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Carbon and Its Compounds
Class 10 Science | Chapter 4 | Complete Detailed Notes
š Overview and Occurrence of Carbon
- Abundance in Nature: Carbon constitutes 0.02% in the Earth's crust (as minerals like carbonates, hydrogen carbonates, coal and petroleum) and 0.03% in the atmosphere (as carbon dioxide).
- Everyday Importance: Found in all living organisms, food, clothing, medicines, paper, books and fuels.
- Core Focus: Chemical bonding, versatile nature, functional groups, chemical reactions and industrial importance of carbon compounds.
1. Bonding in Carbon — The Covalent Bond
Carbon compounds are organic substances made primarily of carbon atoms bonded with hydrogen, oxygen, nitrogen and other non-metals. These compounds exhibit distinct physical and chemical properties that set them apart from ionic compounds.
Properties of Carbon Compounds
- Poor Electrical Conductivity: Most carbon compounds do not conduct electricity in liquid or dissolved states due to the absence of free ions.
- Low Melting and Boiling Points: Carbon compounds generally possess low melting and boiling points compared to ionic compounds.
- Weak Intermolecular Forces: The intermolecular forces of attraction between molecules are relatively weak.
- Non-Ionic Bonding: Bonding in carbon compounds does not give rise to charged ions.
Electron Distribution and Tetravalency
- Atomic Structure: Carbon has atomic number 6. Its electron configuration is 2 (K shell) and 4 (L shell).
- Valence Electrons: Carbon has 4 valence electrons in its outermost shell and requires 4 more electrons to achieve a stable noble gas configuration (octet rule).
Reactivity and Noble Gas Configuration
To achieve a stable octet configuration similar to neon, carbon could hypothetically gain or lose 4 electrons, but both paths face major energy barriers:
- Gaining 4 electrons (C⁴⁻): Gaining 4 electrons to form a C⁴⁻ anion is extremely difficult because a nucleus with 6 protons cannot easily hold 10 electrons.
- Losing 4 electrons (C⁴⁺): Losing 4 electrons to form a C⁴⁺ cation requires a massive amount of ionisation energy to remove 4 electrons, leaving a tiny cation with 6 protons holding just 2 electrons.
The Covalent Solution: Carbon overcomes this limitation by sharing its 4 valence electrons with other carbon atoms or atoms of other elements, resulting in covalent bonding.
What is a Covalent Bond?
A covalent bond is a chemical bond formed by the mutual sharing of one or more electron pairs between two atoms. Molecules formed by covalent bonding have strong covalent bonds within the molecule (intramolecular forces) but relatively weak forces between different molecules (intermolecular forces).
General Properties of Covalent Compounds
- Melting and Boiling Points: Low melting and boiling points due to weak intermolecular attraction.
- Solubility: Insoluble or poorly soluble in water and polar solvents, but soluble in non-polar organic solvents.
- Electrical Conductivity: Non-conductors of electricity in solid, liquid or dissolved states because no ions or free electrons are present.
Types and Examples of Covalent Molecules
Single covalent bondSharing 1 pair of electrons.
Hydrogen: H—H
Hydrogen chloride: H—Cl
Hydrogen: H—H
Hydrogen chloride: H—Cl
Double covalent bondSharing 2 pairs of electrons.
Oxygen: O=O
Carbon dioxide: O=C=O
Oxygen: O=O
Carbon dioxide: O=C=O
Triple covalent bondSharing 3 pairs of electrons.
Nitrogen: N≡N
Nitrogen: N≡N
Methane (CH₄)Carbon shares its 4 valence electrons with 4 individual hydrogen atoms.
Water (H₂O)Oxygen shares electron pairs with two hydrogen atoms, forming H—O—H.
