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

Okay, let's begin with the detailed notes for Chapter 4: Carbon and its Compounds, focusing on aspects important for government exam preparation.

Chapter 4: Carbon and its Compounds - Detailed Notes for Exam Preparation

1. Introduction to Carbon

• Symbol: C
• Atomic Number: 6
• Electronic Configuration: 2, 4 (K shell: 2 electrons, L shell: 4 electrons)
• Valency: 4 (Tetravalent) - It has 4 valence electrons.
• Nature: Non-metal
• Occurrence: Found in the earth's crust (approx. 0.02%) as minerals (carbonates, hydrogen carbonates, coal, petroleum) and in the atmosphere (0.03%) as carbon dioxide. All living organisms (plants and animals) are made up of carbon-based compounds (organic compounds).

2. Bonding in Carbon – The Covalent Bond

• Why Covalent Bonds?
  • Carbon cannot form C⁴⁺ ion (by losing 4 electrons) as it requires a very large amount of energy to remove 4 electrons.
  • Carbon cannot form C⁴⁻ ion (by gaining 4 electrons) as the nucleus with 6 protons cannot hold onto 10 electrons.
  • Therefore, 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 is called a covalent bond. Sharing results in both atoms achieving a stable noble gas configuration.
• Types of Covalent Bonds:
  • Single Bond: Formed by sharing one pair of electrons (e.g., H₂, Cl₂, CH₄). Represented by a single line (–).
  • Double Bond: Formed by sharing two pairs of electrons (e.g., O₂, CO₂, C₂H₄ - Ethene). Represented by a double line (=).
  • Triple Bond: Formed by sharing three pairs of electrons (e.g., N₂, C₂H₂ - Ethyne). Represented by a triple line (≡).
• Electron Dot Structures: Useful for visualizing valence electrons and bond formation (e.g., Methane CH₄, Ethene C₂H₄, Ethyne C₂H₂).
• Properties of Covalent Compounds:
  • Generally have low melting and boiling points (due to weak intermolecular forces).
  • Generally poor conductors of electricity (as electrons are shared, no free ions or electrons are present).
  • Usually soluble in organic solvents but insoluble in water (exceptions exist, like ethanol, glucose).

3. Allotropes of Carbon

• Allotropy: The phenomenon where an element exists in two or more different physical forms with similar chemical properties but different physical properties.
• Allotropes of Carbon:
  • Diamond:
    • Structure: Each carbon atom is bonded to four other carbon atoms forming a rigid tetrahedral three-dimensional structure.
    • Properties: Hardest known natural substance, high melting point, electrical insulator (no free electrons), transparent, high refractive index.
    • Uses: Cutting tools, jewellery, abrasives.
  • Graphite:
    • Structure: Each carbon atom is bonded to three other carbon atoms in the same plane giving hexagonal arrays. These 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, greyish-black.
    • Uses: Lubricant (especially dry lubricant), pencil leads, electrodes in batteries and electrolysis.
  • Fullerenes (e.g., Buckminsterfullerene C₆₀):
    • Structure: Spherical or cage-like molecules. C₆₀ resembles a soccer ball (geodesic dome) with carbon atoms arranged in hexagons and pentagons.
    • Properties: Dark solids at room temperature.
    • Discovery: Identified later than diamond and graphite.

4. Versatile Nature of Carbon

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

• Catenation: The property of self-linking of atoms of an element through covalent bonds to form long straight chains, branched chains, or rings. Carbon exhibits catenation to the maximum extent due to the strong C-C bond strength. Silicon shows this property to a limited extent (compounds with up to 7-8 Si atoms).
• Tetravalency: Carbon has a valency of 4. It can form covalent bonds with four other atoms (carbon, hydrogen, oxygen, nitrogen, sulfur, halogens, etc.), allowing for a huge variety of structures.

