Class 10 Notes

Class 10 Science Notes Chapter 4 (Chapter 4) – Examplar Problems (English) Book

Philoid

Philoid

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Examplar Problems (English)
Detailed Notes with MCQs of Chapter 4, 'Carbon and its Compounds', from your NCERT Exemplar book. This is a crucial chapter, not just for your board exams but also for various government competitive exams. Pay close attention to the details.

Chapter 4: Carbon and its Compounds - Detailed Notes for Competitive Exams

1. Introduction to Carbon

  • Symbol: C
  • Atomic Number: 6
  • Electronic Configuration: 2, 4 (K shell - 2, L shell - 4)
  • Valency: 4 (Tetravalent). It has 4 valence electrons.
  • Occurrence: Found in elemental form (diamond, graphite, fullerenes) and combined form (carbonates, fossil fuels, organic compounds, CO2 in the atmosphere ~0.03%).
  • Uniqueness: Forms the backbone of a vast number of compounds (organic chemistry) due to its unique properties.

2. Bonding in Carbon – The Covalent Bond

  • Carbon cannot form C⁴⁺ ions (requires huge energy to remove 4 electrons) or C⁴⁻ ions (difficult for the nucleus with 6 protons to hold 10 electrons).
  • Therefore, Carbon achieves a stable noble gas configuration by sharing its valence electrons with other atoms (including other carbon atoms).
  • Covalent Bond: The bond formed by the mutual sharing of electron pairs between two atoms.
    • Single Bond: Sharing one pair of electrons (e.g., H₂, Cl₂, CH₄). Represented by a single line (–).
    • Double Bond: Sharing two pairs of electrons (e.g., O₂, CO₂, C₂H₄). Represented by a double line (=).
    • Triple Bond: Sharing three pairs of electrons (e.g., N₂, C₂H₂). Represented by a triple line (≡).
  • Properties of Covalent Compounds:
    • Generally have low melting and boiling points (weak intermolecular forces).
    • Generally poor conductors of electricity (no free ions or electrons).
    • Solubility: Often soluble in organic solvents, insoluble in water (exceptions exist, e.g., ethanol, glucose).
  • Lewis Dot Structures: Representing valence electrons as dots around the atomic symbol to show bond formation. (Practice drawing for CH₄, CO₂, C₂H₆, C₂H₄, C₂H₂).

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.
  • Carbon Allotropes:
    • Diamond:
      • Structure: Each carbon atom is bonded to four other carbon atoms forming a rigid three-dimensional tetrahedral structure.
      • Properties: Hardest natural substance known, high melting point, poor conductor of electricity (no free electrons), good conductor of heat, 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, forming hexagonal layers. Layers are held by weak van der Waals forces. One valence electron per carbon is relatively free.
      • Properties: Soft, slippery/greasy, good conductor of electricity (due to free electrons), high melting point, opaque.
      • Uses: Lubricant (especially at high temperatures), pencil leads, electrodes in batteries and electrolysis.
    • Fullerenes (e.g., Buckminsterfullerene C₆₀):
      • Structure: Spherical or cage-like molecules. C₆₀ resembles a soccer ball with carbon atoms arranged in hexagons and pentagons.
      • Properties: Discovered relatively recently. Research ongoing for potential applications (superconductors, catalysts, medical uses).

4. Versatile Nature of Carbon

  • Carbon forms millions of compounds due to:
    • Catenation: The unique ability of carbon atoms to link with other carbon atoms through covalent bonds to form long chains (straight or branched) and rings. This is possible due to the strong C-C bond. Silicon shows some catenation but forms unstable compounds.
    • Tetravalency: Carbon has a valency of 4, allowing it to bond with four other atoms (monovalent, divalent, or trivalent elements, or other carbon atoms). This leads to a wide 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, bicarbonates, carbides are usually studied under inorganic chemistry.
  • Hydrocarbons: Organic compounds containing only Carbon and Hydrogen. They are the simplest organic compounds.
    • Saturated Hydrocarbons (Alkanes):
      • Contain only single 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. Undergo substitution reactions.
    • Unsaturated Hydrocarbons:
      • Contain at least one double or triple bond between carbon atoms.
      • 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).
      • More reactive than alkanes due to the presence of double/triple bonds. Undergo addition reactions.

