9.6 Carbon
🎯 Learning outcomes
Learners should be able to:
- Describe the crystalline and amorphous allotropes of carbon, including fullerenes, with their structure, general properties, and uses.
- State the properties of carbon monoxide: reducing action, reaction with metals, and nonmetals.
- List uses of carbon monoxide.
Occurrence
- Free state: diamond, graphite, coal, etc.
- Combined state: carbonate (CaCO₃, MgCO₃) organic compounds, etc.
Carbon makes up about 18% of the human body. It is the backbone of every living molecule — proteins, fats, sugars, and DNA all contain carbon.
Allotropes are different physical forms of the same element. They have the same chemical properties, different structures & physical properties.
Diamond
- The purest form of carbon is a crystalline allotrope.
- Structure: Each C is sp³ hybridised, bonded to 4 other C atoms → tetrahedral, bond angle 109°28′.
- The hardness, high melting point, and high density (3.5) of a diamond are due to the extension of these strong covalent bonds to form a giant three-dimensional covalent network, as shown in the figure.
- High refractive index (2.45) → sparkles due to total internal reflection.
- No free electrons → All the valence electrons of carbon are involved in bond formation; it does not conduct electricity.
- Chemically unreactive, but burns at ~900 °C to give CO₂.
Uses:
- In jewellery as a gem because it reflects light.
- For cutting glass and as a borer for rock drilling.
- For grinding and polishing of hard materials.
Graphite
- Crystalline allotrope of carbon.
- Structure: Each C is sp² hybridised, bonded to 3 other C atoms in hexagonal layers.
- Out of 4 valence electrons, three are involved in covalent bonds with other carbon atoms. 1 delocalized electron per C makes it a conductor of electricity.
- Adjacent hexagonal layers of carbon atoms, held together by weak Van der Waals’ force, these layers can slide over each other. This property of graphite makes it useful as a lubricant.
Uses:
- Lubricants for machines operating at high temperatures (where oil would burn away).
- Making electrodes, as it is a good conductor of electricity.
- Making pencil ‘lead.’
Fullerene
Fullerene is a fascinating family of carbon molecules in which atoms are arranged in hollow cage-like structures.
C₆₀ looks exactly like a football! It was discovered in 1985 and earned its discoverers the Nobel Prize in Chemistry in 1996.
- The most common form of fullerene is C₆₀
- It is composed of a fused system of 5-membered and 6-membered rings
- Each carbon atom is sp² hybridized and has delocalized electrons. Thus, fullerenes are conductors of electricity (semiconductors). These could be doped to make a superconductor.
Uses:
- In biomedical/research.
- Lubricant for micro machines.
- Making superconductors.
Graphene
- It is one of the crystalline allotropes of carbon, one atom thick. (It is, in fact, a single layer of graphite)
- It is a good conductor of electricity and is about 100 times stronger than steel.
- Used in solar panels, flexible screens, medical sensors, etc.
Scientists first isolated graphene in 2004 using ordinary sticky tape to peel layers off graphite. They won the Nobel Prize in Physics in 2010 for this discovery!
Charcoal
- Amorphous form of carbon and has a high surface area.
- Depending upon the source of origin, charcoal may be classified as plant, animal, or activated charcoal.
- Plant charcoal is obtained from wood, and animal charcoal from animal blood or bones.
- Activated charcoal is derived by treating charcoal with high-temperature steam, which dramatically increases its surface area.
Uses:
- As a fuel.
- Adsorbing gases and purifying and clarifying liquids.
- As a reductant
- As an ingredient of gunpowder
- For refining oil, sugar, etc.
Lamp Black
- It is obtained by burning natural and other carbon compounds in a limited supply of air.
- Uses: printer ink, shoe polish, and black paint.
Gas Carbon
- It is carbon deposited on the interior part of the retort during the manufacture of coal gas.
- It is a good conductor of electricity.
Coal
- Coal is formed over millions of years from the partial decomposition of plant matter buried under layers of rock, subjected to high pressure and temperature. It is not pure carbon — it also contains hydrogen, sulphur, and other elements. Coal exists in grades of increasing carbon content: peat, lignite, bituminous coal, and anthracite. It is used as fuel.
Coke
- It is obtained as a form of residue after the destructive distillation of coal.
- It is used as fuel as well as a reducing agent.
Carbon monoxide is a colourless, odourless, and tasteless gas — you cannot see, smell, or taste it, which makes it particularly dangerous. It is often called the “silent killer”.
Carbon monoxide combines with haemoglobin in the blood approximately 200–250 times more readily than oxygen does. This forms a stable compound called carboxyhaemoglobin, which cannot carry oxygen. The result is oxygen starvation, leading to suffocation and death.
Reducing Action
One of the most important properties of carbon monoxide is that it acts as a reducing agent — it removes oxygen from other compounds. This property is central to the extraction of metals from their ores.
- It is used to reduce metallic oxide to metal in metallurgical processes.
- Carbon monoxide can reduce iodine oxide to iodine
- Carbon monoxide reduces Tollen’s reagent to metallic silver. Ammoniacal silver nitrate solution is Tollen’s reagent. Its molecular form is [Ag(NH₃)₂]OH.
- CO reduces Fehling’s solution to produce a red precipitate of cuprous oxide. Fehling solution is basic copper sulphate in the presence of sodium potassium tartarate.
Reaction with Metals — Carbonyl Formation
Carbon monoxide, when passed through some metals like nickel, iron, and cobalt, forms carbonyl compounds.
E.g., when carbon monoxide is passed into finely divided nickel at about 80 °C, nickel tetra carbonyl is formed, which on further heating to 180 °C decomposes into metallic nickel.
This is used for the purification of nickel.
Action with Non-metals
Action with hydrogen: When a mixture of carbon monoxide gas and hydrogen gas is passed into a ZnO/Cr₂O₃ mixture as a catalyst at 300 °C and 200 atmospheres pressure, methyl alcohol is produced.
Action with Cl₂: Carbon monoxide reacts with chlorine in the presence of sunlight to produce poisonous phosgene gas.
Action with sulphur: When CO is heated with sulphur, carbonyl sulphide is formed.
Uses of Carbon Monoxide
- As a reducing agent in blast furnaces to extract metals from their ores.
- In the purification of metals like nickel, iron, etc.
- As a component of industrial fuel gases. E.g., Water gas (CO + H₂) and producer gas (CO + N₂).
- To prepare carbonyl compounds, which are used in organic synthesis.
- Unfortunately, misused in warfare to prepare phosgene gas.
Test your understanding
Take a short quiz on carbon and its allotropes.