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

Detailed Notes with MCQs of Chapter 11, 'The p-Block Elements', specifically covering Groups 13 and 14 from your NCERT Class 11 Chemistry Exemplar book. This is a crucial chapter for various government exams, so pay close attention to the details, trends, and specific compounds.

Chapter 11: The p-Block Elements (Groups 13 & 14)

Introduction:

• p-Block elements are those in which the last electron enters the outermost p orbital.
• They encompass groups 13 to 18 in the periodic table.
• Their valence shell electronic configuration is ns²np¹⁻⁶ (except for He).
• This chapter focuses on Group 13 (Boron family) and Group 14 (Carbon family).
• p-Block contains metals, non-metals, and metalloids, showing great diversity in properties.

General Trends (Groups 13 & 14):

1. Electronic Configuration:
   • Group 13: ns²np¹
   • Group 14: ns²np²
2. Atomic Radii:
   • Generally increases down the group due to the addition of a new shell.
   • Anomaly in Group 13: Atomic radius of Ga is less than that of Al. This is due to the poor shielding effect of the 10 d-electrons in Ga, which increases the effective nuclear charge.
   • Group 14: Increase from C to Si is significant, followed by a smaller increase from Si to Ge, Sn, and Pb due to poor shielding by d- and f-electrons.
3. Ionization Enthalpy (IE):
   • Generally decreases down the group.
   • Anomalies in Group 13: IE does not decrease smoothly. ΔᵢH₁ order: B > Al < Ga > In < Tl. The discontinuity between Al and Ga is again due to poor d-electron shielding in Ga. The increase from In to Tl is due to poor f-electron shielding (lanthanoid contraction effect).
   • Group 14: Decrease from C to Si is sharp, then decreases slightly from Si to Ge, Sn. IE of Pb is slightly higher than Sn due to poor f-electron shielding.
4. Electronegativity:
   • Group 13: Decreases from B to Al, then increases slightly for Ga, and then remains almost constant or slightly decreases for In and Tl. B (2.0) > Tl (1.8) ≈ In (1.7) ≈ Ga (1.6) > Al (1.5). The variations are due to discrepancies in atomic size.
   • Group 14: Decreases from C to Si, then remains almost constant for Ge, Sn, and Pb. C (2.5) > Si (1.8) ≈ Ge (1.8) ≈ Sn (1.8) < Pb (1.9).
5. Oxidation States:
   • Group 13: Expected +3. Boron and Aluminium predominantly show +3. Due to the inert pair effect (reluctance of ns² electrons to participate in bonding), the stability of the +1 oxidation state increases down the group (Al < Ga < In < Tl). Tl(+1) is more stable than Tl(+3). Group 13 compounds in the +3 state are electron-deficient and act as Lewis acids (e.g., BF₃, AlCl₃).
   • Group 14: Expected +4. Carbon and Silicon mainly show +4. Due to the inert pair effect, the stability of the +2 oxidation state increases down the group (C < Si < Ge < Sn < Pb). Pb(+2) is more stable and common than Pb(+4). Sn(+2) acts as a reducing agent, while Pb(+4) acts as an oxidizing agent.
6. Nature of Oxides:
   • Group 13: B₂O₃ (acidic), Al₂O₃ & Ga₂O₃ (amphoteric), In₂O₃ & Tl₂O₃ (basic). Acidity decreases down the group.
   • Group 14: CO (neutral), CO₂ & SiO₂ (acidic), GeO₂, SnO₂, PbO₂ (amphoteric). Acidity decreases down the group. Oxides in lower oxidation states (CO, SnO, PbO) are less acidic or neutral/amphoteric compared to higher oxidation states (CO₂, SnO₂, PbO₂).