6. Structural Isomerism

  • Isomers: Compounds having the same molecular formula but different structural arrangements of atoms.
  • Structural Isomers: Isomers that differ in the connectivity of atoms (i.e., different structures).
  • Example: Butane (C₄H₁₀) has two structural isomers:
    • n-Butane (straight chain): CH₃-CH₂-CH₂-CH₃
    • Isobutane (branched chain): CH₃-CH(CH₃)-CH₃ (IUPAC name: 2-methylpropane)
  • The number of possible isomers increases rapidly with the number of carbon atoms.

7. Functional Groups

  • An atom or group of atoms present in an organic molecule which determines its characteristic chemical properties.

  • The hydrocarbon chain mainly determines physical properties.

  • Common Functional Groups:

    Class Functional Group Formula Example (IUPAC Name)
    Haloalkanes Halo (F, Cl, Br, I) -X CH₃Cl (Chloromethane)
    Alcohols Hydroxyl -OH CH₃OH (Methanol)
    Aldehydes Aldehyde -CHO HCHO (Methanal)
    Ketones Carbonyl/Keto >C=O CH₃COCH₃ (Propanone)
    Carboxylic Acids Carboxyl -COOH HCOOH (Methanoic Acid)
    Alkenes Double Bond >C=C< C₂H₄ (Ethene)
    Alkynes Triple Bond -C≡C- C₂H₂ (Ethyne)

8. 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.
    • Successive members differ by a -CH₂ group (or 14 atomic mass units).
    • Show similar chemical properties (due to the same functional group).
    • Show a gradual change in physical properties (like melting point, boiling point, density) as molecular mass increases.
  • Examples: Alkane series (CH₄, C₂H₆, C₃H₈...), Alcohol series (CH₃OH, C₂H₅OH, C₃H₇OH...).

9. Nomenclature of Carbon Compounds (Basic IUPAC Rules)

  • Identify the parent chain: Longest continuous chain of carbon atoms.

  • Identify the functional group: Determines the suffix (ending) of the name.

  • Number the carbon atoms: Start from the end closer to the functional group or substituent.

  • Identify and name substituents (branches): Use prefixes like methyl (-CH₃), ethyl (-C₂H₅), chloro (-Cl), etc., along with their position number.

  • Assemble the name: Prefix(es) + Word Root (based on no. of C atoms) + Suffix (based on C-C bonds and functional group).

    • Word Roots: Meth (1C), Eth (2C), Prop (3C), But (4C), Pent (5C), Hex (6C)...
    • Suffixes: -ane (alkane), -ene (alkene), -yne (alkyne), -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid).

10. Chemical Properties of Carbon Compounds

  • a) Combustion:
    • Burning of carbon compounds in air (oxygen) 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.
    • Unsaturated hydrocarbons generally burn with a yellow, sooty flame (due to incomplete combustion).
  • b) Oxidation:
    • Controlled addition of oxygen or removal of hydrogen.
    • Alcohols can be oxidized to carboxylic acids using oxidizing agents like alkaline KMnO₄ (potassium permanganate) or acidified K₂Cr₂O₇ (potassium dichromate).
    • CH₃CH₂OH (Ethanol) --[Alkaline KMnO₄ + Heat or Acidified K₂Cr₂O₇ + Heat]--> CH₃COOH (Ethanoic Acid) + H₂O
  • c) Addition Reaction:
    • Characteristic reaction of unsaturated hydrocarbons (alkenes and alkynes).
    • Atoms/groups are added across the double or triple bond, converting it into a single bond.
    • Hydrogenation: Addition of hydrogen to unsaturated hydrocarbons in the presence of a catalyst (like Nickel, Palladium, or Platinum) to form saturated hydrocarbons.
    • CH₂=CH₂ (Ethene) + H₂ --[Ni catalyst]--> CH₃-CH₃ (Ethane)
    • Application: Hydrogenation of vegetable oils (unsaturated fats) to form vanaspati ghee (saturated fats).
  • d) Substitution Reaction:
    • Characteristic reaction of saturated hydrocarbons (alkanes).
    • One or more hydrogen atoms are replaced by another atom or group (e.g., halogens).
    • Requires energy like sunlight or heat.
    • CH₄ (Methane) + Cl₂ --[Sunlight]--> CH₃Cl (Chloromethane) + HCl
    • Further substitution can occur (CH₂Cl₂, CHCl₃, CCl₄).

