Class 10 Science Notes Chapter 4 (Carbon and its compounds) – Science Book

Okay, let's prepare detailed notes for Chapter 4: Carbon and its Compounds, focusing on aspects relevant for government exam preparation based on the NCERT Class 10 syllabus.

Chapter 4: Carbon and its Compounds - Detailed Notes

1. Introduction to Carbon

• Symbol: C
• Atomic Number: 6
• Electronic Configuration: 2, 4
• Valence Electrons: 4
• Occurrence:
  • Earth's crust: ~0.02% (as minerals like carbonates, hydrogen carbonates, coal, petroleum).
  • Atmosphere: ~0.03% (as Carbon Dioxide).
  • Essential component of all living organisms (organic compounds).

2. Bonding in Carbon - The Covalent Bond

• Problem: Carbon has 4 valence electrons.
  • It cannot gain 4 electrons to form C⁴⁻ anion (nucleus with 6 protons cannot hold 10 electrons).
  • It cannot lose 4 electrons to form C⁴⁺ cation (requires a large amount of energy).
• Solution: Carbon overcomes this problem by sharing its valence electrons with other atoms (of carbon or other elements).
• Covalent Bond: The bond formed by the mutual sharing of electron pairs between two atoms. Shared electrons belong to the outer shells of both atoms, helping both attain a stable noble gas configuration.
• Characteristics of Covalent Compounds:
  • Generally have low melting and boiling points (weak intermolecular forces).
  • Generally poor conductors of electricity (no free ions or electrons).
• Formation of Simple Covalent Molecules (Electron Dot Structures are important):
  • Hydrogen (H₂): Each H shares 1 electron. (H : H)
  • Oxygen (O₂): Each O shares 2 electrons (Double Bond). (O :: O) or (O=O)
  • Nitrogen (N₂): Each N shares 3 electrons (Triple Bond). (N ::: N) or (N≡N)
  • Methane (CH₄): Carbon shares its 4 valence electrons with 4 Hydrogen atoms. (Tetrahedral shape)
  • Water (H₂O): Oxygen shares 1 electron with each of the two Hydrogen atoms.
  • Ammonia (NH₃): Nitrogen shares 1 electron with each of the three Hydrogen atoms.

3. Versatile Nature of Carbon

Carbon forms a vast number of compounds due to two primary properties:

• (i) Catenation:
  • The unique ability of carbon to form strong covalent bonds with other carbon atoms, giving rise to large molecules.
  • These can be long chains, branched chains, or rings of carbon atoms.
  • Carbon-Carbon bonds are very strong and stable.
  • Silicon (Si) shows some catenation but forms compounds that are highly reactive, and the Si-Si bond is weaker.
• (ii) Tetravalency:
  • Carbon has a valency of 4.
  • It is capable of bonding with four other atoms (monovalent like H, Cl) or atoms of other elements (like O, N, S).
  • This allows for a wide variety of compounds with different elements attached to carbon.
  • The small size of the carbon atom enables its nucleus to hold onto the shared pairs of electrons strongly, contributing to the stability of its compounds.

4. Allotropes of Carbon

• Allotropes: Different structural forms of the same element in the same physical state. They have different physical properties but similar chemical properties.
• Key Allotropes of Carbon:
  • (a) Diamond:
    • Structure: Each carbon atom is bonded to four other carbon atoms, forming a rigid three-dimensional tetrahedral network.
    • Properties: Hardest natural substance known, high melting point, electrical insulator (no free electrons), transparent.
    • Uses: Cutting tools, jewellery.
  • (b) Graphite:
    • Structure: Each carbon atom is bonded to three other carbon atoms in the same plane, forming hexagonal layers. Layers are held by weak van der Waals forces. One valence electron per carbon atom is relatively free.
    • Properties: Smooth, slippery, good conductor of electricity (due to free electrons), opaque, less dense than diamond.
    • Uses: Lubricant, pencil 'lead', electrodes.
  • (c) Buckminsterfullerene (C₆₀):
    • Structure: Spherical molecule consisting of 60 carbon atoms arranged in interlocking hexagonal (20) and pentagonal (12) rings, resembling a soccer ball.
    • Properties: Dark solid at room temperature.
    • Discovery: First identified C₆₀ molecule. Other fullerenes exist.

