Chemistry Notes – Set 9: Detailed Guide for UPSC, PCS, SSC Competitive Exams

Class 9: Chemical Properties of Substances

Detailed Concepts:

• Chemical Properties: Characteristics observed during chemical reactions, where substances change into new ones.
  • Reactivity: Tendency to react with other substances (e.g., Na reacts vigorously with water: 2Na + 2H₂O → 2NaOH + H₂).
  • Combustion: Reaction with O₂ producing heat/light (e.g., CH₄ + 2O₂ → CO₂ + 2H₂O, methane burning).
  • Corrosion: Degradation by environmental reaction (e.g., rusting: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O).
  • Acidity/Basicity: Ability to release H⁺ (acids) or OH⁻ (bases) (e.g., vinegar (CH₃COOH) reacts with baking soda (NaHCO₃) → CO₂).
• Observable Chemical Changes:
  • Gas Evolution: Bubbles (e.g., CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂, limestone with acid).
  • Color Change: E.g., CuSO₄·5H₂O (blue) → CuSO₄ (white) on heating, reversible with water.
  • Precipitation: Formation of insoluble solids (e.g., AgNO₃ + NaCl → AgCl↓ + NaNO₃, white precipitate).
• Everyday Examples:
  • Food: Fermentation (C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂, yeast in dough).
  • Cleaning: Bleach (NaOCl) oxidizes stains (e.g., organic dyes → colorless products).
  • Materials: Burning fuels (e.g., LPG: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O).
• Chemical vs. Physical Properties:
  • Chemical: Involve reactions (e.g., burning, rusting).
  • Physical: No new substance (e.g., melting, dissolving).
• Applications in Exams: Understanding chemical properties and their manifestations in daily life is key for objective and descriptive questions, especially for linking to environmental or industrial contexts.

Formulas:

• Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O.
• Rusting: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O.
• Neutralization (Simplified): Acid + Base → Salt + Water (e.g., CH₃COOH + NaOH → CH₃COONa + H₂O).
• Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
• Precipitation: AgNO₃ + NaCl → AgCl↓ + NaNO₃.

Applications:

• Competitive Exams:
  • UPSC/PCS: Questions on chemical properties in environmental contexts (e.g., corrosion in infrastructure) or daily life (e.g., fermentation in food).
  • SSC: Objective questions on chemical vs. physical properties or reaction indicators (e.g., gas evolution).
  • Descriptive: Explain rusting prevention or chemical changes in cooking.
• Real-World:
  • Daily Life: Bleach in stain removal, baking soda in cooking.
  • Environment: Corrosion in pipelines, CO₂ from combustion in climate change.
  • Industry: Fuel combustion for energy, chemical synthesis in food processing.
• Exam Tips:
  • Focus on observable signs of chemical reactions (e.g., color change, gas evolution).
  • Link to environmental science (e.g., corrosion, combustion emissions) for mains.

Diagram (Textual Description):

• Rusting Process: Show an iron nail exposed to air and water, forming red-brown rust (Fe₂O₃·xH₂O). Label Fe reacting with O₂ and H₂O, with arrows showing electron transfer (oxidation: Fe → Fe³⁺). Include prevention methods (e.g., Zn coating for galvanization).

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Class 10: Periodic Classification of Elements

Detailed Concepts:

• Note: Revisiting Set 5’s “Periodic Classification of Elements” with a focus on periodic trends, group applications, and industrial/environmental relevance to avoid redundancy, tailored for Class 10 level and exam needs.
• Purpose of Classification: Organizes elements by properties for prediction and study.
• Historical Development:
  • Dobereiner’s Triads: Three elements with similar properties, middle element’s atomic mass ≈ average (e.g., Cl, Br, I: atomic masses 35.5, 80, 127; 80 ≈ (35.5+127)/2).
  • Newlands’ Octaves: Every eighth element repeats properties, limited scope.
  • Mendeleev’s Table: Based on atomic mass, predicted elements (e.g., eka-aluminum → Ga), but had anomalies (e.g., Ar before K).
• Modern Periodic Table:
  • Based on atomic number (Z), per Moseley’s discovery.
  • Periods: 1–7, indicate energy shells.
  • Groups: 1–18, indicate valence electrons.
  • Blocks: s, p, d, f based on orbital filling.
• Periodic Trends:
  • Atomic Radius: Decreases across period (higher nuclear charge), increases down group (more shells).
  • Ionization Energy: Increases across period (smaller size), decreases down group (larger size).
  • Electronegativity: Increases across period (e.g., F = 4.0), decreases down group.
  • Metallic Character: Decreases across period (metals → non-metals), increases down group.
• Group Characteristics:
  • Group 1 (Alkali Metals): Soft, reactive, form basic oxides (e.g., Na₂O).
  • Group 17 (Halogens): Reactive non-metals, form acidic oxides (e.g., Cl₂O₇).
  • Group 18 (Noble Gases): Inert, stable (e.g., He, Ne).
• Applications in Exams: Trends and group properties are key for objective and descriptive questions, especially for industrial or environmental applications.

