Class 11 Chemistry Notes Chapter 2 (Hydrogen) – Chemistry Part-II Book

Detailed Notes with MCQs of Chapter 9: Hydrogen from your Chemistry Part-II book. This is a unique element, and understanding its properties and compounds is crucial, not just for your Class 11 syllabus but also for various competitive government exams. Pay close attention as we break down the key concepts.

Chapter 9: Hydrogen - Detailed Notes for Exam Preparation

1. Position in the Periodic Table:

• Unique Position: Hydrogen is the lightest element, with electronic configuration 1s¹. Its position is anomalous.
• Resemblance with Alkali Metals (Group 1):
  • Has one electron in the outermost shell (1s¹ like ns¹).
  • Forms unipositive ion (H⁺) like alkali metals (Na⁺, K⁺).
  • Forms oxides (H₂O), halides (HX), and sulphides (H₂S) similar to alkali metals (Na₂O, NaCl, Na₂S).
  • Acts as a reducing agent.
• Resemblance with Halogens (Group 17):
  • Needs one electron to complete its valence shell (like halogens needing one electron for ns²np⁶).
  • Forms uninegative ion (H⁻, hydride ion) like halogens (F⁻, Cl⁻).
  • Exists as a diatomic molecule (H₂) like halogens (F₂, Cl₂).
  • Has high ionization enthalpy (though lower than halogens).
  • Forms covalent compounds (e.g., CH₄, SiH₄) like halogens (CCl₄, SiCl₄).
• Differences: Unlike alkali metals, H⁺ does not exist freely (highly polarizing). Unlike halogens, it has low reactivity and lacks unshared electron pairs.
• Conclusion: Due to these unique characteristics, it's best placed separately at the top of the periodic table.

2. Isotopes of Hydrogen:

• Hydrogen has three isotopes:
  • Protium (¹₁H): Most abundant (~99.985%). No neutrons. Nucleus = 1 proton. Non-radioactive.
  • Deuterium (²₁H or D): Abundance ~0.015%. Nucleus = 1 proton + 1 neutron. Non-radioactive. Used in heavy water (D₂O).
  • Tritium (³₁H or T): Trace amounts. Nucleus = 1 proton + 2 neutrons. Radioactive (emits low-energy β⁻ particles, half-life ~12.33 years).

3. Preparation of Dihydrogen (H₂):

• Laboratory Methods:
  • Reaction of granulated Zinc with dilute acids (e.g., dil. HCl or dil. H₂SO₄):
    Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)
  • Reaction of Zinc with aqueous alkali (e.g., NaOH):
    Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g) (Sodium zincate)
• Commercial Production:
  • Electrolysis of acidified or alkaline water: Using platinum electrodes.
    2H₂O(l) --(Electrolysis, trace acid/base)--> 2H₂(g) + O₂(g)
  • By-product in Chlor-alkali process: Electrolysis of brine (aqueous NaCl solution).
    2NaCl(aq) + 2H₂O(l) --(Electrolysis)--> 2NaOH(aq) + Cl₂(g) + H₂(g)
  • Reaction of steam on hydrocarbons or coke (Coal Gasification): At high temperatures (1270 K) in the presence of a catalyst (Ni).
    CH₄(g) + H₂O(g) --(1270 K, Ni)--> CO(g) + 3H₂(g)
C(s) + H₂O(g) --(1270 K)--> CO(g) + H₂(g)
    The mixture of CO and H₂ is called Water gas or Syngas (Synthesis gas).
  • Water-gas Shift Reaction: To increase H₂ production, CO from syngas is reacted with steam in the presence of an iron chromate catalyst (673 K).
    CO(g) + H₂O(g) --(673 K, Catalyst)--> CO₂(g) + H₂(g)
    CO₂ is removed by scrubbing with sodium arsenite solution.