Group 13 Elements (Boron Family: B, Al, Ga, In, Tl)

1. Anomalous Behaviour of Boron:
   • Small size, high IE, high electronegativity.
   • Non-metal, forms covalent compounds.
   • Maximum covalency is 4 (due to unavailability of d-orbitals). Al and others can expand their covalency beyond 4.
   • Forms acidic oxide (B₂O₃), whereas others form amphoteric or basic oxides.
   • Boron halides (BX₃) exist as monomers and are strong Lewis acids; AlCl₃ exists as a dimer (Al₂Cl₆) in non-polar solvents/vapour phase and is ionic in the solid state.
2. Reactivity:
   • Towards Air: Boron is unreactive in crystalline form. Aluminium forms a protective oxide layer. Amorphous Boron and Aluminium on heating form B₂O₃ and Al₂O₃. Down the group, reactivity increases. Tl forms Tl₂O. They also form nitrides (BN, AlN) at high temperatures.
   • Towards Acids & Alkalis: Boron is unreactive with non-oxidizing acids but reacts with strong oxidizing acids (like conc. HNO₃, conc. H₂SO₄). It also reacts with fused alkali (NaOH/KOH). Aluminium dissolves in dilute acids and aqueous alkalis, showing amphoteric character.
     • 2Al(s) + 6HCl(aq) → 2Al³⁺(aq) + 6Cl⁻(aq) + 3H₂(g)
     • 2Al(s) + 2NaOH(aq) + 6H₂O(l) → 2Na⁺[Al(OH)₄]⁻(aq) + 3H₂(g) (Sodium tetrahydroxoaluminate(III))
   • Towards Halogens: React to form trihalides (MX₃). Tl also forms stable TlX.
3. Important Compounds of Boron:
   • Borax (Sodium tetraborate decahydrate): Na₂B₄O₇·10H₂O. Correct formula: Na₂[B₄O₅(OH)₄]·8H₂O. Contains tetranuclear units [B₄O₅(OH)₄]²⁻. Two B atoms are sp² hybridized (trigonal planar), and two are sp³ hybridized (tetrahedral).
     • Borax Bead Test: Used to identify coloured metal ions. Na₂B₄O₇ (on heating) → Na₂B₄O₇ (anhydrous) → 2NaBO₂ (Sodium metaborate) + B₂O₃ (Boric anhydride). B₂O₃ forms a glassy bead, which reacts with metal oxides to form coloured metaborates (e.g., CoO + B₂O₃ → Co(BO₂)₂ (blue)).
     • Hydrolysis: Borax solution is alkaline (Na₂B₄O₇ + 7H₂O → 2NaOH + 4H₃BO₃).
   • Orthoboric Acid (H₃BO₃): White crystalline solid, soapy touch. Weak monobasic acid; acts as a Lewis acid by accepting OH⁻ from water: B(OH)₃ + 2H₂O ⇌ [B(OH)₄]⁻ + H₃O⁺. It has a layered structure where planar BO₃ units are joined by hydrogen bonds.
     • Effect of Heat: H₃BO₃ (heat, >100°C) → HBO₂ (Metaboric acid) (heat, >160°C) → H₂B₄O₇ (Tetraboric acid) (heat, red hot) → B₂O₃ (Boric anhydride).
   • Diborane (B₂H₆): Colourless, highly toxic gas. Prepared by:
     • 4BF₃ + 3LiAlH₄ → 2B₂H₆ + 3LiF + 3AlF₃ (Lab method)
     • 2NaBH₄ + I₂ → B₂H₆ + 2NaI + H₂ (Industrial method)
     • Structure: Electron deficient. Contains two bridging hydrogen atoms (B-H-B bonds). These are 3-center-2-electron (3c-2e) bonds (banana bonds). There are four terminal B-H bonds which are normal 2-center-2-electron (2c-2e) covalent bonds. Each Boron atom is sp³ hybridized.
     • Reactions: Readily hydrolysed by water (B₂H₆ + 6H₂O → 2B(OH)₃ + 6H₂). Reacts with Lewis bases (like NH₃, CO) undergoing cleavage reactions.
       • With excess NH₃ at low temp: B₂H₆·2NH₃ or [BH₂(NH₃)₂]⁺[BH₄]⁻
       • With excess NH₃ at high temp: B₃N₃H₆ (Borazine or 'Inorganic Benzene')
       • With CO: BH₃·CO

Group 14 Elements (Carbon Family: C, Si, Ge, Sn, Pb)

