Class 11 Chemistry Notes Chapter 10 (Chapter 10) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 10: The s-Block Elements from your NCERT Exemplar. This is a crucial chapter for various government exams as questions frequently test the trends, properties, and reactions of these elements and their compounds. Pay close attention to the details and exceptions.

Chapter 10: The s-Block Elements - Detailed Notes for Exam Preparation

1. Introduction

• Definition: Elements in which the last electron enters the outermost s-orbital.
• Groups: Includes Group 1 (Alkali Metals) and Group 2 (Alkaline Earth Metals).
• Position in Periodic Table: Extreme left side.
• General Electronic Configuration:
  • Group 1: [Noble gas] ns¹
  • Group 2: [Noble gas] ns²
• General Characteristics:
  • Highly reactive metals (reactivity increases down the group).
  • Low ionization enthalpies.
  • Large atomic and ionic radii (increase down the group).
  • Highly electropositive character.
  • Form predominantly ionic compounds (except Li and Be compounds which show covalent character).
  • Good reducing agents.
  • Exhibit characteristic flame colours (except Be and Mg).
  • Soft metals with low densities, melting points, and boiling points (compared to other metals).

2. Group 1: Alkali Metals (Li, Na, K, Rb, Cs, Fr)

• Physical Properties:

  • Atomic/Ionic Radii: Increase down the group (addition of a new shell). Cations are smaller than parent atoms.
  • Ionization Enthalpy (IE): Very low. Decreases down the group (due to increasing size and screening effect). Second IE is very high (removal from stable noble gas core).
  • Hydration Enthalpy: Decreases down the group (Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺). Smaller ions have higher charge density and are more heavily hydrated. This high hydration enthalpy makes Li⁺ salts often hydrated (e.g., LiCl·2H₂O) while other alkali metal chlorides are anhydrous. It also contributes to Li⁺ having the most negative E° value (strongest reducing agent in aqueous solution).
  • Electronegativity: Low, decreases down the group.
  • Melting & Boiling Points: Low, decrease down the group (weak metallic bonding due to large size and only one valence electron).
  • Density: Generally low and increase down the group (Li, Na, K are lighter than water). Exception: K is lighter than Na due to an unusual increase in atomic size from Na to K.
  • Flame Coloration: Impart characteristic colours to the flame due to easy excitation of the outer electron to higher energy levels.
    • Li: Crimson Red
    • Na: Golden Yellow
    • K: Pale Violet (Lilac)
    • Rb: Red Violet
    • Cs: Blue Violet
  • Appearance: Silvery-white, soft (can be cut with a knife), and light metals.

• Chemical Properties:

  • Reactivity: Highly reactive due to low IE. Reactivity increases down the group. Stored under kerosene or paraffin oil to prevent reaction with air/moisture.
  • Reaction with Air/Oxygen: Tarnish rapidly. Form different oxides depending on the metal:
    • Li: Forms mainly oxide (Li₂O) and some nitride (Li₃N) directly with N₂ of air.
    • Na: Forms mainly peroxide (Na₂O₂) and some oxide (Na₂O).
    • K, Rb, Cs: Form mainly superoxides (KO₂, RbO₂, CsO₂).
    • Oxides: Basic (Li₂O, Na₂O)
    • Peroxides: Contain O₂²⁻ ion.
    • Superoxides: Contain O₂⁻ ion (paramagnetic). Stability of peroxide/superoxide increases down the group due to stabilization of larger anions by larger cations (lattice energy effect).
  • Reaction with Water: React vigorously (explosively for lower members) to form hydroxides and liberate H₂ gas. Reactivity increases down the group.
    • 2M + 2H₂O → 2MOH + H₂ (M = Alkali metal)
    • Hydroxides (MOH) are strong bases. Basicity increases down the group (LiOH < NaOH < KOH < RbOH < CsOH).
  • Reaction with Dihydrogen (H₂): Form ionic hydrides (M⁺H⁻) when heated with H₂. Stability decreases down the group. These are strong reducing agents.
    • 2M + H₂ → 2MH
  • Reaction with Halogens (X₂): React vigorously to form ionic halides (M⁺X⁻). Reactivity decreases F₂ > Cl₂ > Br₂ > I₂. All halides are high melting, colourless crystalline solids, typically soluble in water (except LiF due to high lattice energy; CsI due to low hydration energy). Li halides (except LiF) show some covalent character and are soluble in organic solvents.
  • Reducing Nature: Strong reducing agents. Li is the strongest reducing agent in aqueous solution (most negative E° value: -3.04 V) due to its very high hydration enthalpy, despite having the highest IE. Reducing power generally increases down the group in the gaseous state (opposite of E° trend).
  • Solutions in Liquid Ammonia: Dissolve to give deep blue, conducting, and paramagnetic solutions.
    • M + (x+y)NH₃ → [M(NH₃)ₓ]⁺ + [e(NH₃)y]⁻ (Ammoniated cation and ammoniated electron)
    • The blue colour is due to the ammoniated electron which absorbs energy in the visible region.
    • The solution is paramagnetic due to the unpaired ammoniated electron.
    • On standing, the solution slowly liberates H₂, forming amide: 2M + 2NH₃ → 2MNH₂ + H₂
    • At higher concentrations (>3M), the solution becomes bronze-coloured and diamagnetic due to the formation of electron pairs/clusters.

