Class 11 Notes

Class 11 Chemistry Notes Chapter 9 (Chapter 9) – Examplar Problems (English) Book

Philoid

Philoid

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Examplar Problems (English)
Detailed Notes with MCQs of Chapter 9, Hydrogen. This chapter is quite fundamental and often features in various government examinations. We'll cover the key concepts as presented in the NCERT Exemplar, which will help you tackle trickier questions.

Chapter 9: Hydrogen - Detailed Notes for Government Exam Preparation

1. Position in the Periodic Table:

  • Hydrogen has an electronic configuration of 1s¹.
  • Resemblance with Alkali Metals (Group 1):
    • Forms unipositive ion (H⁺) by losing its only electron.
    • Forms oxides (H₂O), halides (HX), and sulphides (H₂S).
    • Shows +1 oxidation state.
  • Resemblance with Halogens (Group 17):
    • Needs one electron to achieve the stable configuration of the next noble gas (He).
    • Forms uninegative ion (H⁻, hydride ion).
    • Exists as a diatomic molecule (H₂) like halogens (X₂).
    • Forms covalent compounds (e.g., CH₄, SiH₄) similar to halogens (e.g., CCl₄, SiCl₄).
    • High Ionization Enthalpy, closer to halogens than alkali metals.
  • Unique Position: Due to these dual characteristics and significant differences (e.g., H⁺ is a bare proton, unlike M⁺ ions; H⁻ is less stable than X⁻; H₂ lacks lone pairs unlike X₂), hydrogen is best placed separately in the periodic table.

2. Isotopes of Hydrogen:

  • Atoms of the same element with the same atomic number but different mass numbers.
  • Protium (¹H₁): Most abundant (99.985%). No neutrons. Mass ≈ 1 amu.
  • Deuterium (²H₁ or D): Abundance ≈ 0.015%. One neutron. Mass ≈ 2 amu. Also called heavy hydrogen. Non-radioactive.
  • Tritium (³H₁ or T): Trace amounts. Two neutrons. Mass ≈ 3 amu. Radioactive (β-emitter, half-life ≈ 12.33 years).
  • Key Differences: Isotopes have similar chemical properties (due to same electronic configuration) but differ significantly in physical properties (mass, density, melting point, boiling point, enthalpy of fusion/vaporization) and reaction rates due to the 'isotope effect' (mass difference).

3. Dihydrogen (H₂): Preparation

  • Laboratory Methods:
    • Reaction of granulated zinc with dilute acids: Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)
    • Reaction of zinc with aqueous alkali: Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g) (Sodium zincate)
  • Commercial Production:
    • Electrolysis of acidified water: 2H₂O(l) --(Electrolysis, trace acid/base)--> 2H₂(g) + O₂(g) (Very pure H₂ obtained from electrolysis of warm Ba(OH)₂ solution between nickel electrodes).
    • Steam Reforming of Hydrocarbons: Reaction of hydrocarbons (like methane) with steam at high temperature (1270 K) in presence of a catalyst (Ni).
      CH₄(g) + H₂O(g) --(Ni, 1270K)--> CO(g) + 3H₂(g)
    • Water-Gas Shift Reaction: CO produced above is reacted with steam in presence of iron chromate catalyst (673 K) to produce more H₂.
      CO(g) + H₂O(g) --(FeCrO₄, 673K)--> CO₂(g) + H₂(g)
      (CO + H₂ mixture is called Water Gas or Syngas)
      CO₂ is removed by scrubbing with sodium arsenite solution.
    • Coal Gasification: Reaction of steam on coke (coal) at high temperature (1270 K).
      C(s) + H₂O(g) --(1270K)--> CO(g) + H₂(g) (Syngas)

