Carbon

Allotropes of Carbon

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

It can be also defined as the existence of the same element in different physical forms with similar chemical properties.

The different forms of the same element in the same physical state are known as allotropes.

Allotropes of carbon are classified as follows:

Crystalline allotropes

(a) Diamond

In diamond each carbon atom is chemically bonded to four other carbon atoms in a tetrahedral shape. The bonds are single covalent bonds.

Physical Properties

1. It is the hardest naturally occurring substance.
2. It has high melting point of 3550⁰C and high boiling point of 4289⁰C.
3. It is a poor conductor of electricity. Because there are no free electrons (mobile electrons/delocalized electrons) in its structure. Each electron is firmly held in covalent bonds.
4. It is denser than graphite, due to close packing of carbon atoms in the structure of diamond.
5. It is colourless, transparent and sparkling.

Uses of Diamond

1. Due to its hardness, it is used for making
   • Abrasive tools
   • Drilling tools eg. drill bits used in quarrying and water drilling.
   • Glass cutter tools
2. It is used in making jewellery because of its shining property when polished.
3. It can also be used as an insulator because it is a poor conductor of electricity.

(b) Graphite

In graphite, each carbon atom is bonded to three other carbon atoms in a hexagonal ring in one plane.

Therefore, since carbon atom has four valence electrons, in graphite three participates in the bond formation and one electron (the forth one) is free to move or it is delocalized electron in the graphite structure.

Physical properties of graphite

1. It is a good conductor of electricity due to the presence of free and delocalized electrons in its structure.
2. It is less dense than diamond.
3. It has very high melting point due to weak forces holding the parallel layers.
4. It is soft and greasy.

Uses of graphite

1. It is used in the manufacture of pencil lead.
   It made out of graphite and clay. It writes well on paper.
2. It is used to make electrodes in cells and batteries due to its good conductivity of electricity.
3. It is used as a lubricant, because it is soft and slippery.

Amorphous allotropes (without shapes)

(a) Coal

Apart from carbon it contains other elements such as sulphur and nitrogen.

It is classified based on the percentage of carbon in it:

• Peat → 50 -60% of carbon.
• Lignite →60-70% of carbon.
• Bituminous → 70 – 80% of carbon.
• Anthracite → 80 – 90% of carbon.

Uses of coal

• It is used as fuel.
• It is used in the preparation of other fuels eg. coke and coal gas.

(b) Coke

Uses

(c ) Charcoal

a. Wood charcoal

Uses of wood charcoal

• It is used as fuel for domestic use.
• It used in gas masks to absorb poisonous gas.

b. Animal charcoal

c. Sugar charcoal

(d)Lampblack/ soot

It is made when petroleum products are burned in a limited supply of air.

Uses of Lampblack

Chemical properties of carbon

1. Combustion reaction. When burnt in plenty supply of oxygen, carbon dioxide is formed.
   C(s) + O₂(g) → CO₂(g)
   When oxygen is in limited supply carbon monoxide is formed.
   2C(s) + O₂(g) →2 CO(g)
2. Reducing property
   Carbon is a good reducing agent because it removes oxygen from metal oxides.
   CuO(s)(black) + C (s)→Cu(s)(brown) + CO₂(g)
   2PbO(s) (yellow) + C → 2Pb(grey) + CO₂(g)
   2Fe₂O₃(s)(red brown) + 3C (s) → Fe(s)(grey) + 3CO₂(g)
   2ZnO(s)(white)+ C(s) → 2Zn(s)(grey) +CO₂(g)
   
3. Reaction with concentrated sulphuric and nitric acid
   C(s) + H₂SO₄ (l)→ CO₂ (g)+ 2SO₂ (g)+ 2H₂O(l)
   C (s) + 4HNO₃(l) → CO₂(g) + 4NO₂ (g) + 2H₂O(l)
   

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