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

Carbon is a non-metallic element that occurs as a free element (diamond and graphite) and in many compounds in rocks, air, fuels and living things.

Behavioural objectives

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

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

Where does carbon occur in nature?

Carbon occurs both as a free element and in the combined state.

• Free state: as diamond and graphite.
• Impure form: as coal.
• Fuels and wood: as petroleum, natural gas and wood.
• Minerals: as limestone (CaCO₃) and dolomite (CaCO₃.MgCO₃).
• Air: as carbon dioxide (CO₂) in the atmosphere.
• Living things: it is an essential constituent of all plant and animal life.

What is allotropy?

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

• Allotropes: the different forms of the element.
• Chemical properties: allotropes of an element are the same.
• Physical properties: allotropes of an element are different, for example hardness and conductivity.

What are the allotropes of carbon?

Carbon has two groups of allotropes: crystalline and non-crystalline (amorphous).

• Crystalline allotropes: diamond and graphite, with atoms in a regular pattern.
• Non-crystalline allotropes: coal, charcoal, coke, lampblack and carbon black (soot).

What are the non-crystalline (amorphous) allotropes?

Amorphous carbon is carbon with no definite crystal shape, found in coal, charcoal, coke, lampblack and soot.

• Coal: impure carbon from decayed plants, used as fuel.
• Charcoal: made by heating wood in limited air, used as fuel and to remove colour and odour.
• Coke: made by heating coal in the absence of air, used as fuel and as a reducing agent in smelting.
• Lampblack: soot from burning oil or fat in limited air, used in inks and shoe polish.
• Carbon black (soot): fine black powder from incomplete burning of hydrocarbons, used in printing ink and rubber tyres.

What is diamond?

Diamond is the purest natural form of carbon and a crystalline allotrope with a giant covalent structure.

• Crystal shape: octahedral.
• Bonding: each carbon atom is joined to four others by strong covalent bonds.
• Structure: a giant molecule with closely packed atoms in a tetrahedral arrangement.

What are the properties of diamond?

• Hardness: extremely hard and strong, the hardest known substance, because of its strong covalent bonds.
• Melting point: very high, for the same reason.
• Density: high, because the crystal is compact.
• Chemical resistance: very resistant to chemical action and temperature, because all four valence electrons are bonded.
• Electrical conduction: a non-conductor, because there are no free electrons in the lattice.
• Light: transparent and highly refractive, so it sparkles as a jewel.

What are the uses of diamond?

• Mining drills: diamond is dense and hard.
• Sharpening: used to sharpen very hard tools.
• Cutting: used for cutting glass and metals.
• Instruments and wires: used as pivot supports in precision instruments and as dies for drawing wires.
• Jewellery: its high refractive index and dispersion power give a sparkling brilliance when cut and polished.

How are artificial diamonds made?

Artificial diamonds are made by heating graphite under very high pressure in the presence of a metal catalyst.

1. Graphite is placed under very high pressure.
2. It is heated to a very high temperature for several hours.
3. A nickel or rhodium catalyst is present.
4. The graphite changes into diamond.

What is graphite?

Graphite is a crystalline allotrope of carbon in which the atoms are arranged in flat layers.

• Crystal shape: hexagonal.
• Layers: flat sheets of carbon atoms lie parallel, one above the other, to form the lattice.
• Forces between layers: weak, so the layers slide over each other.
• Bonding within a layer: each carbon atom is bonded to three others, leaving one electron free to move.

What are the properties of graphite?

• Softness: soft and slippery, because the weak forces between layers let them slide. Hence it acts as a lubricant.
• Density: lower than diamond, because of its open layered structure.
• Chemical attack: more open to chemical attack than diamond, for the same reason.
• Electrical conduction: a good conductor, because of free delocalised (mobile) electrons in the lattice.
• Heat and reactions: withstands high temperatures and is fairly inert.

What are the uses of graphite?

• Lubricant: used on bicycle chains and in the bearings of some motor cars.
• High-temperature lubricant: graphite mixed with oil.
• Electrodes: used in electroplating and dry cells, as it conducts well and is fairly inert.
• Conducting coat: coating a non-conductor with graphite makes it conduct.
• Crucible lining: lines crucibles for making high-grade steel and other alloys, since it withstands high temperatures.
• Pencils: graphite mixed with clay makes the “lead” of a pencil.
• Pigment: a black pigment in paints.
• Nuclear reactors: a neutron moderator in atomic piles.

How is graphite prepared industrially?

Graphite is made industrially from coke by the Acheson process in an electric furnace.

1. Coke is packed into an electric furnace.
2. The coke is covered with sand to keep air out.
3. It is heated to a very high temperature for about 20 to 30 hours.
4. The coke changes into graphite, which is very pure and free from grit.

What are the differences between graphite and diamond?

Graphite has layers of atoms that slide and conduct, while diamond has a rigid network of atoms that is hard and does not conduct.

• X-rays: 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

List three uses of diamond and three uses of graphite, then explain why graphite is used as a lubricant.