1. Anomalous Behaviour of Carbon:
   • Small size, high electronegativity, high IE, unavailability of d-orbitals.
   • Maximum covalency is 4. Others can expand (e.g., [SiF₆]²⁻, [GeCl₆]²⁻).
   • Unique ability to form pπ-pπ multiple bonds with itself (C=C, C≡C) and other small atoms (C=O, C=N, C≡N). Heavier elements do not form stable pπ-pπ bonds due to larger size and diffuse p-orbitals.
   • Catenation: Exceptional ability to link with itself through covalent bonds to form long chains and rings. Order: C >> Si > Ge ≈ Sn >> Pb. Bond enthalpy (C-C > Si-Si > Ge-Ge > Sn-Sn > Pb-Pb) is a major factor.
2. Allotropes of Carbon:
   • Diamond: Crystalline, sp³ hybridized carbon atoms linked tetrahedrally. Hardest substance, electrical insulator, thermal conductor. C-C bond length 154 pm. Rigid 3D network structure.
   • Graphite: Layered structure. Layers held by weak van der Waals forces. Within layers, C atoms are sp² hybridized, forming hexagonal rings. C-C bond length 141.5 pm. Fourth valence electron is delocalized (π-system) within the layer, making graphite a good conductor of electricity. Soft, lubricant. Thermodynamically most stable allotrope.
   • Fullerenes: Cage-like molecules (e.g., C₆₀ - Buckminsterfullerene). sp² hybridized carbon atoms. Spherical structure with 20 six-membered rings and 12 five-membered rings. Aromatic character. C-C distances are 143.5 pm and 138.3 pm.
   • Other allotropes: Carbon black, coke, charcoal (amorphous forms); Graphene (single layer of graphite).
3. Reactivity:
   • Generally less reactive than Group 13.
   • Towards Oxygen: Form monoxides (MO) and dioxides (MO₂). CO₂, SiO₂, GeO₂ are acidic; SnO₂, PbO₂ are amphoteric. CO is neutral. Stability of higher oxides decreases down the group.
   • Towards Water: C, Si, Ge are unaffected. Sn decomposes steam (Sn + 2H₂O(steam) → SnO₂ + 2H₂). Pb is unaffected (protective oxide layer).
   • Towards Halogens: Form tetrahalides (MX₄). Stability decreases down the group (PbCl₄ decomposes easily). Except C, others react directly. CCl₄ is resistant to hydrolysis due to lack of d-orbitals on C. SiCl₄ is readily hydrolysed (SiCl₄ + 4H₂O → Si(OH)₄ (Silicic acid) + 4HCl) due to the presence of d-orbitals on Si which can accept lone pairs from water. Heavier halides (GeX₄, SnX₄, PbX₄) are also hydrolysed. Dihalides (MX₂) stability increases down the group (inert pair effect).
4. Important Compounds of Carbon & Silicon:
   • Carbon Monoxide (CO): Colourless, odourless, toxic gas. Powerful reducing agent (used in metallurgy). Forms metal carbonyls [e.g., Ni(CO)₄]. Lewis base (C≡O:). Prepared by incomplete combustion or dehydration of formic acid with conc. H₂SO₄.
   • Carbon Dioxide (CO₂): Colourless, odourless gas. Acidic oxide. Used in photosynthesis, fire extinguishers, dry ice (solid CO₂). Linear molecule (sp hybridization). Exists as discrete molecules.
   • Silicon Dioxide (Silica, SiO₂): Covalent, 3D network solid. Each Si is bonded tetrahedrally to 4 oxygen atoms, and each oxygen is bonded to 2 Si atoms (Si-O-Si linkage). High melting point. Exists in various crystalline forms (quartz, cristobalite, tridymite). Acidic oxide (reacts with NaOH/HF). HF is used for etching glass (SiO₂ + 4HF → SiF₄ + 2H₂O). Unlike CO₂, SiO₂ is a solid network structure.
   • Silicones: Organosilicon polymers containing R₂SiO repeating units. General formula (R₂SiO)n. Have -Si-O-Si- linkages. Prepared by hydrolysis of alkyl or aryl substituted chlorosilanes (e.g., R₂SiCl₂) followed by polymerization. Water repellent, chemically inert, heat resistant, good electrical insulators. Used as sealants, greases, electrical insulators, water-proofing agents.
   • Silicates: Basic structural unit is the tetrahedral SiO₄⁴⁻ anion. Si is sp³ hybridized. Silicates involve sharing of oxygen atoms between tetrahedra. Types include:
     • Orthosilicates: Discrete SiO₄⁴⁻ units (e.g., Willemite Zn₂SiO₄).
     • Pyrosilicates: Two units share one oxygen (Si₂O₇⁶⁻).
     • Cyclic Silicates: (SiO₃)n²ⁿ⁻ units (e.g., Beryl Be₃Al₂Si₆O₁₈).
     • Chain Silicates (Pyroxenes): Single chains (SiO₃)n²ⁿ⁻; Double chains (Amphiboles): (Si₄O₁₁)n⁶ⁿ⁻.
     • Sheet Silicates (Phyllosilicates): 2D sheets (Si₂O₅)n²ⁿ⁻ (e.g., Talc, Mica).
     • Framework Silicates (Tectosilicates): 3D network (SiO₂)n (e.g., Quartz, Feldspar, Zeolites).
   • Zeolites: Microporous aluminosilicates (framework silicates where some Si⁴⁺ ions are replaced by Al³⁺ ions). General formula: Mₓ/ₙ[(AlO₂)ₓ(SiO₂)y]·zH₂O. Have cage-like structures. Used as catalysts in petrochemical industries (e.g., ZSM-5 for converting alcohols to gasoline), ion exchangers (water softening), and adsorbents.