• Anomalous Behavior of Lithium: Differs from other alkali metals due to:

  • Exceptionally small size of atom and ion.
  • High polarizing power (charge/radius ratio).
  • High IE and electronegativity compared to others.
  • Absence of d-orbitals.
  • Points of Difference: Harder, higher MP/BP, least reactive but strongest reducing agent in solution, forms monoxide & nitride with air, LiCl is deliquescent and soluble in ethanol/acetone (covalent character), LiHCO₃ does not exist as solid, LiNO₃ decomposes to Li₂O (unlike other nitrates forming nitrites), Li₂CO₃ and LiOH are less stable to heat.

• Diagonal Relationship (Li and Mg): Li shows similarities to Mg due to similar ionic size (Li⁺ ≈ 0.76 Å, Mg²⁺ ≈ 0.72 Å) and charge/radius ratio (polarizing power).

  • Similarities: Harder than other Group 1 metals, form nitrides directly, oxides/hydroxides are less soluble, carbonates decompose easily on heating, form covalent organometallic compounds, chlorides are deliquescent and soluble in ethanol.

• Important Compounds of Sodium:

  • Sodium Carbonate (Washing Soda, Na₂CO₃·10H₂O):
    • Preparation: Solvay Process (Ammonia-Soda Process). Key steps:
      1. Ammoniation of brine: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃(s) + NH₄Cl
      2. Filtration of NaHCO₃ (sparingly soluble).
      3. Calcination: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
      4. Recovery of NH₃: 2NH₄Cl + Ca(OH)₂ → 2NH₃ + CaCl₂ + 2H₂O
         Note: K₂CO₃ cannot be made by Solvay process as KHCO₃ is too soluble.
    • Properties: White crystalline solid, efflorescent (loses water of crystallization), aqueous solution is alkaline (hydrolysis: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻).
    • Uses: Water softening, laundering, cleaning, manufacturing glass, soap, borax, caustic soda.
  • Sodium Chloride (Common Salt, NaCl):
    • Source: Sea water (evaporation), rock salt deposits.
    • Properties: High melting ionic solid, slightly hygroscopic due to impurities (MgCl₂, CaCl₂).
    • Uses: Essential dietary component, preservative, source of Na, Cl₂, NaOH, Na₂CO₃.
  • Sodium Hydroxide (Caustic Soda, NaOH):
    • Preparation: Castner-Kellner Process (electrolysis of brine using mercury cathode).
      • Cathode (Hg): Na⁺ + e⁻ → Na(amalgam)
      • Anode (Graphite): Cl⁻ → ½ Cl₂ + e⁻
      • Amalgam + Water: 2Na(Hg) + 2H₂O → 2NaOH + H₂ + 2Hg
    • Properties: White, translucent, deliquescent solid. Highly corrosive. Strong base. Absorbs CO₂ from air (forms Na₂CO₃).
    • Uses: Manufacturing soap, paper, artificial silk (rayon), petroleum refining, bauxite purification, laboratory reagent.
  • Sodium Bicarbonate (Baking Soda, NaHCO₃):
    • Preparation: Saturating Na₂CO₃ solution with CO₂ OR intermediate in Solvay process.
      • Na₂CO₃ + H₂O + CO₂ → 2NaHCO₃
    • Properties: White crystalline powder, sparingly soluble, decomposes on heating (~100°C) releasing CO₂. Aqueous solution is mildly alkaline.
    • Uses: Baking powder (along with an acidic substance like tartaric acid), effervescent drinks, fire extinguishers (releases CO₂), mild antiseptic.