1.1 Allotropes of Carbon
Allotropy is the property by which an element can exist in multiple physical forms having identical chemical properties but different physical arrangements and properties.
| Allotrope | Structure and Bonding | Physical Properties | Electrical Conductivity |
|---|---|---|---|
| Diamond | Each carbon atom is bonded to 4 other carbon atoms in a rigid 3D tetrahedral structure. | Hardest natural substance known; high density and very high melting point. | Non-conductor (no free electrons). |
| Graphite | Each carbon atom is bonded to 3 other carbon atoms in hexagonal planar layers held by weak Van der Waals forces. | Soft, slippery, smooth lubricant; lower density than diamond. | Good conductor (1 free electron per carbon atom). |
| Fullerenes (C-60) | Carbon atoms arranged in a hollow geodesic dome/football shape (20 hexagons and 12 pentagons). | Dark solid, soft as graphite, named “Bucky Ball” after Buckminster Fuller. | Semiconductor / specialised properties. |
Note: Carbon also exists in amorphous forms such as coke, charcoal, lampblack and gas carbon.
2. Versatile Nature of Carbon
More than 3 million carbon compounds are known today. Carbon forms such an immense variety of compounds due to three unique key characteristics:
1. CatenationThe unique self-linking ability of carbon atoms to join with one another via covalent bonds to form long straight chains, branched chains or closed rings.
2. TetravalencyWith 4 valence electrons, carbon can form strong covalent bonds with 4 other monovalent atoms or polyvalent non-metals (H, O, N, S and halogens).
3. Small Atomic SizeCarbon's small atomic size allows its nucleus to hold shared pairs of electrons strongly, forming exceptionally stable and durable covalent bonds.
2.1 Saturated and Unsaturated Carbon Compounds
Hydrocarbons are organic compounds containing only carbon and hydrogen atoms.
(i) Saturated Hydrocarbons (Alkanes)
- Definition: Hydrocarbons where carbon atoms are linked exclusively by single covalent bonds (—C—C—).
- General Formula: CāH₂ā₊₂
- Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀).
(ii) Unsaturated Hydrocarbons (Alkenes and Alkynes)
- Definition: Hydrocarbons containing at least one carbon-carbon double bond (—C=C—) or triple bond (—C≡C—).
- Alkenes (Double Bond): CāH₂ā — examples: Ethene (C₂H₄), Propene (C₃H₆).
- Alkynes (Triple Bond): CāH₂ā₋₂ — examples: Ethyne (C₂H₂), Propyne (C₃H₄).
| Type | Bond between carbon atoms | General formula | Example |
|---|---|---|---|
| Alkane | Single (C—C) | CāH₂ā₊₂ | Ethane, C₂H₆ |
| Alkene | Double (C=C) | CāH₂ā | Ethene, C₂H₄ |
| Alkyne | Triple (C≡C) | CāH₂ā₋₂ | Ethyne, C₂H₂ |
2.2 Chains, Branches, Rings and Structural Isomerism
- Straight Chains: Carbon atoms joined in a single continuous linear line (e.g., Propane C₃H₈, Pentane C₅H₁₂).
- Branched Chains: Carbon chains containing side branches attached to the main backbone.
- Structural Isomers: Compounds having the same molecular formula but different structural formulas (e.g., n-pentane, isopentane and neopentane all share formula C₅H₁₂).
- Cyclic Hydrocarbons: Carbon atoms arranged in a closed ring shape (e.g., Cyclohexane C₆H₁₂, Benzene C₆H₆).
2.3 Heteroatoms and Functional Groups
In hydrocarbon chains, one or more hydrogen atoms can be replaced by other elements known as heteroatoms (such as halogens, oxygen, nitrogen or sulfur).
A functional group is an atom or group of atoms attached to a carbon chain that determines the chemical properties of the organic compound, regardless of the length or nature of the carbon chain.
| Heteroatom | Functional Group | Formula / Group Symbol | Example Compound |
|---|---|---|---|
| Halogen (Cl, Br, I) | Halo (Chloro/Bromo) | —Cl, —Br, —I | Chloromethane (CH₃Cl) |
| Oxygen | Alcohol | —OH | Ethanol (CH₃CH₂OH) |
| Oxygen | Aldehyde | —CHO | Methanal / Formaldehyde (HCHO) |
| Oxygen | Ketone | >C=O | Propanone / Acetone (CH₃COCH₃) |
| Oxygen | Carboxylic Acid | —COOH | Ethanoic Acid (CH₃COOH) |
2.4 Homologous Series
A homologous series is a family of organic compounds having the same functional group, similar chemical properties and the same general formula.
