Carbon and Its Compounds

Carbon is a non-metal located in Group 4 of the periodic table. It naturally occurs as diamond and graphite—two physically distinct forms of the same element known as allotropes. Other elements that exhibit allotropy include tin, sulphur, and phosphorus. Carbon is also found in impure forms like coal and in combined states such as petroleum, wood, and natural gas.

Atomic Structure of Carbon

Carbon has an atomic number of 6. Its nucleus contains 6 protons and 6 neutrons, surrounded by 6 electrons. These electrons are distributed with 2 in the first energy level (K-shell) and 4 in the second energy level (L-shell), giving it the configuration:

1s² 2s² 2p²

Carbon has four valence electrons, allowing it to form covalent bonds. It can form chains by bonding with itself—a property known as catenation.

Allotropes of Carbon

Definition

Allotropy refers to the existence of an element in different forms in the same physical state. The different forms are called allotropes. Carbon exhibits allotropy in forms like diamond, graphite (crystalline), and charcoal (amorphous).

Diamond

Structure and Bonding

Each carbon atom in diamond is covalently bonded to four other carbon atoms in a tetrahedral arrangement, forming a giant 3D structure. The strong, continuous covalent bonds give diamond its exceptional hardness and high melting point.

Physical Properties of Diamond

• Hard, colourless, and transparent with a high refractive index
• Forms octahedral crystals
• Density: 3.5 g/cm³; Melting point: 3600°C
• Does not conduct electricity (no free electrons)
• Insoluble in all solvents

Uses of Diamond

• Used in jewellery due to its sparkle and luster
• Employed in cutting glass, drilling rocks, boring holes, engine bearings, and abrasives

Graphite

Structure and Bonding

In graphite, each carbon atom bonds with three others to form flat hexagonal layers. These layers are held together by weak van der Waals forces, allowing them to slide over one another, making graphite soft and slippery. One valence electron per atom remains unbonded, allowing electrical conductivity.

Physical Properties of Graphite

• Forms soft, black, opaque hexagonal crystals
• Density: 2.3 g/cm³; Melting point: ~3500°C
• Good conductor of heat and electricity
• Insoluble in any solvent

Uses of Graphite

• Used as a lubricant due to its layered structure
• Serves as electrodes in electrolysis and brushes in electric motors
• Mixed with clay to make pencil leads
• Used to manufacture crucibles due to its high melting point
• Employed in nuclear reactors

Comparison Between Diamond and Graphite

Chemical Properties of Carbon

1. Combustion

In limited air:

2C(s) + O₂(g) → 2CO(g)

In excess air:

C(s) + O₂(g) → CO₂(g)

When charcoal burns, it releases heat—an exothermic reaction used for cooking.

2. Combination Reactions

Carbon reacts with certain elements at high temperatures:

C + 2S → CS₂

2C + Ca → CaC₂

3. As a Reducing Agent

Carbon reduces metal oxides, especially of less reactive metals:

Fe₂O₃ + 3C → 2Fe + 3CO

4. Reaction with Strong Oxidizing Agents

Carbon reacts with concentrated trioxonitrate(V) acid:

4HNO₃ + C → CO₂ + 4NO₂ + 2H₂O

Note:

Carbon is widely found in nature both in elemental forms (diamond, graphite) and in compounds (coal, petroleum, natural gas).

Coal and Carbon Compounds

Introduction to Coal

Coal is a naturally occurring black or brownish-black sedimentary rock that forms from the remains of plants over millions of years under heat and pressure. It is mainly composed of carbon, along with hydrogen, sulfur, oxygen, and nitrogen.

Types of Coal

Coal is classified based on its carbon content and energy content:

• Peat: Lowest grade of coal; partially decayed plant matter with low carbon content.
• Lignite: Soft brown coal with low carbon and high moisture content.
• Bituminous Coal: Commonly used; contains more carbon and burns with a smoky flame.
• Anthracite: Highest grade of coal; hard, glossy, and burns cleanly with high energy output.

Destructive Distillation of Coal

When coal is heated in the absence of air, it breaks down into several useful products. This process is called destructive distillation. The main products are:

• Coke: Solid residue rich in carbon, used as a fuel and in steel manufacturing.
• Coal Tar: A thick, black liquid containing aromatic hydrocarbons used to make dyes, explosives, and drugs.
• Ammoniacal Liquor: A solution of ammonia in water, used in fertilizers.
• Coal Gas: A mixture of gases like hydrogen, methane, and carbon monoxide, used as a fuel.

Carbon Compounds

Carbon compounds are mainly organic compounds made of carbon atoms bonded with hydrogen, oxygen, nitrogen, and other elements. They form long chains and rings due to carbon’s tetravalency and catenation property.