Formulas:

• No direct formulas, but key relationships:
  • Atomic Radius: ∝ 1/Nuclear charge (across period), ∝ Shells (down group).
  • Ionization Energy: ∝ Nuclear charge, ∝ 1/Size.
  • Electronegativity: Pauling scale (e.g., F = 4.0, O = 3.5).

Applications:

• Competitive Exams:
  • UPSC/PCS: Questions on periodic trends in industrial applications (e.g., halogens in disinfectants) or environmental chemistry (e.g., noble gases in lighting).
  • SSC: Objective questions on trends, group properties, or Mendeleev’s contributions.
  • Descriptive: Explain periodic trends or uses of Group 17 elements.
• Real-World:
  • Industry: Na in chemical synthesis, Cl₂ in water treatment.
  • Technology: Ne in neon lights, Si in semiconductors.
  • Environment: Halogens in ozone depletion (CFCs).
• Exam Tips:
  • Memorize trends (radius, ionization energy, electronegativity).
  • Link group properties to environmental science (e.g., CFCs) for mains.

Diagram (Textual Description):

• Periodic Trends: A simplified periodic table (Groups 1, 2, 13–18; Periods 1–4). Arrows show atomic radius (increases down, decreases across), ionization energy (decreases down, increases across), electronegativity (increases across). Label key elements (e.g., Li, F, Cs).

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Class 11: p-Block Elements

Detailed Concepts:

• p-Block Elements: Groups 13–18, valence electrons in p-orbitals (ns²np¹⁻⁶), include metals (Al), metalloids (B, Si), non-metals (C, N, O, F).
• Group-Wise Properties:
  • Group 13 (Boron Family):
    • B: Metalloid, Al: Metal, others (Ga, In, Tl) less common.
    • Form covalent compounds (e.g., BCl₃), Al forms Al₂O₃ (amphoteric).
    • Boron’s electron deficiency leads to unique bonding (e.g., B₂H₆, diborane).
  • Group 14 (Carbon Family):
    • C: Non-metal (allotropes: diamond, graphite), Si, Ge: Metalloids, Sn, Pb: Metals.
    • C forms covalent bonds (e.g., CO₂), Sn/Pb form ionic/covalent compounds.
  • Group 15 (Nitrogen Family):
    • N, P: Non-metals, As, Sb: Metalloids, Bi: Metal.
    • N₂ stable due to triple bond, P forms P₄ or allotropes (white, red).
    • NH₃ (basic), HNO₃ (oxidizing acid).
  • Group 16 (Oxygen Family):
    • O, S: Non-metals, Se, Te: Metalloids, Po: Metal.
    • O₂ stable, S forms S₈ or allotropes (rhombic, monoclinic).
    • SO₂ forms H₂SO₃ (acid rain contributor).
  • Group 17 (Halogens):
    • F, Cl, Br, I: Non-metals, highly reactive.
    • Form HX acids (e.g., HCl), reactivity decreases down group (F > Cl > Br > I).
  • Group 18 (Noble Gases):
    • He, Ne, Ar, Kr, Xe, Rn: Inert due to full valence shell, Xe forms compounds (e.g., XeF₂).
• Key Compounds:
  • Boric Acid (H₃BO₃): Weak acid, antiseptic.
  • Silicates: SiO₄⁴⁻ units, in glass, cement.
  • Ammonia (NH₃): Fertilizer production (Haber process).
  • Sulphuric Acid (H₂SO₄): Industrial acid, contact process.
  • Chlorine (Cl₂): Disinfectant, bleaching.
• Trends:
  • Atomic Radius: Increases down group, decreases across period.
  • Electronegativity: Decreases down group, high in Group 17 (F = 4.0).
  • Metallic Character: Increases down group, decreases across period.
• Applications in Exams: Group properties, compounds, and trends are key for objective and descriptive questions.

Formulas:

• Ammonia Synthesis: N₂ + 3H₂ ⇌ 2NH₃ (Haber process).
• Contact Process: 2SO₂ + O₂ → 2SO₃; SO₃ + H₂O → H₂SO₄.
• Boric Acid Ionization: H₃BO₃ + H₂O ⇌ B(OH)₄⁻ + H⁺.
• Chlorine Disproportionation: Cl₂ + H₂O → HCl + HClO.