4. Dihydrogen (H₂): Properties and Uses

  • Physical Properties: Colourless, odourless, tasteless, combustible gas. Lighter than air. Insoluble in water.
  • Chemical Properties:
    • Relatively inert at room temperature due to high H-H bond enthalpy (435.9 kJ mol⁻¹). Reactivity increases at high temperatures or with catalysts.
    • Reaction with Halogens: H₂(g) + X₂(g) → 2HX(g) (Reaction with F₂ is violent even in dark; with Cl₂ requires light; with Br₂ requires heat; with I₂ requires catalyst and is reversible).
    • Reaction with Dioxygen: 2H₂(g) + O₂(g) --(Heat/Catalyst)--> 2H₂O(l) (Highly exothermic).
    • Reaction with Dinitrogen (Haber Process): N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (High T & P, Fe catalyst). Key industrial process.
    • Reaction with Metals: Reacts with active metals (s-block) at high temperatures to form ionic hydrides.
      2Na(s) + H₂(g) → 2NaH(s)
      Ca(s) + H₂(g) → CaH₂(s)
    • Reaction with Metal Ions and Oxides: Reduces some metal ions in aqueous solution and oxides of less reactive metals.
      H₂(g) + Pd²⁺(aq) → Pd(s) + 2H⁺(aq)
      H₂(g) + CuO(s) → Cu(s) + H₂O(l)
    • Hydrogenation of Organic Compounds: Adds across double/triple bonds in presence of catalysts (Ni, Pt, Pd). Used in vegetable oil hydrogenation (vanaspati ghee production).
      CH₂=CH₂(g) + H₂(g) --(Ni/Pt/Pd)--> CH₃-CH₃(g)
      Vegetable Oils (l) + H₂(g) → Vegetable Ghee (s)
  • Uses:
    • Manufacture of ammonia (Haber process).
    • Manufacture of vanaspati fat.
    • Manufacture of methanol (CO + 2H₂ → CH₃OH).
    • Manufacture of metal hydrides.
    • Preparation of hydrogen chloride (HCl).
    • Metallurgical processes (reducing metal oxides).
    • Atomic hydrogen and oxy-hydrogen torches (cutting and welding).
    • Rocket fuel (liquid hydrogen + liquid oxygen).
    • Fuel cells for generating electricity.

5. Hydrides:

  • Binary compounds of hydrogen with other elements. Classified into three types:
    • (i) Ionic or Saline Hydrides:
      • Formed with most s-block elements (highly electropositive). Eg: LiH, NaH, KH, CaH₂, SrH₂.
      • Crystalline, non-volatile, non-conducting solids.
      • Conduct electricity in molten state (liberate H₂ at anode, confirming H⁻ ion).
      • React violently with water producing H₂ gas: NaH(s) + H₂O(l) → NaOH(aq) + H₂(g).
      • Strong reducing agents.
    • (ii) Covalent or Molecular Hydrides:
      • Formed with most p-block elements. Eg: CH₄, NH₃, H₂O, HF, SiH₄, PH₃.
      • Mostly volatile compounds with low melting/boiling points (except those with H-bonding like NH₃, H₂O, HF).
      • Further classified based on electron availability:
        • Electron-deficient: Less electrons than required for Lewis structure. Eg: B₂H₆ (Group 13). Act as Lewis acids.
        • Electron-precise: Exact number of electrons for Lewis structure. Eg: CH₄ (Group 14). Tetrahedral geometry.
        • Electron-rich: More electrons (as lone pairs) than required for Lewis structure. Eg: NH₃, H₂O, HF (Groups 15, 16, 17). Act as Lewis bases.
    • (iii) Metallic or Non-stoichiometric (Interstitial) Hydrides:
      • Formed by many d-block and f-block elements.
      • Metals of groups 7, 8, 9 do not form hydrides (Hydride Gap).
      • Hydrogen occupies interstitial sites in the metal lattice.
      • Often non-stoichiometric (e.g., LaH₂.₈₇, TiH₁.₅₋₁.₇, ZrH₁.₃₋₁.₇₅, PdH₀.₆₋₀.₈). Exception: Stoichiometric hydrides formed by f-block elements.
      • Conduct heat and electricity (though less than parent metal).
      • Used in hydrogen storage and as hydrogenation catalysts.