5. Organic Compounds & Hydrocarbons

• Organic Compounds: Compounds of carbon (originally thought to be derived only from living organisms). Exceptions: Oxides of carbon (CO, CO₂), carbonates, hydrogen carbonates, carbides are usually studied under inorganic chemistry.
• Hydrocarbons: Organic compounds containing only Carbon and Hydrogen. They are the simplest organic compounds.
• Classification of Hydrocarbons:
  • Saturated Hydrocarbons (Alkanes):
    • Contain only single covalent bonds between carbon atoms.
    • General Formula: C<0xE2><0x82><0x99>H₂<0xE2><0x82><0x99>₊₂ (where n = number of carbon atoms)
    • Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀).
    • Relatively unreactive.
  • Unsaturated Hydrocarbons:
    • Contain at least one double or triple covalent bond between carbon atoms.
    • More reactive than saturated hydrocarbons.
    • Alkenes: Contain at least one C=C double bond.
      • General Formula: C<0xE2><0x82><0x99>H₂<0xE2><0x82><0x99>
      • Examples: Ethene (C₂H₄), Propene (C₃H₆), Butene (C₄H₈).
    • Alkynes: Contain at least one C≡C triple bond.
      • General Formula: C<0xE2><0x82><0x99>H₂<0xE2><0x82><0x99>₋₂
      • Examples: Ethyne (C₂H₂), Propyne (C₃H₄), Butyne (C₄H₆).
• Structural Isomers: Compounds having the same molecular formula but different structural arrangements of atoms (e.g., Butane C₄H₁₀ exists as n-butane (straight chain) and iso-butane (branched chain)). Isomers have different physical properties.
• Cyclic Hydrocarbons: Carbon atoms arranged in a ring structure.
  • Saturated Cyclic: Cyclohexane (C₆H₁₂)
  • Unsaturated Cyclic: Benzene (C₆H₆) - has alternating double and single bonds.

6. Functional Groups

• An atom or group of atoms present in a molecule which largely determines its chemical properties.
• Hydrocarbon chain/part determines physical properties; functional group determines chemical reactivity.
• Common Functional Groups:

7. 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 (or a molecular mass of 14u).
• 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.
  • They have the same functional group.
  • They show similar chemical properties.
  • There is a gradual change in physical properties (like melting point, boiling point, density) with increasing molecular mass.
• Examples: Series of Alkanes, Alkenes, Alkynes, Alcohols, Carboxylic Acids.

8. Nomenclature of Carbon Compounds (IUPAC System)

• IUPAC: International Union of Pure and Applied Chemistry. Provides systematic naming.
• Basic Rules:
  1. Identify the longest continuous carbon chain (Parent Chain): This gives the 'word root' (Meth- for 1C, Eth- for 2C, Prop- for 3C, But- for 4C, Pent- for 5C, Hex- for 6C, etc.).
  2. Identify the Functional Group: If present, it is indicated by a '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 Chain: Start numbering from the end that gives the lowest possible number to the carbon atom bearing the functional group (or multiple bond, or substituent).
  4. Indicate Position: The position of the functional group, multiple bond, or substituent is indicated by writing the number before the suffix/prefix (e.g., Propan-2-ol, But-1-ene).
  5. Name Substituents (Branches): Alkyl groups (like methyl -CH₃, ethyl -C₂H₅) or halogens are named as prefixes with their position number (e.g., 2-Methylpropane, 1-Chlorobutane).
  6. Multiple Groups: If the same substituent/functional group appears more than once, use prefixes di-, tri-, tetra- (e.g., 2,3-Dimethylbutane).