4. Properties of Dihydrogen (H₂):

• 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 with temperature or catalyst.
  • Reaction with Halogens (X₂): Forms hydrogen halides (HX).
    H₂(g) + X₂(g) → 2HX(g) (Reactivity: F₂ > Cl₂ > Br₂ > I₂)
  • Reaction with Dioxygen (O₂): Forms water. Highly exothermic.
    2H₂(g) + O₂(g) --(Heat/Catalyst)--> 2H₂O(l); ΔH = -285.9 kJ mol⁻¹
  • Reaction with Dinitrogen (N₂): Forms ammonia (Haber Process). Requires high temperature (~673 K), high pressure (200 atm), and catalyst (Iron, with Molybdenum as promoter).
    N₂(g) + 3H₂(g) ⇌ 2NH₃(g); ΔH = -92.6 kJ mol⁻¹
  • Reaction with Metals: Forms hydrides (MH or MH₂).
    2M + H₂ → 2MH (M = Alkali metal)
    M + H₂ → MH₂ (M = Ca, Sr, Ba)
  • Reaction with Metal Ions and Oxides (Reducing Action): Reduces oxides of metals less reactive than iron.
    CuO(s) + H₂(g) --(Heat)--> Cu(s) + H₂O(l)
Pd²⁺(aq) + H₂(g) → Pd(s) + 2H⁺(aq)
  • Reactions with Organic Compounds:
    • Hydrogenation: Addition of H₂ across double/triple bonds in presence of catalysts (Ni, Pd, Pt). E.g., Vegetable oils (unsaturated) to vegetable ghee/margarine (saturated).
    • Hydroformylation: Olefins react with H₂ and CO to form aldehydes, which can be further reduced to alcohols.
      RCH=CH₂ + H₂ + CO → RCH₂CH₂CHO

5. Hydrides:

• Binary compounds of hydrogen with other elements (EHₓ or EₘHₙ).
• Types:
  • (i) Ionic or Saline Hydrides: Formed with most s-block elements (highly electropositive). Stoichiometric compounds (e.g., LiH, NaH, CaH₂). Crystalline, non-volatile solids. Conduct electricity in molten state (liberate H₂ at anode). React violently with water producing H₂.
    NaH(s) + H₂O(l) → NaOH(aq) + H₂(g)
  • (ii) Covalent or Molecular Hydrides: Formed with most p-block elements. Volatile compounds (e.g., CH₄, NH₃, H₂O, HF).
    • Electron-deficient: Less electrons than required for Lewis structure (e.g., B₂H₆ - Group 13). Act as Lewis acids.
    • Electron-precise: Exact number of electrons for Lewis structure (e.g., CH₄ - Group 14). Tetrahedral geometry.
    • Electron-rich: More electrons (as lone pairs) than required for Lewis structure (e.g., NH₃, H₂O, HF - Groups 15-17). Act as Lewis bases. Exhibit hydrogen bonding.
  • (iii) Metallic or Non-stoichiometric (Interstitial) Hydrides: Formed by many d-block and f-block elements (Groups 3, 4, 5, 6 (Cr only), 10, 11, 12). Deficient in hydrogen (e.g., LaH₂.₈₇, TiH₁.₅₋₁.₈). Hydrogen occupies interstices in the metal lattice. Conduct heat and electricity (less than parent metal). Used in hydrogen storage. Hydride Gap: Elements of Groups 7, 8, 9 do not form hydrides.