6. Water (H₂O):

  • Structure: Bent/V-shape. O is sp³ hybridized with 2 bond pairs and 2 lone pairs. H-O-H angle is 104.5°. Highly polar molecule. Extensive intermolecular hydrogen bonding.
  • Physical Properties: Colourless, tasteless liquid. High freezing point (0°C), boiling point (100°C), specific heat, heat of vaporization, surface tension due to H-bonding. Ice is less dense than water (H-bonds create open cage-like structure in ice). Max density at 4°C (277 K). Universal solvent (high dielectric constant).
  • Chemical Properties:
    • Amphoteric Nature: Acts as both acid and base.
      H₂O(l) + NH₃(aq) ⇌ OH⁻(aq) + NH₄⁺(aq) (Water acts as acid)
      H₂O(l) + H₂S(aq) ⇌ H₃O⁺(aq) + HS⁻(aq) (Water acts as base)
    • Redox Reactions: Can be easily reduced or oxidized.
      • Reduced to H₂ by active metals: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
      • Oxidized to O₂ by fluorine: 2F₂(g) + 2H₂O(l) → 4H⁺(aq) + 4F⁻(aq) + O₂(g)
    • Hydrolysis Reactions: Dissolves many ionic compounds. Hydrates salts. Hydrolyzes oxides, halides, hydrides etc.
      P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)
      SiCl₄(l) + 2H₂O(l) → SiO₂(s) + 4HCl(aq)
    • Hydrates Formation: Forms hydrated salts from aqueous solutions.
      • Coordinated water: [Cr(H₂O)₆]³⁺ 3Cl⁻
      • Interstitial water: BaCl₂·2H₂O
      • Hydrogen-bonded water: [Cu(H₂O)₄]²⁺ SO₄²⁻·H₂O (in CuSO₄·5H₂O)

7. Hard and Soft Water:

  • Soft Water: Lathers readily with soap.
  • Hard Water: Does not lather readily with soap. Contains dissolved salts of Calcium (Ca²⁺) and Magnesium (Mg²⁺). Soap (sodium stearate, C₁₇H₃₅COONa) reacts with these ions to form insoluble scum (calcium/magnesium stearate).
    2C₁₇H₃₅COONa(aq) + M²⁺(aq) → (C₁₇H₃₅COO)₂M(s)↓ + 2Na⁺(aq) (where M = Ca or Mg)
  • Types of Hardness:
    • Temporary Hardness: Due to presence of bicarbonates [Mg(HCO₃)₂ and Ca(HCO₃)₂]. Can be removed by:
      • Boiling: Bicarbonates decompose into insoluble carbonates/hydroxides.
        Mg(HCO₃)₂(aq) --(Heat)--> Mg(OH)₂(s)↓ + 2CO₂(g)
        Ca(HCO₃)₂(aq) --(Heat)--> CaCO₃(s)↓ + H₂O(l) + CO₂(g)
      • Clark's Method: Adding calculated amount of slaked lime [Ca(OH)₂].
        Ca(HCO₃)₂(aq) + Ca(OH)₂(aq) → 2CaCO₃(s)↓ + 2H₂O(l)
        Mg(HCO₃)₂(aq) + 2Ca(OH)₂(aq) → 2CaCO₃(s)↓ + Mg(OH)₂(s)↓ + 2H₂O(l)
    • Permanent Hardness: Due to presence of chlorides and sulphates of Ca²⁺ and Mg²⁺ (CaCl₂, MgCl₂, CaSO₄, MgSO₄). Cannot be removed by boiling. Removed by:
      • Treatment with Washing Soda (Na₂CO₃): Precipitates Ca²⁺/Mg²⁺ as carbonates.
        MCl₂(aq) + Na₂CO₃(aq) → MCO₃(s)↓ + 2NaCl(aq) (M = Ca, Mg)
        MSO₄(aq) + Na₂CO₃(aq) → MCO₃(s)↓ + Na₂SO₄(aq) (M = Ca, Mg)
      • Calgon's Method: Calgon is Sodium hexametaphosphate (Na₆P₆O₁₈). It forms soluble complexes with Ca²⁺/Mg²⁺ ions, keeping them in solution but unavailable to react with soap.
        Na₆P₆O₁₈ → 2Na⁺ + Na₄P₆O₁₈²⁻
        Na₄P₆O₁₈²⁻ + M²⁺ → [Na₂(MP₆O₁₈)]²⁻ + 2Na⁺ (M = Ca, Mg) (Soluble complex)
      • Ion-Exchange Method (Zeolite/Permutit Process): Permutit is hydrated sodium aluminium silicate (NaAlSiO₄), represented as NaZ. Ca²⁺/Mg²⁺ ions are exchanged for Na⁺ ions.
        2NaZ(s) + M²⁺(aq) → MZ₂(s) + 2Na⁺(aq) (M = Ca, Mg)
        Zeolite is regenerated by treating with brine (NaCl solution).
        MZ₂(s) + 2NaCl(aq) → 2NaZ(s) + MCl₂(aq)
      • Synthetic Resins Method: More efficient than zeolites.
        • Cation exchange resins (RSO₃H): Exchange H⁺ for Ca²⁺/Mg²⁺.
          2RH(s) + M²⁺(aq) → MR₂(s) + 2H⁺(aq)
        • Anion exchange resins (RNH₃⁺OH⁻): Exchange OH⁻ for anions (Cl⁻, SO₄²⁻, HCO₃⁻).
          RNH₃⁺OH⁻(s) + X⁻(aq) → RNH₃⁺X⁻(s) + OH⁻(aq)
          H⁺ and OH⁻ released combine to form water (H⁺ + OH⁻ → H₂O). Resins are regenerated with dilute acid and alkali solutions. This process yields demineralised/deionised water.