9. Chemical Properties of Carbon Compounds

• a) Combustion:
  • Carbon compounds burn in oxygen (air) to produce CO₂, H₂O, heat, and light.
  • C + O₂ → CO₂ + Heat + Light
  • CH₄ + 2O₂ → CO₂ + 2H₂O + Heat + Light
  • C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Heat + Light
  • Saturated hydrocarbons generally burn with a clean blue flame (complete combustion).
  • Unsaturated hydrocarbons generally burn with a yellow, sooty flame (incomplete combustion, due to higher carbon content). Soot is unburnt carbon.
  • Incomplete combustion (in limited O₂) produces Carbon Monoxide (CO), which is poisonous.
• b) Oxidation:
  • Controlled oxidation using oxidizing agents.
  • Alcohols can be oxidized to Carboxylic Acids using oxidizing agents like alkaline Potassium Permanganate (KMnO₄) or acidified Potassium Dichromate (K₂Cr₂O₇).
    • CH₃CH₂OH (Ethanol) --[Alkaline KMnO₄ + Heat or Acidified K₂Cr₂O₇ + Heat]--> CH₃COOH (Ethanoic Acid) + H₂O
  • Oxidizing agents add oxygen to the substance.
• c) Addition Reaction:
  • Characteristic reaction of unsaturated hydrocarbons (alkenes and alkynes).
  • Addition of hydrogen (Hydrogenation): Unsaturated hydrocarbons add hydrogen in the presence of catalysts like Nickel (Ni), Palladium (Pd), or Platinum (Pt) to form saturated hydrocarbons.
    • CH₂=CH₂ (Ethene) + H₂ --[Ni catalyst]--> CH₃-CH₃ (Ethane)
  • Industrial Application: Hydrogenation of vegetable oils (unsaturated fats, liquid) to form vanaspati ghee (saturated fats, solid).
  • Addition of halogens (e.g., Br₂, Cl₂) and hydrogen halides (e.g., HBr) also occurs across the double/triple bond. Bromine water test (reddish-brown Br₂ water decolorizes) is used to detect unsaturation.
• d) Substitution Reaction:
  • Characteristic reaction of saturated hydrocarbons (alkanes).
  • One or more hydrogen atoms are replaced by another atom or group (e.g., halogen).
  • Requires energy like sunlight (UV light) or heat.
  • Example: Reaction of Methane with Chlorine in sunlight.
    • CH₄ + Cl₂ --[Sunlight]--> CH₃Cl (Chloromethane) + HCl
    • The reaction can continue further: CH₃Cl + Cl₂ → CH₂Cl₂ + HCl ; CH₂Cl₂ + Cl₂ → CHCl₃ + HCl ; CHCl₃ + Cl₂ → CCl₄ + HCl

10. Important Carbon Compounds: Ethanol and Ethanoic Acid

• a) Ethanol (C₂H₅OH) - Ethyl Alcohol:
  • Physical Properties: Colourless liquid, pleasant smell, burning taste, soluble in water, volatile, low boiling point (78°C or 351K), neutral compound.
  • Chemical Reactions:
    • Reaction with Sodium: 2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa (Sodium ethoxide) + H₂ (Test for alcohol - evolution of H₂ gas).
    • Dehydration (Reaction with conc. H₂SO₄): Ethanol heated with excess concentrated sulphuric acid at 443 K (170°C) gives Ethene. H₂SO₄ acts as a dehydrating agent (removes water).
      • CH₃CH₂OH --[Conc. H₂SO₄, 443K]--> CH₂=CH₂ (Ethene) + H₂O
  • Uses: Solvent (in medicines like tincture iodine, cough syrups, tonics), fuel (mixed with petrol - gasohol), alcoholic beverages, antiseptic (sterilize wounds), manufacture of other organic compounds.
  • Harmful Effects: Consumption affects the central nervous system, causes drowsiness, impairs judgment, addiction, liver damage (cirrhosis). Methanol (CH₃OH) consumption is lethal or causes blindness.
  • 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:
  • Physical Properties: Colourless liquid, sour taste, pungent smell (like vinegar), soluble in water. 5-8% solution in water is called Vinegar. Melting point is 290 K (17°C), so it often freezes in winter (called Glacial Acetic Acid in pure form). Weak acid (partially ionizes in water).
  • Chemical Reactions:
    • Esterification: Reaction with an alcohol in the presence of an acid catalyst (conc. H₂SO₄) to form an ester (sweet-smelling compound).
      • 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 a base (like NaOH) to give back the alcohol and the sodium salt of the carboxylic acid. This reaction is used in soap making.
      • CH₃COOC₂H₅ (Ester) + NaOH → C₂H₅OH (Ethanol) + CH₃COONa (Sodium ethanoate - Soap component if long chain)
    • Reaction with Base (Neutralization): Reacts with bases like NaOH to form salt and water.
      • CH₃COOH + NaOH → CH₃COONa (Sodium ethanoate/acetate) + H₂O
    • Reaction with Carbonates and Hydrogen Carbonates: Reacts to produce salt, water, and carbon dioxide (effervescence). Test for carboxylic acids.
      • 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
      • CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
  • Uses: As vinegar in food preservation (pickles) and flavouring, solvent, manufacture of esters, rayon, plastics, dyes.