11. Important Carbon Compounds: Ethanol and Ethanoic Acid

  • Ethanol (C₂H₅OH): Ethyl alcohol.
    • Properties: Colourless liquid, pleasant smell, burning taste, soluble in water, volatile, neutral.
    • Reactions:
      • Combustion (already discussed).
      • Oxidation to Ethanoic acid (already discussed).
      • Reaction with Sodium: 2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa (Sodium ethoxide) + H₂ (Test for alcohol - evolution of H₂ gas).
      • Dehydration: Reaction with concentrated H₂SO₄ at 443 K (170°C) to form ethene.
        CH₃CH₂OH --[Conc. H₂SO₄, 443 K]--> CH₂=CH₂ + H₂O (Conc. H₂SO₄ acts as a dehydrating agent).
    • Uses: Solvent, alcoholic beverages, antiseptic (tincture of iodine), fuel additive (gasohol), manufacture of other chemicals.
    • Denatured Alcohol: Ethanol made unfit for drinking by adding poisonous substances like methanol, pyridine, copper sulphate.
  • Ethanoic Acid (CH₃COOH): Acetic acid.
    • Properties: Colourless liquid, pungent smell (vinegar), sour taste, soluble in water. 5-8% solution in water is called vinegar. Freezing point is 290 K (17°C), often freezes in winter (called glacial acetic acid). Weak acid (partially ionizes in water).
    • Reactions:
      • Acidic Nature: Turns blue litmus red, reacts with bases, carbonates, and hydrogen carbonates.
        • With Base (NaOH): CH₃COOH + NaOH → CH₃COONa (Sodium ethanoate/acetate) + H₂O (Neutralization)
        • With Carbonates (Na₂CO₃): 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
        • With Hydrogen Carbonates (NaHCO₃): CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂ (Effervescence due to CO₂ - test for carboxylic acids)
      • Esterification: Reaction with an alcohol in the presence of concentrated H₂SO₄ to form a sweet-smelling ester.
        CH₃COOH (Ethanoic acid) + CH₃CH₂OH (Ethanol) --[Conc. H₂SO₄]--> CH₃COOC₂H₅ (Ethyl ethanoate - Ester) + H₂O
        (Reversible reaction; Conc. H₂SO₄ acts as catalyst and dehydrating agent).
      • Saponification: Reaction of an ester with an alkali (like NaOH) to give back the alcohol and the sodium salt of the carboxylic acid (soap). It is the reverse of esterification.
        CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa (Sodium ethanoate)
        Used in soap making.
    • Uses: Preservative in pickles (vinegar), solvent, manufacture of esters, rayon, plastics.