Common Carbon Compounds and Their Uses

• Carbon Tetrachloride (CCl₄): Used as a cleaning agent and in fire extinguishers.
• Chloroform (CHCl₃): Once used as an anesthetic; now used in laboratories.
• Methane (CH₄): A natural gas used as fuel and starting material in organic synthesis.
• Ethene (C₂H₄): Used in the manufacture of plastics like polythene.

Combustion of Carbon Compounds

Carbon compounds burn in air to form carbon dioxide and water, releasing energy.

Example:

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

Environmental Effects

• Burning coal and carbon compounds releases CO₂, a greenhouse gas.
• Incomplete combustion produces carbon monoxide (CO), which is poisonous.
• Coal combustion may release sulfur dioxide (SO₂), leading to acid rain.

Synthetic Gas

Synthetic gas, like water gas, is a mixture of carbon monoxide (CO) and hydrogen (H₂). It is produced by passing natural gas (methane) over steam or air in the presence of nickel as a catalyst at about 900°C.

Reactions Involved:

CH₄(g) + H₂O(g)   Ni, 900°C → CO(g) + 3H₂(g)
2CH₄(g) + O₂(g)   Ni, 900°C → 2CO(g) + 4H₂(g)

Reaction with Chlorine:

CO(g) + Cl₂(g) → COCl₂(g)

Separation of Constituents of Producer Gas and Water Gas

Producer gas is a mixture of carbon monoxide (CO) and nitrogen (N₂), while water gas is a mixture of CO and hydrogen (H₂).

Types of Fuels

• Solid fuels: Wood, coal, coke
• Liquid fuels: Kerosene, alcohol (methylated spirit)
• Gaseous fuels: Producer gas, water gas, coal gas, natural gas, paraffin gas, synthetic gas
• Human fuels: Food substances like fats, oils, proteins, starch, and sugars
• Atomic fuel: Uranium

Metallic Trioxocarbonates: Occurrence, Preparation, and Uses

Preparation

Trioxocarbonates (carbonates) of most metals are insoluble in water, except those of sodium (Na), potassium (K), and ammonium (NH₄), which are soluble. They can be prepared using the double decomposition method:

Na₂CO₃(aq) + CuSO₄(aq) → CuCO₃(s) + Na₂SO₄(aq)
Na₂CO₃(aq) + ZnCl₂(aq) → ZnCO₃(s) + 2NaCl(aq)

Alternative Method:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Properties of Trioxocarbonates

1. All trioxocarbonates are insoluble in water, except those of Na, K, and NH₄.
2. They form alkaline solutions because they are salts of strong bases and weak acids, and they hydrolyze in water.
   K₂CO₃(s) + 2H₂O(l) → 2KOH(aq) + H₂CO₃(aq)
3. All trioxocarbonates (except Na and K salts) decompose on heating to release CO₂ gas.
   ZnCO₃(s) → ZnO(s) + CO₂(g)
   (NH₄)₂CO₃(s) → 2NH₃(g) + H₂O(g) + CO₂(g)

Note: Na₂CO₃ and K₂CO₃ are stable to heat and do not decompose.

Reaction with Acids:

ZnCO₃(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l) + CO₂(g)
CO₃²⁻(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g)

Trioxocarbonate(IV) Acid (Carbonic Acid)

Carbonic acid, H₂CO₃, is a very weak, dibasic acid formed when CO₂ dissolves in water. It is unstable and cannot be isolated.

CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq) ⇌ 2H⁺(aq) + CO₃²⁻(aq)

Types of Salts Formed:

• Normal salts (e.g., Na₂CO₃)
• Acid salts (e.g., NaHCO₃)

Uses of Trioxocarbonate(IV) Salts:

• Sodium carbonate (Na₂CO₃) is used in water softening, soap and glass making, and as a base in titrations.
• Calcium carbonate (CaCO₃) is used in the production of cement, glass, and lime, and in agriculture to reduce soil acidity (liming).

Chemical Tests for Carbonates (CO₃²⁻)

Dry Test (On a Solid Sample)

1. Add a few drops of dilute hydrochloric acid to a solid carbonate in a test tube.
2. Effervescence occurs and a colorless, odorless gas is released.
3. The gas turns moist blue litmus paper slightly red and turns limewater milky — confirming the gas is CO₂.

Wet Test (On a Solution)

1. Add barium chloride solution to a solution of the salt.
2. A white precipitate of barium trioxocarbonate (BaCO₃) forms:
   CO₃²⁻(aq) + BaCl₂(aq) → BaCO₃(s) + 2Cl⁻(aq)
3. The precipitate dissolves in dilute hydrochloric acid to form a colorless solution:
   BaCO₃(s) + 2HCl(aq) → BaCl₂(aq) + H₂CO₃(aq)