• Biological Importance:

  • Na⁺: Principal cation in extracellular fluids. Involved in nerve signal transmission, regulating water flow across membranes, transport of sugars and amino acids into cells.
  • K⁺: Principal cation within cells. Activates many enzymes, participates in glucose oxidation (ATP production), nerve signal transmission.
  • Sodium-Potassium Pump: Biological process using ATP to pump 3 Na⁺ ions out of the cell for every 2 K⁺ ions pumped in, maintaining concentration gradients crucial for nerve and muscle function.

3. Group 2: Alkaline Earth Metals (Be, Mg, Ca, Sr, Ba, Ra)

• Physical Properties:

  • Atomic/Ionic Radii: Smaller than corresponding alkali metals (due to higher nuclear charge). Increase down the group. Cations (M²⁺) are smaller than parent atoms.
  • Ionization Enthalpy (IE): Higher than alkali metals but still relatively low. IE₁ > IE₁ (Group 1); IE₂ < IE₂ (Group 1). IE decreases down the group.
  • Hydration Enthalpy: Higher than alkali metals (due to smaller size and higher charge). Decreases down the group (Be²⁺ > Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺). High hydration enthalpy of Be²⁺ and Mg²⁺ leads to hydrated salts (e.g., MgCl₂·6H₂O, CaCl₂·6H₂O).
  • Electronegativity: Low, decrease down the group (higher than alkali metals).
  • Melting & Boiling Points: Higher than alkali metals (stronger metallic bonding due to two valence electrons). Trend is not regular (Mg has lowest MP/BP).
  • Density: Low, generally increase down the group with some irregularities (Ca < Mg < Be).
  • Flame Coloration: Impart characteristic colours (except Be, Mg - electrons too tightly bound).
    • Ca: Brick Red
    • Sr: Crimson Red
    • Ba: Apple Green
  • Appearance: Silvery-white, harder than alkali metals.

• Chemical Properties:

  • Reactivity: Less reactive than alkali metals but still quite reactive. Reactivity increases down the group (Be < Mg < Ca < Sr < Ba).
  • Reaction with Air/Oxygen: Less rapid tarnishing than alkali metals. Form oxides (MO) and nitrides (M₃N₂) on heating in air (except Be).
    • 2M + O₂ → 2MO
    • 3M + N₂ → M₃N₂
    • Be is relatively unreactive towards air/water due to a protective oxide layer.
  • Reaction with Water: React with water (less vigorously than alkali metals) to form hydroxides and H₂. Reactivity increases down the group.
    • M + 2H₂O → M(OH)₂ + H₂
    • Be does not react with cold water. Mg reacts with hot water. Ca, Sr, Ba react readily with cold water.
    • Hydroxides M(OH)₂: Basicity increases down the group (Be(OH)₂ < Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂). Be(OH)₂ is amphoteric. Solubility also increases down the group.
  • Reaction with Dihydrogen (H₂): Form hydrides (MH₂) on heating (except Be). BeH₂ is prepared indirectly. CaH₂, SrH₂, BaH₂ are ionic; BeH₂ and MgH₂ are covalent (polymeric).
  • Reaction with Halogens (X₂): React readily to form halides (MX₂). BeF₂ is prepared differently. Be halides are covalent; others are ionic but MgCl₂ shows some covalent character. Ionic character increases down the group. Covalent BeCl₂ exists as a dimer/polymer in solid state.
  • Reducing Nature: Strong reducing agents, but weaker than alkali metals. Reducing strength (E° values become more negative) increases down the group (Be has less negative E° due to high atomization enthalpy and IE).
  • Solutions in Liquid Ammonia: Like alkali metals, dissolve to give deep blue-black solutions containing ammoniated cations and electrons.