5. Organic Compounds

• Initially defined as compounds obtained from living sources.
• Modern Definition: Compounds of carbon, except for oxides of carbon (CO, CO₂), carbonates, hydrogen carbonates, and carbides.
• Hydrocarbons: The simplest organic compounds containing only Carbon (C) and Hydrogen (H).

6. Classification of Hydrocarbons

• (A) Saturated Hydrocarbons (Alkanes):
  • Contain only single covalent bonds between carbon atoms.
  • General Formula: C_nH_2n+2 (where n = number of carbon atoms)
  • Examples:
    • Methane (CH₄)
    • Ethane (C₂H₆)
    • Propane (C₃H₈)
    • Butane (C₄H₁₀)
  • Generally less reactive. Undergo substitution reactions.
• (B) Unsaturated Hydrocarbons:
  • Contain at least one double or triple covalent bond between carbon atoms.
  • More reactive than alkanes. Undergo addition reactions.
  • (i) Alkenes: Contain at least one Carbon-Carbon double bond (C=C).
    • General Formula: C_nH_2n
    • Examples:
      • Ethene (C₂H₄)
      • Propene (C₃H₆)
      • Butene (C₄H₈)
  • (ii) Alkynes: Contain at least one Carbon-Carbon triple bond (C≡C).
    • General Formula: C_nH_2n-2
    • Examples:
      • Ethyne (Acetylene) (C₂H₂)
      • Propyne (C₃H₄)
      • Butyne (C₄H₈)

7. Structural Variations in Carbon Compounds

• Chains: Carbon atoms linked in a straight line (e.g., Propane, Butane).
• Branches: Carbon chain with side chains attached (e.g., Isobutane).
• Rings (Cyclic Hydrocarbons): Carbon atoms arranged in a ring.
  • Saturated Cyclic: Cyclohexane (C₆H₁₂) - single bonds only.
  • Unsaturated Cyclic: Benzene (C₆H₆) - alternating single and double bonds (often shown as a hexagon with a circle inside representing delocalized electrons).
• Structural Isomers:
  • Compounds having the same molecular formula but different structural arrangements of atoms.
  • They have different physical and chemical properties.
  • Example: Butane (C₄H₁₀) exists as:
    • n-Butane (straight chain): CH₃-CH₂-CH₂-CH₃
    • Isobutane (branched chain): CH₃-CH(CH₃)-CH₃ (IUPAC: 2-methylpropane)

8. Functional Groups

• An atom or group of atoms present in a molecule which largely determines its chemical properties.
• They replace one or more hydrogen atoms in a hydrocarbon chain.

9. Homologous Series

• A series of organic compounds having the same functional group and similar chemical properties, in which successive members differ by a -CH₂ group.
• Characteristics:
  • All members can be represented by the same general formula (e.g., Alkanes: CnH₂n+₂, Alcohols: CnH₂n+₁OH).
  • Successive members differ by a -CH₂ group (or a molecular mass of 14 u).
  • They have the same functional group.
  • They show similar chemical properties.
  • They show a gradual change (gradation) in physical properties (like melting point, boiling point, density) as the molecular mass increases.
• Examples: Series of Alkanes, Alkenes, Alkynes, Alcohols, Carboxylic Acids, etc.

