Unit 12: Carbon and its Compounds
From the brilliance of diamond to the chemistry of the air we breathe.
12.27 Allotropes of Carbon (Diamond, Graphite, Fullerenes)
Carbon is unique in its ability to form several allotropes—different structural forms of the same element. The bonding in each allotrope gives it vastly different properties.
- Diamond: In this giant covalent structure, each carbon atom is bonded to four other carbon atoms in a rigid tetrahedral network. This makes diamond extremely hard, gives it a very high melting point, and makes it a poor electrical conductor as all electrons are held in strong covalent bonds.
- Graphite: In this giant covalent structure, each carbon atom is bonded to three others in flat hexagonal layers. The layers are held together by weak intermolecular forces, allowing them to slide over each other, which makes graphite soft and slippery. Each carbon atom has one delocalized electron per layer, allowing graphite to conduct electricity.
- Fullerenes: These are large molecules of carbon. The most famous is Buckminsterfullerene ($C_{60}$), with atoms arranged in a spherical shape like a soccer ball. Other forms include nanotubes, which are cylindrical. They have applications in nanotechnology and materials science.
Solved Examples:
-
Why is diamond used in cutting tools, while graphite is used as a
lubricant?
Solution: Diamond's rigid, 3D network of strong covalent bonds makes it the hardest known natural substance, ideal for cutting. Graphite's layered structure, with weak forces between layers, allows the layers to slide easily, making it an excellent lubricant. -
Explain why graphite can conduct electricity but diamond
cannot.
Solution: In graphite, each carbon atom has one delocalized valence electron that is free to move along the layers and carry an electric current. In diamond, all four valence electrons of each carbon atom are locked in strong covalent bonds and are not free to move.
12.28 Coal & Coke (Formation, Destructive Distillation)
Coal is an impure form of carbon formed over millions of years from the compression of dead plant matter. The quality of coal (from peat to lignite to anthracite) increases with its carbon content.
Coke is a much purer form of carbon (over 90%) produced from coal by a process called destructive distillation (or coking). This involves heating coal to a very high temperature in the absence of air. This process drives off volatile components, leaving behind the solid coke.
Coke is a vital industrial material, primarily used as a reducing agent in the blast furnace to extract iron from its ore.
Solved Examples:
-
What is destructive distillation?
Solution: It is the process of heating a solid material, like coal, to a high temperature in the absence of air to break it down into more useful products. -
What is the main industrial use of coke?
Solution: It is used as the reducing agent and a source of heat in the blast furnace for the extraction of iron.
12.29 Carbon Dioxide (COâ‚‚) & Carbonate/Hydrogencarbonate Ions
Carbon dioxide ($CO_2$) is a colorless, odorless gas. It is an
acidic oxide that reacts with alkalis to form carbonate ($CO_3^{2-}$) or
hydrogencarbonate ($HCO_3^-$) salts. Its most famous reaction is with limewater (calcium
hydroxide), which it turns milky due to the formation of insoluble calcium carbonate:
$CO_2(g) + Ca(OH)_2(aq) \rightarrow CaCO_3(s) + H_2O(l)$
Carbonate ions ($CO_3^{2-}$) are found in rocks like limestone and chalk
($CaCO_3$). They react with acids to produce carbon dioxide gas, which is the basis for the
test for carbonates.
$CO_3^{2-}(aq) + 2H^+(aq) \rightarrow CO_2(g) + H_2O(l)$
Sodium hydrogencarbonate ($NaHCO_3$), or baking soda, is useful in baking because it thermally decomposes on heating to release $CO_2$ gas, which causes dough to rise.
Solved Examples:
-
How would you test a rock sample to see if it is limestone
($CaCO_3$)?
Solution: Add a few drops of a dilute acid, like HCl. If the rock fizzes (produces a gas), and that gas turns limewater milky when bubbled through it, the rock is a carbonate like limestone. -
Why is carbon dioxide used in fire extinguishers?
Solution: Carbon dioxide is non-flammable and is denser than air. It sinks onto a fire, displacing the oxygen and suffocating the flames. -
Write the equation for the thermal decomposition of baking soda
($NaHCO_3$).
Solution: $2NaHCO_3(s) \rightarrow Na_2CO_3(s) + CO_2(g) + H_2O(g)$
12.30 Carbon Monoxide (CO)
Carbon monoxide (CO) is a colorless, odorless, and highly toxic gas. It is
formed during the incomplete combustion of carbon-containing fuels, which
occurs when there is a limited supply of oxygen.
$$ 2C(s) + O_2(g) \rightarrow 2CO(g) $$
It is a neutral oxide but is an important reducing agent.
It is the main reducing agent in the blast furnace, reducing iron ore to iron metal.
$Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(l) + 3CO_2(g)$
Its toxicity is due to its ability to bind irreversibly to hemoglobin in red blood cells, preventing them from carrying oxygen around the body.
Solved Examples:
-
Under what conditions is carbon monoxide formed?
Solution: It is formed during the incomplete combustion of fuels, meaning when there is not enough oxygen for complete combustion to carbon dioxide. -
Why is it dangerous to use a charcoal barbecue indoors?
Solution: A charcoal barbecue produces carbon monoxide gas. In an enclosed space without proper ventilation, this toxic gas can build up to lethal concentrations. -
What is the role of carbon monoxide in a blast furnace?
Solution: It acts as the primary reducing agent, removing oxygen from the iron ore ($Fe_2O_3$) to produce molten iron.