CARBON: OCCURRENCE, ALLOTROPES, USES & PROPERTIES

OCCURRENCE OF CARBON

It occurs naturally as diamond and graphite.
It occurs in an impure form as coal.
It occurs in the combined state as petroleum, wood and natural gases.
It occurs in minerals such as limestone (CaCO₃) and dolomite (MgCO₃)
It occur in the atmosphere (air) as CO₂
It is an essential constituent of all forms of plant and animal life.

ALLOTROPES OF CARBON

Allotropy is the phenomenon whereby an element exists in two or more different forms in the same physical state. The different forms of the elements are known as allotropes. They have the same chemical properties but different physical properties.

Carbon exists in several allotropic forms:

(1). Crystalline Allotropes e.g Diamond and graphite

(2). Non-crystalline Allotropes/Amorphous carbon e.g coal, charcoal, coke, lampblack and carbon black (soot)

Crystalline Allotropes of carbon

Diamond: Diamond is the purest form of carbon. The diamond crystal is octahedral in shape. It is actually a giant molecule in which the carbon atoms are closely packed and held together by strong covalent bonds.

Basic Tetrahedral Shape in Diamond Crystals

PROPERTIES OF DIAMOND

(1) Diamond is extremely hard and strong with high melting point because of strong covalent bond. Diamond is the hardest substance known in the world.

(2) It has a high density because of its compactness of crystal.

(3) It is a very resistant to chemical action and temperature because all four valence electrons are saturated bonded.

(4) It is a non-conductor of electricity because there are no free valence electrons in the crystal lattice.

(5) Transparent and highly refractive, hence it is used as a jewel and sparkling substance.

USES OF DIAMOND

(1) They are used industrially in drills for in mining since they are dense and hard.

(2) They are used to sharpen very hard tools.

(3) They are used for cutting glass and metals.

(4) They are also used as pivot supports in precision instruments and as dies for drawing wires

(5) It is valuable in making jewellery (i.e. its high refractive index and dispersion power give it a sparkling brilliance when it is cut and polished).

Artificial diamond: They are made by subjecting graphite to a very high temperature and pressure for several hours in the presence of nickel or rhodium catalyst.

GRAPHITE

The graphite crystal is hexagonal in shape. The carbon atoms in graphite form flat layers. These layers are arranged in parallel, one above the other to form a crystal lattice.

PROPERTIES OF GRAPHITE

(1) Graphite is soft and slippery because of weak forces holding its layers. Each layer can slide over one another. Hence, graphite acts as a lubricant.

(2) It is less dense and prone to chemical attack due to its open structures in layers.

(3) It is a good conductor of electricity because of the presence of free delocalized electrons (mobile electron) in the crystal lattice.

(4) It is inert and used to absorb radiations in nuclear station in atomic pile.

USES OF GRAPHITE

(1) It is usually used on bicycle chains and for the bearings of some motor cars.

(2) It is used as a non greasy lubricant (i.e combining it with oil makes a high temperature lubricant).

(3) It is used as electrodes in electroplating and in dry cells (since it is a good conductor of electricity and relatively inert).

(4) Graphite can be used to make a non-conductor conductive by coating with it.

(5) It is used to line crucibles for making high-grade steel and other alloys (since it can withstand high temperature).

(6) It is used in making lead pencils i.e. combining it with clay makes lead in pencils.

(7) It is used as a black pigment in paints.

(8) It is used as a neutron moderator in atomic piles.

INDUSTRIAL PREPARATION OF GRAPHITE

Graphite is produced industrially by heating coke in an electric furnace to a very high temperature for about 20 to 30 hours. This process is called the Acheson process. Acheson process is a process of producing graphite from coke at high temperature. Air is excluded by covering the coke with sand. The graphite produced is very pure and free from grit.