10. Nomenclature of Carbon Compounds (IUPAC System - Basics)

• Steps:
  1. Identify the longest continuous carbon chain (parent chain). Determine the word root based on the number of C atoms (Meth-1, Eth-2, Prop-3, But-4, Pent-5, Hex-6...).
  2. Identify the functional group. Determine the appropriate suffix or prefix. If a suffix starting with a vowel (a, e, i, o, u) is added, the terminal 'e' of the parent alkane name is dropped (e.g., Propane + -ol -> Propanol).
  3. Number the carbon atoms in the parent chain starting from the end that gives the lowest number to the carbon atom bearing the functional group (or substituent/branch).
  4. Indicate the position of the functional group, substituents, or multiple bonds using the number of the carbon atom they are attached to.
  5. Name substituents (like -CH₃ methyl, -C₂H₅ ethyl) with their position numbers as prefixes.
• Examples:
  • CH₃CH₂CH₂Cl: 1-Chloropropane
  • CH₃CH(OH)CH₃: Propan-2-ol
  • CH₃CH₂CHO: Propanal
  • CH₃COCH₃: Propanone (or Propan-2-one)
  • CH₃CH₂COOH: Propanoic acid

11. Chemical Properties of Carbon Compounds

• (a) Combustion:
  • Burning carbon compounds in air (oxygen) produces Carbon Dioxide (CO₂), Water (H₂O), Heat, and Light.
  • Example: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat + Light
  • Example: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Heat + Light
  • Complete Combustion: Occurs with sufficient oxygen supply. Produces a blue, non-sooty flame (e.g., LPG stove). Saturated hydrocarbons generally give clean flames.
  • Incomplete Combustion: Occurs with limited oxygen supply. Produces Carbon Monoxide (CO), Carbon (soot), Water, Heat, and Light. Produces a yellow, sooty flame. Unsaturated hydrocarbons often burn with a sooty flame.
• (b) Oxidation:
  • Controlled addition of oxygen. Oxidizing agents add oxygen to other substances.
  • Common Oxidizing Agents: Alkaline Potassium Permanganate (KMnO₄), Acidified Potassium Dichromate (K₂Cr₂O₇).
  • Example: Ethanol can be oxidized to Ethanoic Acid.
    CH₃CH₂OH + [O] (from Alk. KMnO₄ or Acid. K₂Cr₂O₇) → CH₃COOH + H₂O
• (c) Addition Reaction:
  • Characteristic reaction of unsaturated hydrocarbons (alkenes and alkynes).
  • An atom or group adds across the double or triple bond, converting it into a single bond (making the compound saturated).
  • Hydrogenation: Addition of hydrogen (H₂) in the presence of a catalyst like Nickel (Ni), Palladium (Pd), or Platinum (Pt).
    • Example: Ethene + H₂ --(Ni catalyst)--> Ethane
      CH₂=CH₂ + H₂ --(Ni)--> CH₃-CH₃
    • Application: Hydrogenation of vegetable oils (unsaturated fats, liquid) to form vegetable ghee/vanaspati (saturated fats, solid).
• (d) Substitution Reaction:
  • Characteristic reaction of saturated hydrocarbons (alkanes).
  • One or more hydrogen atoms are replaced by another atom or group (like halogens).
  • Requires energy like sunlight (UV light) or heat.
  • Example: Methane reacts with Chlorine in the presence of sunlight.
    CH₄ + Cl₂ --(Sunlight)--> CH₃Cl (Chloromethane) + HCl
    (Reaction can continue to replace more H atoms: CH₂Cl₂, CHCl₃, CCl₄)