Characteristics of a Homologous Series
- General Formula: All members share a common general molecular formula (e.g., CāH₂ā₊₂ for alkanes, CāH₂ā₊₁OH for alcohols).
- Formula Difference: Any two adjacent members differ by a —CH₂— (methylene) unit.
- Mass Difference: Two adjacent members differ in molecular mass by 14 atomic mass units (14 u).
- Chemical Properties: Members exhibit similar chemical properties due to the presence of the same functional group.
- Gradual Physical Gradation: Physical properties (melting point, boiling point and density) show a gradual gradation as molecular mass increases down the series.
2.5 Nomenclature of Carbon Compounds (IUPAC Rules)
- Step 1 — Chain Length: Identify the number of carbon atoms in the longest chain: 1 Meth, 2 Eth, 3 Prop, 4 But, 5 Pent, 6 Hex.
- Step 2 — Saturation: Determine saturated (“-ane”) or unsaturated (“-ene” for double bond, “-yne” for triple bond) status.
- Step 3 — Functional Group: Indicate functional groups via prefix or suffix. If the suffix begins with a vowel (a, e, i, o, u), drop the final “e” from the carbon chain name (e.g., Propane + -one = Propanone).
3. Chemical Properties of Carbon Compounds
3.1 Combustion
Carbon and its compounds react with oxygen to burn in air, releasing large amounts of heat and light energy along with carbon dioxide and water vapour.
C + O₂ → CO₂ + Heat + Light
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat + Light
CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O + Heat + Light
- Flame Types (Saturated): Saturated hydrocarbons burn with a clear, blue, non-sooty flame due to complete combustion.
- Flame Types (Unsaturated): Unsaturated hydrocarbons burn with a luminous, yellow, sooty flame due to high carbon content and incomplete oxidation.
3.2 Oxidation
Carbon compounds can be oxidised by adding oxygen or removing hydrogen using specialised oxidising agents.
š Oxidation of Alcohol to Carboxylic Acid
Chemical equation:
CH₃CH₂OH ──[Alkaline KMnO₄ + heat OR Acidified K₂Cr₂O₇ + heat]──> CH₃COOH
Oxidising agents: Substances that add oxygen or remove hydrogen. Examples: Alkaline Potassium Permanganate (KMnO₄) or Acidified Potassium Dichromate (K₂Cr₂O₇).3.3 Addition Reaction
Unsaturated hydrocarbons add atoms across their double or triple bonds in the presence of catalysts such as Nickel (Ni) or Palladium (Pd) to yield saturated hydrocarbons.
Industrial Application: Industrial hydrogenation of vegetable oils (unsaturated fats) into vegetable ghee (saturated fats) using a Nickel catalyst at elevated temperature.
3.4 Substitution Reaction
Saturated hydrocarbons are generally unreactive, but in the presence of sunlight, chlorine replaces hydrogen atoms one by one in a rapid halogenation process.
CH₄ + Cl₂ ──[Sunlight]──> CH₃Cl (Chloromethane) + HCl
4. Some Important Carbon Compounds — Ethanol and Ethanoic Acid
4.1 Ethanol (Ethyl Alcohol — CH₃CH₂OH)
Physical Properties
- Appearance: Colourless liquid with a pleasant alcoholic odour and burning taste.
- Boiling Point: 351 K (78°C); melting point 156 K.
- Solubility: Completely miscible with water in all proportions due to hydrogen bonding.
- Litmus Test: Neutral to litmus paper (does not contain free OH⁻ ions).
Chemical Reactions of Ethanol
1. Reaction with Sodium
Ethanol reacts with sodium metal with effervescence to release hydrogen gas (which burns with a pop sound).
2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa (Sodium Ethoxide) + H₂↑
2. Dehydration Reaction
Heating ethanol at 443 K with excess concentrated sulfuric acid (Conc. H₂SO₄) removes water to form ethene.
CH₃CH₂OH ──[Hot Conc. H₂SO₄ at 443 K]──> CH₂=CH₂ (Ethene) + H₂O
4.2 Ethanoic Acid (Acetic Acid — CH₃COOH)
Physical Properties
- Appearance and Smell: Colourless liquid with a sour taste and distinct vinegar smell.