6. Water (H₂O):

• Structure: Bent/V-shape. O is sp³ hybridized. Bond angle ~104.5°. Polar molecule. Extensive intermolecular hydrogen bonding.
• Physical Properties: Colourless, tasteless liquid. Unusually high freezing point (0°C), boiling point (100°C), heat of vaporization, heat of fusion, specific heat capacity, surface tension, and dielectric constant – all due to extensive H-bonding. Ice has a highly ordered, open cage-like structure (lower density than liquid water). Max density at 4°C (277 K).
• Chemical Properties:
  • Amphoteric Nature: Can act as an acid (with bases like NH₃) and a base (with acids like H₂S).
    H₂O(l) + NH₃(aq) ⇌ OH⁻(aq) + NH₄⁺(aq) (Water as acid)
    H₂O(l) + H₂S(aq) ⇌ H₃O⁺(aq) + HS⁻(aq) (Water as base)
  • Redox Reactions: Can be easily reduced to H₂ by electropositive metals. Can be oxidized to O₂ during photosynthesis or by fluorine.
    2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
2F₂(g) + 2H₂O(l) → 4H⁺(aq) + 4F⁻(aq) + O₂(g)
  • Hydrolysis: Due to high dielectric constant, dissolves many ionic compounds and hydrolyzes certain covalent/ionic compounds.
    P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)
SiCl₄(l) + 2H₂O(l) → SiO₂(s) + 4HCl(aq)
  • Hydrate Formation: Water associates with salts in different ways:
    • Coordinated water: e.g., [Cr(H₂O)₆]³⁺ 3Cl⁻
    • Interstitial water: e.g., BaCl₂·2H₂O
    • Hydrogen-bonded water: e.g., [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. Contains dissolved salts of Calcium (Ca²⁺) and Magnesium (Mg²⁺).
• 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.
      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)
  • 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): Reacts with soluble Ca/Mg salts to form insoluble carbonates.
      MgCl₂(aq) + Na₂CO₃(aq) → MgCO₃(s)↓ + 2NaCl(aq)
CaSO₄(aq) + Na₂CO₃(aq) → CaCO₃(s)↓ + Na₂SO₄(aq)
    • Calgon's Method: Calgon = Sodium hexametaphosphate (Na₆P₆O₁₈). It keeps Ca²⁺/Mg²⁺ ions in solution as soluble complexes.
      Na₆P₆O₁₈ → 2Na⁺ + Na₄P₆O₁₈²⁻
M²⁺ + Na₄P₆O₁₈²⁻ → [Na₂(MP₆O₁₈)]²⁻ + 2Na⁺ (M = Mg, Ca)
    • Ion-Exchange Method: Using zeolites (hydrated sodium aluminium silicate - NaAlSiO₄, also called permutit) or synthetic resins.
      2NaZ(s) + M²⁺(aq) → MZ₂(s) + 2Na⁺(aq) (Z = zeolite/resin)
      Resin can be regenerated by treating with NaCl solution. Cation exchange (H⁺ form) and anion exchange (OH⁻ form) resins remove all mineral ions, producing demineralised/deionised water.

8. Hydrogen Peroxide (H₂O₂):

• Preparation:
  • Acidifying Barium peroxide (BaO₂) with H₂SO₄ or H₃PO₄.
    BaO₂·8H₂O(s) + H₂SO₄(aq) → BaSO₄(s) + H₂O₂(aq) + 8H₂O(l)
  • Electrolysis of cold 50% H₂SO₄ followed by hydrolysis of peroxodisulphate obtained.
  • Auto-oxidation of 2-alklylanthraquinols.
• Physical Properties: Almost colourless (very pale blue) liquid. Miscible with water.
• Structure: Non-planar, "open book" structure. Dihedral angle ~111.5° (gas), ~90.2° (solid).
• Chemical Properties: Acts as both an oxidizing and a reducing agent in both acidic and basic media.
  • Oxidizing action (acidic): Fe²⁺ → Fe³⁺, PbS → PbSO₄
2Fe²⁺(aq) + H₂O₂(aq) + 2H⁺(aq) → 2Fe³⁺(aq) + 2H₂O(l)
  • Reducing action (acidic): MnO₄⁻ → Mn²⁺, HOCl → Cl⁻
2MnO₄⁻(aq) + 5H₂O₂(aq) + 6H⁺(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 5O₂(g)
  • Oxidizing action (basic): Fe²⁺ → Fe³⁺, Mn²⁺ → Mn⁴⁺
2Fe²⁺(aq) + H₂O₂(aq) → 2Fe³⁺(aq) + 2OH⁻(aq)
  • Reducing action (basic): I₂ → I⁻, MnO₄⁻ → MnO₂
I₂(s) + H₂O₂(aq) + 2OH⁻(aq) → 2I⁻(aq) + 2H₂O(l) + O₂(g)
• Storage: Decomposes slowly on standing (2H₂O₂ → 2H₂O + O₂). Decomposition is catalyzed by light, metal surfaces, and alkali. Stored in dark, wax-lined glass or plastic bottles. Stabilizers like urea, phosphoric acid, or glycerol are added.
• Strength: Usually expressed as 'Volume strength'. E.g., '10 volume' H₂O₂ means 1 Litre of this H₂O₂ solution will give 10 Litres of O₂ at STP upon decomposition.
  • Molarity of 'V volume' H₂O₂ = V / 11.2
  • Normality of 'V volume' H₂O₂ = V / 5.6
• Uses: Antiseptic (perhydrol), bleaching agent (hair, textiles, paper pulp), synthesis of chemicals (hydroquinone, tartaric acid), pollution control (treatment of cyanide, oxidation of pollutants), rocket propellant.