12. Soaps and Detergents

  • Soaps: Sodium or Potassium salts of long-chain carboxylic acids (fatty acids). E.g., Sodium stearate (C₁₇H₃₅COONa).
  • Detergents: Generally Ammonium or Sulphonate salts of long-chain carboxylic acids. Can also be non-ionic. E.g., Sodium n-dodecylbenzene sulphonate.
  • Structure of Soap/Detergent Molecule: Has two parts:
    • Hydrophobic Tail: Long hydrocarbon chain (non-polar), repelled by water but attracted to oil/grease.
    • Hydrophilic Head: Ionic part (polar -COO⁻Na⁺ or -SO₃⁻Na⁺), attracted to water.
  • Cleansing Action (Micelle Formation):
    1. Soap/detergent dissolves in water. The molecules arrange themselves radially with hydrophobic tails pointing inwards and hydrophilic heads outwards. This cluster is called a micelle.
    2. The hydrophobic tails trap the oil/grease particle (dirt) inside the micelle.
    3. The hydrophilic heads remain dissolved in water.
    4. When water is agitated or rinsed, the micelles containing the dirt get washed away.
  • Hard Water vs Soft Water:
    • Hard Water: Contains dissolved salts of Calcium (Ca²⁺) and Magnesium (Mg²⁺) ions.
    • Soft Water: Free from Ca²⁺ and Mg²⁺ ions.
  • Problem with Soaps in Hard Water: Soaps react with Ca²⁺ and Mg²⁺ ions in hard water to form insoluble precipitates called scum. This wastes soap and hinders cleaning.
    2C₁₇H₃₅COONa (Soap) + Ca²⁺ → (C₁₇H₃₅COO)₂Ca (Scum - Calcium Stearate) + 2Na⁺
  • Advantage of Detergents: Detergents do not form scum with hard water because their Calcium and Magnesium salts are soluble in water. Hence, they are effective even in hard water.

Multiple Choice Questions (MCQs)

  1. Which property of carbon is responsible for the formation of a very large number of organic compounds?
    a) Tetravalency only
    b) Catenation only
    c) Both Tetravalency and Catenation
    d) Allotropy

  2. Vinegar is a solution of:
    a) 50%–60% acetic acid in alcohol
    b) 5%–8% acetic acid in water
    c) 5%–8% acetic acid in alcohol
    d) 50%–60% acetic acid in water

  3. Which of the following hydrocarbons undergoes addition reactions?
    a) C₂H₆
    b) C₃H₈
    c) C₄H₈
    d) CH₄

  4. The functional group present in propanone is:
    a) -OH (Alcohol)
    b) -COOH (Carboxylic Acid)
    c) >C=O (Ketone)
    d) -CHO (Aldehyde)

  5. During the esterification reaction between ethanol and ethanoic acid in the presence of conc. H₂SO₄, the role of conc. H₂SO₄ is:
    a) Oxidizing agent only
    b) Dehydrating agent only
    c) Catalyst only
    d) Both catalyst and dehydrating agent

  6. Which of the following is NOT an allotrope of carbon?
    a) Diamond
    b) Graphite
    c) Fullerene
    d) Methane

  7. Soaps form scum with hard water due to the presence of which ions?
    a) Na⁺ and K⁺
    b) Ca²⁺ and Mg²⁺
    c) Cl⁻ and SO₄²⁻
    d) H⁺ and OH⁻

  8. The general formula for alkynes is:
    a) C<0xE2><0x82><0x99>H₂<0xE2><0x82><0x99>₊₂
    b) C<0xE2><0x82><0x99>H₂<0xE2><0x82><0x99>
    c) C<0xE2><0x82><0x99>H₂<0xE2><0x82><0x99>₋₂
    d) C<0xE2><0x82><0x99>H₂<0xE2><0x82><0x99>₊₁

  9. The reaction CH₄ + Cl₂ --[Sunlight]--> CH₃Cl + HCl is an example of:
    a) Addition reaction
    b) Substitution reaction
    c) Oxidation reaction
    d) Combustion reaction

  10. Butanone is a four-carbon compound with the functional group:
    a) Carboxylic acid
    b) Aldehyde
    c) Ketone
    d) Alcohol

Answer Key for MCQs:

  1. c
  2. b
  3. c (C₄H₈ fits C<0xE2><0x82><0x99>H₂<0xE2><0x82><0x99>, an alkene)
  4. c
  5. d
  6. d
  7. b
  8. c
  9. b
  10. c

Remember to thoroughly revise these concepts, practice drawing structures, and understand the reactions. This chapter forms the foundation of organic chemistry. Good luck with your preparation!

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