8. Hydrogen Peroxide (H₂O₂):

  • Preparation:
    • Acidifying Barium peroxide (BaO₂) with dilute H₂SO₄:
      BaO₂·8H₂O(s) + H₂SO₄(aq) → BaSO₄(s) + H₂O₂(aq) + 8H₂O(l) (Excess BaSO₄ removed by filtration)
    • Electrolysis of cold 50% H₂SO₄ followed by hydrolysis of peroxodisulphate formed:
      2HSO₄⁻(aq) --(Electrolysis)--> H₂S₂O₈(aq) + 2e⁻ (Peroxodisulphuric acid)
      H₂S₂O₈(aq) + 2H₂O(l) --(Hydrolysis)--> 2H₂SO₄(aq) + H₂O₂(aq)
    • Industrial: Auto-oxidation of 2-alkylanthaquinols.
      2-Alkylanthraquinol --(O₂/Air)--> H₂O₂ + Oxidised product (regenerated by H₂/Pd)
  • Structure: Non-planar, 'open book' structure. Gas phase dihedral angle ≈ 111.5°, Solid phase ≈ 90.2°. O-O single bond.
  • Physical Properties: Almost colourless (very pale blue) liquid. Miscible with water (forms H-bonds). BP = 150.2 °C (extrapolated, decomposes below BP).
  • Chemical Properties: Acts as both an oxidising and reducing agent.
    • Oxidising Agent (Accepts electrons):
      • In acidic medium: H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O (E° = +1.77 V)
        • Oxidises Fe²⁺ to Fe³⁺: 2Fe²⁺(aq) + H₂O₂(aq) + 2H⁺(aq) → 2Fe³⁺(aq) + 2H₂O(l)
        • Oxidises PbS to PbSO₄: PbS(s) + 4H₂O₂(aq) → PbSO₄(s) + 4H₂O(l)
      • In basic medium: H₂O₂ + 2e⁻ → 2OH⁻ (E° = +0.87 V)
        • Oxidises Mn²⁺ to Mn⁴⁺: Mn²⁺(aq) + H₂O₂(aq) + 2OH⁻(aq) → MnO₂(s) + 2H₂O(l)
    • Reducing Agent (Donates electrons):
      • In acidic medium: H₂O₂ → O₂ + 2H⁺ + 2e⁻ (E° = -0.68 V)
        • Reduces MnO₄⁻ to Mn²⁺: 2MnO₄⁻(aq) + 5H₂O₂(aq) + 6H⁺(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 5O₂(g)
        • Reduces HOCl to Cl⁻: HOCl(aq) + H₂O₂(aq) → H₃O⁺(aq) + Cl⁻(aq) + O₂(g)
      • In basic medium: H₂O₂ + 2OH⁻ → O₂ + 2H₂O + 2e⁻ (E° = -0.08 V)
        • Reduces I₂ to I⁻: I₂(s) + H₂O₂(aq) + 2OH⁻(aq) → 2I⁻(aq) + 2H₂O(l) + O₂(g)
        • Reduces MnO₄⁻ to MnO₂: 2MnO₄⁻(aq) + 3H₂O₂(aq) → 2MnO₂(s) + 2H₂O(l) + 3O₂(g) + 2OH⁻(aq)
    • Disproportionation: Decomposes slowly on standing or rapidly on heating/exposure to light/catalysts (metals, MnO₂).
      2H₂O₂(l) → 2H₂O(l) + O₂(g)
  • Storage: Stored in dark, wax-lined glass or plastic bottles. Stabilizers like urea, phosphoric acid, or glycerol are added to slow down decomposition. Kept away from dust and rough surfaces.
  • Uses:
    • Bleaching agent (hair, textiles, paper pulp) - 'mild bleach'.
    • Antiseptic ("Perhydrol" - 30% H₂O₂ solution).
    • Manufacture of chemicals like sodium perborate, sodium percarbonate (used in detergents).
    • Pollution control (treatment of cyanide waste, restoring aerobic conditions).
    • Propellant (rocket fuel).