11. Soaps and Detergents

• Soaps: Sodium (Na⁺) or Potassium (K⁺) salts of long-chain carboxylic acids (fatty acids). Example: Sodium stearate (C₁₇H₃₅COONa).
• Detergents: Generally Ammonium or Sulphonate salts of long-chain carboxylic acids, OR Sodium salts of long-chain alkyl benzene sulphonic acids or alkyl hydrogen sulphates. They have a similar structure to soaps.
• Structure of Soap/Detergent Molecule: Consists of two parts:
  • Long Hydrocarbon Tail: Non-polar, hydrophobic (water-repelling), soluble in oil/grease.
  • Short Ionic Head: Polar, hydrophilic (water-attracting), soluble in water. (e.g., -COONa⁺ in soap, -SO₃Na⁺ in detergent).
• Cleansing Action (Micelle Formation):
  1. When soap/detergent is dissolved in water, the molecules arrange themselves in spherical clusters called micelles.
  2. In a micelle, the hydrophobic tails point inwards (towards the center), and the hydrophilic heads point outwards (towards the water).
  3. Grease/oil/dirt (hydrophobic) gets trapped in the hydrophobic core of the micelle.
  4. The outer hydrophilic layer keeps the micelle suspended in water.
  5. Rinsing with water washes away the micelles containing the dirt.
• Hard Water Problem: Hard water contains Calcium (Ca²⁺) and Magnesium (Mg²⁺) ions.
  • Soaps react with Ca²⁺/Mg²⁺ ions to form insoluble precipitates called scum. Scum hinders cleansing action and wastes soap.
    • 2C₁₇H₃₅COONa (Soap) + CaCl₂ → (C₁₇H₃₅COO)₂Ca (Scum - Calcium Stearate) + 2NaCl
  • Detergents do not form insoluble precipitates with Ca²⁺/Mg²⁺ ions. Their calcium/magnesium salts are soluble in water. Therefore, detergents are effective even in hard water.

Key Definitions for Quick Revision:

• Covalent Bond: Bond formed by sharing of electrons.
• Allotropes: Different physical forms of the same element.
• Catenation: Self-linking property of an element (especially Carbon).
• Tetravalency: Having a valency of four.
• Hydrocarbons: Compounds containing only C and H.
• Saturated Hydrocarbons (Alkanes): Contain only C-C single bonds.
• Unsaturated Hydrocarbons (Alkenes/Alkynes): Contain C=C or C≡C bonds.
• Isomers: Same molecular formula, different structures.
• Functional Group: Atom/group determining chemical properties.
• Homologous Series: Series differing by -CH₂ group, same functional group.
• Combustion: Burning in oxygen.
• Oxidation: Addition of oxygen or removal of hydrogen.
• Addition Reaction: Adding atoms across a multiple bond (characteristic of unsaturated compounds).
• Substitution Reaction: Replacing an atom (usually H) with another atom/group (characteristic of saturated compounds).
• Esterification: Reaction between carboxylic acid and alcohol to form an ester.
• Saponification: Base-hydrolysis of an ester (used in soap making).
• Micelle: Aggregate of soap/detergent molecules in water for cleansing.
• Scum: Insoluble precipitate formed when soap reacts with hard water ions.

Points to Remember for Exams:

• Reasons for Carbon's versatile nature (Catenation, Tetravalency).
• Differences between Diamond and Graphite (Structure, Properties, Conductivity).
• Properties of Covalent compounds (MP/BP, conductivity).
• General formulas of Alkanes, Alkenes, Alkynes.
• Identification and naming (IUPAC) of compounds with functional groups (Alcohol, Aldehyde, Ketone, Carboxylic Acid, Halogens). Practice naming and drawing structures.
• Characteristics of a Homologous Series.
• Distinguishing tests: Bromine water test for unsaturation, Sodium metal test for alcohols, Sodium bicarbonate test for carboxylic acids.
• Key reactions: Combustion (complete/incomplete), Oxidation (Alcohol → Acid), Addition (Hydrogenation), Substitution (Alkanes + Halogen), Esterification, Saponification.
• Properties and uses of Ethanol and Ethanoic Acid.
• Cleansing action of soaps (Micelle formation mechanism).
• Why detergents work better than soaps in hard water. Difference between soap and detergent structure (ionic head part).

These notes cover the core concepts of Chapter 4 comprehensively. Focus on understanding the concepts, practicing nomenclature, and remembering the key reactions and properties for effective exam preparation.