9. Heavy Water (D₂O):

• Prepared by exhaustive electrolysis of ordinary water or as a by-product in fertilizer industry.
• Properties are slightly different from H₂O (higher m.p., b.p., density, viscosity; lower dielectric constant). Chemical reactions are slower than those of H₂O.
• Uses: Primarily as a moderator in nuclear reactors (to slow down fast neutrons), and as a tracer compound to study reaction mechanisms.

10. Dihydrogen as a Fuel (Hydrogen Economy):

• Advantages: High energy release per unit mass compared to other fuels. Combustion product is water (pollution-free).
• Challenges: Production (costly), safe storage (highly inflammable, requires heavy tanks/liquefaction/metal hydrides), and transportation. Research ongoing for efficient and safe utilization.

Multiple Choice Questions (MCQs):

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

2. The commercial production of dihydrogen often involves the 'water-gas shift reaction'. What is the catalyst used in this reaction?
   (A) Nickel
   (B) Iron chromate
   (C) Platinum
   (D) Vanadium pentoxide

3. Which of the following hydrides is electron-deficient?
   (A) CH₄
   (B) NH₃
   (C) B₂H₆
   (D) H₂O

4. Permanent hardness of water is due to the presence of:
   (A) Bicarbonates of Ca²⁺ and Mg²⁺
   (B) Carbonates of Ca²⁺ and Mg²⁺
   (C) Chlorides and Sulphates of Ca²⁺ and Mg²⁺
   (D) Phosphates of Na⁺ and K⁺

5. Calgon, used for softening hard water, is chemically:
   (A) Sodium silicate
   (B) Sodium bicarbonate
   (C) Sodium hexametaphosphate
   (D) Sodium aluminium silicate

6. In the structure of hydrogen peroxide (H₂O₂), the molecule is:
   (A) Linear
   (B) Planar
   (C) Non-planar ('open book' structure)
   (D) Tetrahedral

7. What is the primary use of heavy water (D₂O)?
   (A) As a bleaching agent
   (B) As a moderator in nuclear reactors
   (C) As an antiseptic
   (D) In the Haber process

8. Hydrogen shows resemblance to halogens in which aspect?
   (A) Forms H⁺ ion
   (B) Exists as a diatomic molecule (H₂)
   (C) Has 1 electron in the outermost shell
   (D) Acts as a strong reducing agent

9. The unusually high boiling point of water is primarily due to:
   (A) Its high molecular weight
   (B) Its covalent nature
   (C) Extensive hydrogen bonding between molecules
   (D) Its high dielectric constant

10. What is the oxidation state of oxygen in H₂O₂?
    (A) -2
    (B) -1
    (C) 0
    (D) +1

Answer Key:

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

Study these notes thoroughly. Remember the key reactions, classifications, properties, and uses, especially for compounds like water, hydrogen peroxide, and heavy water, as well as the different types of hydrides. Good luck with your preparation!