared by heating wood in a limited supply of air. It may contain impurities such as sulphur.

(ii) Sugar Charcoal

Sugar charcoal is formed when sugar is dehydrated either by burning the sugar in a limited supply of air or by the action of concentrated sulphuric acid (H₂SO₄). It is one of the purest forms of amorphous carbon.

(iii) Animal Charcoal

Animal charcoal is produced when bones are heated in a limited supply of air. It contains a high percentage of calcium tetraoxophosphate(V), [Ca₃(PO₄)₂], as an impurity.

 Uses of Charcoal

Charcoal has a very porous structure. It allows gas molecules and dyes to become adsorbed onto its internal surface.

1. Charcoal is used as a good fuel.

2. Wood charcoal is used in gas masks for adsorbing poisonous gases.

3. Charcoal is used for the purification of noble gases.

4. It is used for recovering industrial solvents.

5. Animal charcoal is used for decolourising crude sugar and petroleum jelly.

6. Wood charcoal is used as a domestic fuel.

 Carbon Fibres

Carbon fibres are produced by carefully heating fibres of materials such as polymers until they char and form carbon.

Uses of Carbon Fibres

Carbon fibres are used in plastics to produce very light but stiff and strong materials.

                                                  Properties of Carbon

1. Combustion

Carbon burns in excess oxygen to produce carbon(IV) oxide only.

C(s) + O₂(g) → CO₂(g)

When the supply of air is limited, it produces carbon(II) oxide

2C(s) + O₂(g) → 2CO(g)

2. Combination Reaction

Carbon combines directly with certain elements at high temperatures, for example, sulphur, hydrogen, calcium and aluminium.

Carbon + sulphur → carbon disulphide

C(s) + 2S(s) → CS₂(l)

Carbon + hydrogen → methane

C(s) + 2H₂(g) → CH₄(g)

Carbon + calcium → calcium carbide

2C(s) + Ca(s) → CaC₂(s)

Carbon + aluminium → aluminium carbide

3C(s) + 4Al(s) → Al₄C₃(s)

3. Carbon as a Reducing Agent

Carbon is a strong reducing agent. It reduces the oxides of less reactive metals to the metals themselves.

Fe₂O₃(s) + 3C(s) → 2Fe(s) + 3CO(g)

2CuO(s) + C(s) → 2Cu(s) + CO₂(g)

4. Reaction with Strong Oxidising Agents

When carbon is heated with concentrated tetraoxonitrate(V) acid (HNO₃) or concentrated tetraoxosulphate(VI) acid (H₂SO₄), it is oxidised to carbon(IV) oxide.

C(s) + 4HNO₃(aq) → 2H₂O(l) + 4NO₂(g) + CO₂(g)

C(s) + 2H₂SO₄(aq) → 2H₂O(l) + 2SO₂(g) + CO₂(g)

                                        Destructive Distillation of Coal and Wood

Destructive distillation is the heating of a substance strongly in the absence of air.

Coal

Coal → Coke + Ammoniacal Liquor + Coal Tar + Coal Gas

Wood

Wood → Wood Charcoal + Pyroligneous Acid + Wood Tar + Wood Gas

Assignment

1. State five differences between the properties of diamond and graphite.

2. Explain the Acheson process.

3. State the uses of each product of the destructive distillation of coal and wood.

4. Briefly explain the production of artificial diamond.