Carbon and its Compounds I – SS1 Chemistry Lesson Note (Third Term)

Carbon occurs naturally as diamond and graphite, in impure form as coal, and in compounds such as limestone, petroleum and carbon dioxide in air. It exists in different forms called allotropes, which have the same chemical properties but different physical properties. Diamond and graphite are the crystalline allotropes.

Behavioural objectives

By the end of the lesson, students should be able to:

• state where carbon occurs in nature;
• define allotropy and name the crystalline and non-crystalline allotropes of carbon;
• describe the structure, properties and uses of diamond and graphite;
• describe how graphite is made industrially by the Acheson process;
• state the differences in properties between graphite and diamond.

Where does carbon occur?

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

What are the 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 element 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 (amorphous) allotropes, e.g. coal, charcoal, coke, lampblack and carbon black (soot).

What are the crystalline allotropes of carbon?

Diamond

Diamond is the purest form of carbon. The diamond crystal is octahedral in shape. It is 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

• Diamond is extremely hard and strong, with a high melting point, because of its strong covalent bonds. It is the hardest substance known in the world.
• It has a high density because of the compactness of its crystal.
• It is very resistant to chemical action and temperature because all four valence electrons are saturated and bonded.
• It is a non-conductor of electricity because there are no free valence electrons in the crystal lattice.
• It is transparent and highly refractive, hence it is used as a jewel and sparkling substance.

Uses of diamond

• It is used industrially in drills for mining since it is dense and hard.
• It is used to sharpen very hard tools.
• It is used for cutting glass and metals.
• It is also used as pivot supports in precision instruments and as dies for drawing wires.
• It is valuable in making jewellery, because its high refractive index and dispersion power give it a sparkling brilliance when it is cut and polished.

Artificial diamonds

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

Graphite

The graphite crystal is hexagonal. 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

• Graphite is soft and slippery because of the weak forces holding its layers. Each layer can slide over the others. Hence, graphite acts as a lubricant.
• It is less dense and prone to chemical attack due to its open layered structure.
• It is a good conductor of electricity because of the presence of free delocalised (mobile) electrons in the crystal lattice.
• It is inert and is used to absorb radiation in nuclear stations in atomic piles.

Uses of graphite

• It is used on bicycle chains and for the bearings of some motor cars.
• It is used as a non-greasy lubricant. Combining it with oil makes a high-temperature lubricant.
• It is used as electrodes in electroplating and dry cells, since it is a good conductor of electricity and relatively inert.
• It can be used to make a non-conductor conductive by coating it with graphite.
• It is used to line crucibles for making high-grade steel and other alloys, since it can withstand high temperatures.
• It is used in making lead pencils. Combining it with clay makes the “lead” in pencils.
• It is used as a black pigment in paints.
• It is used as a neutron moderator in atomic piles.

How is graphite prepared industrially?

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. Air is excluded by covering the coke with sand. The graphite produced is very pure and free from grit.

What are the differences between graphite and diamond?

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

Evaluation

1. Define allotropy.
2. Name two crystalline allotropes and two amorphous allotropes of carbon.
3. Explain why graphite conducts electricity but diamond does not.
4. State three differences in properties between graphite and diamond.
5. Describe how graphite is produced industrially.

Assignment

1. List three uses of diamond and three uses of graphite.
2. Explain why graphite is used as a lubricant.