Applications:

• Competitive Exams:
  • UPSC/PCS: Questions on p-block compounds in industry (e.g., H₂SO₄) or environment (e.g., SO₂ in acid rain).
  • SSC: Objective questions on group trends, compounds, or reactions.
  • Descriptive: Explain ammonia production or halogen uses in water treatment.
• Real-World:
  • Industry: Si in semiconductors, NH₃ in fertilizers.
  • Environment: SO₂ in air pollution, Cl₂ in water purification.
  • Medicine: H₃BO₃ as antiseptic, Xe in imaging.
• Exam Tips:
  • Master group-wise properties and key compounds.
  • Link to environmental science (e.g., CFCs, SO₂) for mains.

Diagram (Textual Description):

• Diborane Structure (B₂H₆): Two B atoms bridged by two H atoms (3-center-2-electron bonds), each B with two terminal H atoms. Label B–H–B bridge bonds, electron-deficient structure, and sp³ hybridization.

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Class 12: Haloalkanes and Haloarenes

Detailed Concepts:

• Haloalkanes: Alkanes with halogen(s) (R–X, e.g., CH₃Cl, chloroalkanes).
• Haloarenes: Aromatic compounds with halogen(s) (e.g., C₆H₅Cl, chlorobenzene).
• Nomenclature (IUPAC):
  • Haloalkanes: Haloalkane (e.g., CH₃CH₂Br: bromoethane).
  • Haloarenes: Halobenzene (e.g., C₆H₅Cl: chlorobenzene).
• Preparation:
  • Haloalkanes:
    • From alcohols: ROH + HX → RX + H₂O (e.g., C₂H₅OH + HBr → C₂H₅Br, ZnCl₂ catalyst).
    • Halogenation of alkanes: RH + X₂ → RX + HX (UV light, e.g., CH₄ + Cl₂ → CH₃Cl).
    • Addition to alkenes: RCH=CH₂ + HX → RCHXCH₃ (e.g., C₂H₄ + HBr → C₂H₅Br).
  • Haloarenes:
    • Electrophilic substitution: C₆H₆ + Cl₂ → C₆H₅Cl + HCl (FeCl₃ catalyst).
    • Sandmeyer reaction: C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂.
• Chemical Properties:
  • Haloalkanes:
    • Nucleophilic Substitution: RX + Nu⁻ → R–Nu + X⁻.
      • Sₙ1: Two-step, carbocation intermediate, for 3° haloalkanes (e.g., (CH₃)₃CCl → (CH₃)₃COH with OH⁻).
      • Sₙ2: One-step, backside attack, for 1° haloalkanes (e.g., CH₃Br + OH⁻ → CH₃OH).
    • Elimination: Forms alkenes (e.g., CH₃CH₂Br + KOH (alc) → C₂H₄ + HBr).
    • Wurtz Reaction: 2RX + 2Na → R–R + 2NaX (e.g., 2CH₃Br → C₂H₆).
  • Haloarenes:
    • Less reactive due to C–X bond resonance with benzene ring.
    • Electrophilic substitution: C₆H₅Cl → nitration, sulfonation (–Cl directs ortho/para).
    • Dow’s process: C₆H₅Cl + NaOH → C₆H₅OH (high T, P).
• Applications:
  • Chloroform (CHCl₃): Solvent, anesthetic (historical).
  • DDT (C₁₄H₉Cl₅): Pesticide, banned due to environmental persistence.
  • CFCs: Refrigerants, ozone-depleting (e.g., CF₂Cl₂).
• Applications in Exams: Reactions, mechanisms, and environmental impacts are key for objective and descriptive questions.

Formulas:

• Sₙ2 Reaction: RX + Nu⁻ → R–Nu + X⁻ (e.g., CH₃Br + OH⁻ → CH₃OH + Br⁻).
• Elimination: RCH₂CH₂X + OH⁻ → RCH=CH₂ + HX.
• Wurtz Reaction: 2RX + 2Na → R–R + 2NaX.
• Sandmeyer Reaction: C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂.

Applications:

• Competitive Exams:
  • UPSC/PCS: Questions on haloalkanes in synthesis or CFCs in environmental chemistry.
  • SSC: Objective questions on reaction mechanisms or haloarene uses.
  • Descriptive: Explain Sₙ1 vs. Sₙ2 or environmental impact of CFCs.
• Real-World:
  • Industry: Haloalkanes as solvents, intermediates in drug synthesis.
  • Environment: CFCs in ozone depletion, DDT in ecosystem harm.
  • Medicine: Halogenated compounds in anesthetics, pesticides.
• Exam Tips:
  • Master Sₙ1/Sₙ2 mechanisms and haloarene reactivity.
  • Link to environmental science (e.g., CFC ban) for mains.

Diagram (Textual Description):

• Sₙ2 Mechanism: Show CH₃Br + OH⁻ → CH₃OH + Br⁻. Draw transition state with OH⁻ attacking C from opposite side of Br, partial bonds to C, and Br leaving. Label nucleophile (OH⁻), leaving group (Br⁻), and inversion of configuration.