DIFFERENCES IN PROPERTIES BETWEEN GRAPHITE AND DIAMOND

Graphite	Diamond
1. It has a density of 2.3gcm^-3	1. It has a density of 3.5gcm^-3
2. It is a black, opaque solid	2. It is a colourless, transparent solid
3. It is very soft, marks paper	3. It is the hardest known substance.
4. It is a good conductor of electricity	4. It is a non-conductor of electricity
5. Attacked by potassium trioxochlorate (v) and trioxonitrate (v) acid together.	5. Not attacked by these reagents.

Note: Diamond is transparent to x-rays while glass is almost opaque.

EVALUATION

(a) What is allotropy? (b) Mention two allotropes of carbon
Compare the properties of these allotropes of carbon.

AMORPHOUS CARBON

Carbon also occurs in a number of other forms which has no definite crystalline structure. These non-crystalline structures which are not considered to be true allotropes include:

CHARCOAL

This is made by burning wood, bones, sugar etc in a limited amount of air. Charcoal is used to remove colour from substances. Wood charcoal is used in absorbing poisonous gases while animal charcoal is used in absorbing colours.

CARBON BLACK AND LAMP BLACK

Lamp black is obtained by burning the wick of an oil lamp excessively so that it leaves a deposit of soot on the lamp-glass and cover; while carbon black is obtained from burning coal gas, natural gas or petroleum. Carbon black and lamp black are used as an additive to rubber tyres. They are also used in making printer’s ink, carbon paper, black shoe polish, type writing ribbons e.t.c

PHYSICAL PROPERTIES OF CARBON

(1) All the different allotropes of carbon are black or greyish-black solids except diamond and they are odourless and tasteless.

(2) They have a high melting point of about 3500⁰C.

(3) They are insoluble in all common solvents like water, alkalis, acids, petrol and carbon (iv) sulphide (CS₂). This is the reason carbon deposits inside motor engines have to be removed mechanically. This is known as decarbonisation of motor engines.

CHEMICAL PROPERTIES OF CARBON

(1) Combustion:

(a) All forms of carbon burn in excess oxygen to produce carbon (iv) oxide gas.

C_(s) + O_2(g) CO_2(g) ( Complete combustion)

(b) All forms of carbon also burn in a limited supply of air to produce carbon (ii) oxide.

C_(s) + O_2(g) CO_(g) ( Incomplete combustion)

(2) Combination reaction: Carbon combines directly with certain elements such as Sulphur, Hydrogen, Calcium and Aluminium at very high temperatures.

C_(s) + 2S_(s) CS_2(l)

Carbon (iv) sulphide

C_(s) + 2H_2(g) CH_4(g)

Methane

2C_(s) + Ca_(s) CaC_2(s)

Calcium carbide

3C_(s) + 4Al_(s) Al₄C_3(s)

Aluminium carbide.

(3) As a reducing agent: Carbon is a strong reducing agent. It reduces the oxides of the less active metals to the metals, while carbon is itself oxidized to either carbon (iv) oxide or carbon (ii) oxide, depending on the reaction conditions.

Fe₂O_3(s)+ 3C_(s) 2Fe_(s) + 3CO_(g)

2CuO_(s) + C_(s) 2Cu_(s) + CO_2(g)

H₂O_(g) + C_(s) CO_(g)+ H_2(g)

CO_2(g) + C_(s) 2CO_(g)

K Na Ca Mg Al Zn Fe Sn Pb H

Cu Hg Ag Au

The oxide is not reduced The oxide is reduced to the metal when

when heated heated

ZnO_(s) + C_(s) Zn_(s) + CO_(g)

PbO_(s) + C_(s) Pb_(s)+ CO_(g)

(4) Reaction with strong oxidizing agents: When carbon is heated with conc. HNO₃ or conc. H₂SO₄, it is oxidized to Carbon (iv) oxide.

C_(s) + 4HNO_3(aq) 2H₂O_(l) + 4NO_2(g)+ CO_2(g)

C_(s) + 2H₂SO_4(aq) 2H₂O_(l) + 2SO_2(g)+ CO_2(g)