- Vinegar: 5% to 8% solution of ethanoic acid in water is known as vinegar.
- Glacial Acetic Acid: Boiling point of 391 K. Pure ethanoic acid freezes at 290 K (17°C) to form ice-like crystals, earning the name Glacial Acetic Acid.
Chemical Reactions of Ethanoic Acid
1. Esterification
Ethanoic acid reacts with ethanol in the presence of an acid catalyst to form ethyl ethanoate (a sweet-smelling ester).
CH₃COOH + CH₃CH₂OH ──[Acid]──> CH₃COOCH₂CH₃ (Ester) + H₂O
2. Saponification
Hydrolysis of an ester with a base (NaOH) yields alcohol and sodium carboxylate (soap).
CH₃COOCH₂CH₃ + NaOH → CH₃COONa + CH₃CH₂OH
3. Reaction with Base
CH₃COOH + NaOH → CH₃COONa (Sodium Ethanoate) + H₂O
4. Reaction with Carbonates and Bicarbonates
Ethanoic acid reacts with carbonates and bicarbonates, releasing CO₂ gas (which turns limewater milky).
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑
5. Soaps and Detergents
Soaps and detergents are cleansing agents used to dissolve dirt, grease and oil from fabrics and surfaces.
| Property / Feature | Soaps | Synthetic Detergents |
|---|---|---|
| Chemical Nature | Sodium or Potassium salts of long-chain fatty/carboxylic acids (e.g., C₁₇H₃₅COONa). | Sodium or Ammonium salts of long-chain alkyl sulphonic acids. |
| Source | Prepared from natural fats and vegetable oils. | Prepared synthetically from petroleum hydrocarbons. |
| Action in Hard Water | Ineffective in hard water; forms sticky insoluble scum with Ca²⁺ and Mg²⁺ ions. | Effective in both soft and hard water; does not form scum. |
| Biodegradability | Fully biodegradable by microorganisms. | Many synthetic detergents are non-biodegradable. |
Structure of a Soap Molecule
- 1. Hydrophilic Ionic Head: The ionic head (—COO⁻Na⁺) is polar, water-attracting and soluble in water.
- 2. Hydrophobic Carbon Tail: The long hydrocarbon tail (C₁₇H₃₅—) is non-polar, water-repelling and soluble in oil/dirt.
Cleansing Action of Soap and Micelle Formation
When soap is added to water containing oily dirt:
- Micelle Assembly: The hydrophobic tails align inward, attaching to the oil/dirt droplet, while the hydrophilic ionic heads point outward into the surrounding water.
- Micelle Structure: This forms a spherical cluster called a micelle.
- Dirt Removal: The soap micelle entraps the dirt inside its core, lifting it off the fabric into water as an emulsion that is easily rinsed away.
Scum Formation in Hard Water
- Cause of Scum: Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) salts.
- Reaction: Soap molecules react with Ca²⁺ and Mg²⁺ to form an insoluble white precipitate called scum, wasting soap and impeding cleaning.
- Detergent Advantage: Synthetic detergents do not form insoluble precipitates with calcium or magnesium ions, making them superior cleansing agents in hard water.
Remember: The soap molecule has a water-attracting hydrophilic head and an oil-attracting hydrophobic tail. This arrangement helps form micelles and remove oily dirt.
⚡ Quick Revision — Key Points
Carbon atomic number6; electron configuration 2, 4.
Valency of carbon4 (tetravalency).
AlkanesCāH₂ā₊₂; only single bonds.
AlkenesCāH₂ā; at least one double bond.
AlkynesCāH₂ā₋₂; at least one triple bond.
Homologous seriesAdjacent members differ by —CH₂— and 14 u.
Oxidising agentsAlkaline KMnO₄ or acidified K₂Cr₂O₇.
Vinegar5%–8% ethanoic acid in water.
Study tip: Practise writing the chemical equations, learn the functional-group table, and compare soaps and detergents. Revise the definitions of catenation, tetravalency, covalent bond, homologous series and micelle.
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