• Anomalous Behavior of Beryllium: Differs significantly from other Group 2 elements due to:

  • Very small atomic and ionic size.
  • High IE and electronegativity.
  • High polarizing power.
  • Absence of d-orbitals.
  • Points of Difference: Highest MP/BP, does not react with water easily, oxide (BeO) and hydroxide (Be(OH)₂) are amphoteric, forms covalent compounds (e.g., BeCl₂), halides are hygroscopic and fume in air (hydrolysis), does not exhibit coordination number more than 4 (no d-orbitals), salts rarely hydrated beyond BeSO₄·4H₂O.

• Diagonal Relationship (Be and Al): Be shows similarities to Al (Group 13).

  • Similarities: Similar electronegativity, similar charge/radius ratio. Both metals form passive oxide layers, are amphoteric (oxides/hydroxides dissolve in acid and alkali), form covalent halides (dimeric structures, Lewis acids), form covalent alkyls, carbides react with water to give methane (Be₂C, Al₄C₃).

• Important Compounds of Calcium:

  • Calcium Oxide (Quicklime, CaO):
    • Preparation: Heating limestone (CaCO₃) strongly (>1000 K) in a rotary kiln (reversible reaction, CO₂ removed).
      • CaCO₃(s) ⇌ CaO(s) + CO₂(g)
    • Properties: White amorphous solid, high MP, basic oxide. Reacts exothermically with water (slaking): CaO + H₂O → Ca(OH)₂. Reacts with acidic oxides (e.g., SiO₂) and acids. Absorbs moisture and CO₂ from air.
    • Uses: Manufacturing cement, mortar, glass, CaC₂, NaOH (from Na₂CO₃), sugar purification, drying agent (basic).
  • Calcium Hydroxide (Slaked Lime, Ca(OH)₂):
    • Preparation: Slaking of quicklime (CaO + H₂O).
    • Properties: White amorphous powder, sparingly soluble in water. Aqueous solution is 'limewater', suspension is 'milk of lime'. Reacts with CO₂ to form CaCO₃ (milky), excess CO₂ forms soluble Ca(HCO₃)₂. Reacts with Cl₂ to form bleaching powder Ca(OCl)₂·CaCl₂·Ca(OH)₂·2H₂O.
    • Uses: Mortar (building material), whitewashing, glass making, tanning industry, sugar purification, bleaching powder manufacture.
  • Calcium Sulphate (CaSO₄):
    • Occurrence: Gypsum (CaSO₄·2H₂O), Anhydrite (CaSO₄).
    • Plaster of Paris (POP, CaSO₄·½H₂O or (CaSO₄)₂·H₂O):
      • Preparation: Heating gypsum carefully to ~393 K (120°C).
        • 2(CaSO₄·2H₂O) → 2(CaSO₄·½H₂O) + 3H₂O
      • Note: Heating above 393 K gives anhydrous CaSO₄ ('dead burnt plaster') which does not set.
      • Properties: White powder. Sets into a hard mass on mixing with water (rehydration back to gypsum).
        • CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (Setting)
      • Uses: Making casts for statues, pottery, surgical bandages (setting fractured bones), decorative plasterwork.
  • Calcium Carbonate (CaCO₃):
    • Occurrence: Limestone, marble, chalk, coral, calcite, aragonite.
    • Preparation: Passing CO₂ through limewater OR adding Na₂CO₃ to CaCl₂ solution.
      • Ca(OH)₂ + CO₂ → CaCO₃(s) + H₂O
      • CaCl₂ + Na₂CO₃ → CaCO₃(s) + 2NaCl
    • Properties: White fluffy powder or crystalline solid, insoluble in water. Decomposes on heating (~1200 K). Reacts with dilute acids to give CO₂.
      • CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
    • Uses: Building material (marble), quicklime manufacture, flux in metallurgy (removes SiO₂ impurity), antacid, filler in cosmetics, toothpaste, chewing gum, high-quality paper manufacture.