Multiple Choice Questions (MCQs):

1. The reason for the lower atomic radius of Ga compared to Al is:
   (a) Lanthanoid contraction
   (b) Greater screening effect of d-electrons in Ga
   (c) Poor screening effect of d-electrons in Ga
   (d) Inert pair effect

2. Which of the following statements about Boron is incorrect?
   (a) It forms only covalent compounds.
   (b) It exhibits a maximum covalency of 4.
   (c) Its oxide (B₂O₃) is acidic.
   (d) It readily forms B³⁺ ions.

3. Diborane (B₂H₆) has:
   (a) Four 2c-2e bonds and two 3c-2e bonds
   (b) Six 2c-2e bonds
   (c) Two 2c-2e bonds and four 3c-2e bonds
   (d) Four 2c-2e bonds and four 3c-2e bonds

4. The stability of +1 oxidation state among Al, Ga, In, and Tl increases in the sequence:
   (a) Al < Ga < In < Tl
   (b) Tl < In < Ga < Al
   (c) Ga < In < Al < Tl
   (d) Al < Ga < Tl < In

5. Which allotrope of carbon is thermodynamically the most stable?
   (a) Diamond
   (b) Graphite
   (c) Fullerene (C₆₀)
   (d) Carbon black

6. Which of the following halides is most readily hydrolysed?
   (a) CCl₄
   (b) SiCl₄
   (c) GeCl₄
   (d) SnCl₄

7. The basic structural unit in silicates is:
   (a) SiO₃²⁻
   (b) SiO₄²⁻
   (c) SiO₄⁴⁻
   (d) Si₂O₇⁶⁻

8. Which of the following is known as 'inorganic benzene'?
   (a) Borax (Na₂B₄O₇·10H₂O)
   (b) Diborane (B₂H₆)
   (c) Boron nitride (BN)
   (d) Borazine (B₃N₃H₆)

9. Silicones are polymers with repeating units having:
   (a) Si-C linkages
   (b) Si-Si linkages
   (c) Si-N linkages
   (d) Si-O-Si linkages

10. ZSM-5, a type of zeolite, is used as a catalyst in:
    (a) Haber's process
    (b) Polymerization of ethene
    (c) Conversion of alcohols to gasoline
    (d) Ostwald's process

Answer Key for MCQs:

1. (c)
2. (d)
3. (a)
4. (a)
5. (b)
6. (b) (While others are also hydrolysed, SiCl₄ is readily hydrolysed, whereas CCl₄ is resistant).
7. (c)
8. (d)
9. (d)
10. (c)

Study these notes thoroughly, focusing on the trends, anomalies, structures (especially Diborane, Graphite, Diamond, Silicates), and the properties of important compounds like Borax, Boric Acid, Silicones, and Zeolites. Understanding the 'inert pair effect' and Lewis acid character is also vital. Good luck with your preparation!