12. Important Carbon Compounds: Ethanol and Ethanoic Acid

• (A) Ethanol (C₂H₅OH) - Ethyl Alcohol
  • Properties: Colourless liquid, pleasant smell, burning taste, soluble in water, good solvent, volatile, low melting point (156 K), boiling point (351 K). Active ingredient in alcoholic drinks. Used in medicines (tincture of iodine), cough syrups, tonics.
  • Reactions:
    • Reaction with Sodium (Na): Forms Sodium Ethoxide and Hydrogen gas.
      2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa (Sodium ethoxide) + H₂ (gas)
      (Test for alcohols)
    • Dehydration: Reaction to give unsaturated hydrocarbon (Ethene) upon heating with excess concentrated Sulphuric Acid (H₂SO₄) at 443 K (170 °C). Conc. H₂SO₄ acts as a dehydrating agent (removes water).
      CH₃CH₂OH --(Conc. H₂SO₄, 443 K)--> CH₂=CH₂ (Ethene) + H₂O
  • Denatured Alcohol: Ethanol made unfit for drinking by adding poisonous substances like methanol, pyridine, copper sulphate (gives colour). Used for industrial purposes.
• (B) Ethanoic Acid (CH₃COOH) - Acetic Acid
  • Properties: Colourless liquid, sour taste, smell of vinegar. 5-8% solution in water is called Vinegar. Miscible with water. Melting point 290 K (17 °C) - often freezes in cold climates ("glacial acetic acid"). Weak acid (compared to mineral acids like HCl).
  • Reactions:
    • Esterification: Reaction with an alcohol in the presence of an acid catalyst (conc. H₂SO₄) to form an Ester (sweet-smelling compounds).
      CH₃COOH (Ethanoic acid) + CH₃CH₂OH (Ethanol) --(Acid catalyst)--> CH₃COOCH₂CH₃ (Ethyl ethanoate - Ester) + H₂O
      (Esters are used in perfumes and flavouring agents)
      Saponification: Reaction of an ester with an alkali (like NaOH) to get back the alcohol and the sodium salt of the carboxylic acid (soap). CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa
    • Reaction with a Base (Alkali): Forms salt and water (Neutralization).
      CH₃COOH + NaOH → CH₃COONa (Sodium ethanoate/acetate) + H₂O
    • Reaction with Carbonates and Hydrogen Carbonates: Forms salt, water, and Carbon Dioxide (CO₂).
      2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
      CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
      (Test for carboxylic acids - effervescence due to CO₂)

13. Soaps and Detergents

• Soaps: Sodium (Na⁺) or Potassium (K⁺) salts of long-chain carboxylic acids (fatty acids).
  • Example: Sodium Stearate (C₁₇H₃₅COO⁻Na⁺)
  • Structure: Has two parts:
    • (i) Long Hydrocarbon Chain: Non-polar, hydrophobic (water-repelling), soluble in oil/grease.
    • (ii) Ionic Part (-COO⁻Na⁺): Polar, hydrophilic (water-attracting), soluble in water.
• Detergents: Generally Ammonium or Sulphonate salts of long-chain carboxylic acids, or alkyl benzene sulphonates.
  • Structure: Also have a hydrophobic hydrocarbon tail and a hydrophilic head (e.g., -SO₃⁻Na⁺ or -N(CH₃)₃⁺Cl⁻).
• Cleansing Action of Soap (Micelle Formation):
  1. When soap is dissolved in water, the molecules arrange themselves uniquely. The hydrophobic tails cluster together away from water, while the hydrophilic heads face outwards towards the water. This forms spherical aggregates called micelles.
  2. The oily dirt/grease (hydrophobic) is trapped in the centre of the micelle by the hydrocarbon tails.
  3. The outer hydrophilic heads keep the micelle suspended (emulsified) in water.
  4. Rinsing with water washes away the micelles containing the trapped dirt.
• Problem with Soap in Hard Water:
  • Hard water contains Calcium (Ca²⁺) and Magnesium (Mg²⁺) ions.
  • These ions react with soap molecules to form insoluble precipitates called scum.
  • Scum reduces the cleansing efficiency of soap and sticks to clothes/surfaces.
  • Reaction: 2 C₁₇H₃₅COO⁻Na⁺ (Soap) + Ca²⁺ → (C₁₇H₃₅COO)₂Ca (Scum - Insoluble) + 2Na⁺
• Advantage of Detergents:
  • Detergents also form micelles and cleanse effectively.
  • The Calcium and Magnesium salts of detergents (e.g., sulphonates) are generally soluble in water.
  • Therefore, detergents work effectively even in hard water and do not form scum.
  • Disadvantage: Many detergents are non-biodegradable, leading to water pollution.

This chapter is fundamental to organic chemistry and frequently tested in exams. Focus on definitions, general formulas, functional groups, nomenclature rules, properties/reactions of ethanol & ethanoic acid, and the mechanism of soap action. Good luck!