9. Heavy Water (D₂O):

  • Prepared by exhaustive electrolysis of ordinary water or as a by-product in fertilizer industry.
  • Physical properties differ slightly from H₂O (higher density, MP, BP).
  • Used primarily as a moderator in nuclear reactors to slow down fast neutrons.
  • Used in tracer mechanism studies.

10. Dihydrogen as a Fuel (Hydrogen Economy):

  • Advantages: High energy release per unit mass, pollution-free (product is water).
  • Challenges: Cost of production, safe storage (as liquid, gas under pressure, or interstitial hydrides), transportation.
  • Hydrogen economy refers to an energy system where hydrogen is the primary energy carrier.

Multiple Choice Questions (MCQs):

  1. Which isotope of hydrogen is radioactive?
    (A) Protium
    (B) Deuterium
    (C) Tritium
    (D) Ortho-hydrogen

  2. The hydride NaH is an example of:
    (A) Electron-rich covalent hydride
    (B) Interstitial hydride
    (C) Saline hydride
    (D) Electron-deficient covalent hydride

  3. Temporary hardness of water is due to the presence of:
    (A) Chlorides of Ca and Mg
    (B) Sulphates of Ca and Mg
    (C) Bicarbonates of Ca and Mg
    (D) Carbonates of Na and K

  4. In the reaction: 2MnO₄⁻(aq) + 5H₂O₂(aq) + 6H⁺(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 5O₂(g), H₂O₂ acts as:
    (A) An oxidising agent
    (B) A reducing agent
    (C) An acid
    (D) A catalyst

  5. The method used to remove permanent hardness of water using sodium hexametaphosphate is called:
    (A) Clark's method
    (B) Ion-exchange method
    (C) Calgon's method
    (D) Boiling

  6. Commercial production of dihydrogen often involves the reaction of steam on hydrocarbons at high temperature in the presence of a catalyst. This process is known as:
    (A) Haber Process
    (B) Coal Gasification
    (C) Steam Reforming
    (D) Electrolysis

  7. The structure of hydrogen peroxide (H₂O₂) molecule is:
    (A) Linear
    (B) Planar trigonal
    (C) Tetrahedral
    (D) Non-planar (Open book)

  8. Heavy water (D₂O) is primarily used in nuclear reactors as a:
    (A) Fuel
    (B) Moderator
    (C) Coolant
    (D) Shielding material

  9. Which of the following processes uses dihydrogen?
    (A) Synthesis of ammonia
    (B) Hydrogenation of vegetable oils
    (C) Production of methanol
    (D) All of the above

  10. Water acts as an acid when it reacts with:
    (A) H₂S
    (B) HCl
    (C) NH₃
    (D) CH₃COOH

Answer Key:

  1. (C)
  2. (C)
  3. (C)
  4. (B)
  5. (C)
  6. (C)
  7. (D)
  8. (B)
  9. (D)
  10. (C)

Study these notes carefully, paying attention to the reaction conditions, classifications, and specific examples. Understanding the 'why' behind the properties, like H-bonding in water or the redox nature of H₂O₂, is crucial. Good luck with your preparation!

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