• Cement: Complex mixture, primarily calcium silicates and aluminates.

  • Raw Materials: Limestone (provides CaO) and Clay (provides SiO₂, Al₂O₃, Fe₂O₃).
  • Composition (approx): CaO (50-60%), SiO₂ (20-25%), Al₂O₃ (5-10%), MgO (2-3%), Fe₂O₃ (1-2%), SO₃ (1-2%). Good quality cement has SiO₂/Al₂O₃ ratio ~2.5-4 and CaO/(SiO₂+Al₂O₃+Fe₂O₃) ratio ~2.
  • Manufacture: Heating raw materials strongly (~1770-1870 K) in a rotary kiln produces 'cement clinker'. Clinker is cooled and mixed with 2-3% gypsum (CaSO₄·2H₂O) - gypsum controls the setting time.
  • Setting of Cement: Complex hydration reactions. When mixed with water, silicates and aluminates hydrate forming a hard gel-like mass with interlocking crystals (hydrated silicates/aluminates). Gypsum slows down the initial rapid setting caused by tricalcium aluminate (C₃A) hydration.

• Biological Importance:

  • Mg²⁺: Concentrated in animal cells, cofactor for enzymes using ATP (phosphate transfer), essential constituent of chlorophyll (in plants).
  • Ca²⁺: Major component of bones and teeth (as apatite, Ca₃(PO₄)₂). Important in extracellular fluids, blood clotting, muscle contraction, nerve function, maintaining regular heartbeat.

Multiple Choice Questions (MCQs)

1. Which alkali metal forms nitride directly upon reaction with air?
   (a) Na
   (b) K
   (c) Li
   (d) Cs

2. The decreasing order of hydration enthalpy among alkali metal ions is:
   (a) Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺
   (b) Cs⁺ > Rb⁺ > K⁺ > Na⁺ > Li⁺
   (c) Na⁺ > K⁺ > Li⁺ > Rb⁺ > Cs⁺
   (d) K⁺ > Na⁺ > Li⁺ > Rb⁺ > Cs⁺

3. KO₂ (Potassium superoxide) is used in oxygen cylinders in submarines and space because it:
   (a) Absorbs CO₂ and increases O₂ content
   (b) Eliminates moisture
   (c) Absorbs CO₂
   (d) Produces Ozone

4. Which of the following alkaline earth metal hydroxides is amphoteric in nature?
   (a) Ca(OH)₂
   (b) Sr(OH)₂
   (c) Be(OH)₂
   (d) Ba(OH)₂

5. The formula for Plaster of Paris is:
   (a) CaSO₄·2H₂O
   (b) CaSO₄·H₂O
   (c) 2CaSO₄·H₂O
   (d) CaSO₄

6. In the Solvay process for manufacturing sodium carbonate, the raw materials used are:
   (a) NaCl, NH₃, CaCO₃
   (b) NaOH, CO₂, NH₃
   (c) NaCl, CaO, NH₃
   (d) NaHCO₃, NH₃, CaCl₂

7. Which property of Lithium is responsible for its strong reducing character in aqueous solution?
   (a) Low ionization enthalpy
   (b) High lattice energy
   (c) High hydration enthalpy
   (d) Small ionic size

8. Beryllium shows a diagonal relationship with:
   (a) Sodium (Na)
   (b) Magnesium (Mg)
   (c) Boron (B)
   (d) Aluminium (Al)

9. The setting of cement is primarily due to:
   (a) Dehydration process
   (b) Hydration of calcium aluminates and silicates
   (c) Oxidation of components
   (d) Reaction with atmospheric CO₂

10. Which ion plays a crucial role as a cofactor for ATP-utilizing enzymes and is also found in chlorophyll?
    (a) Na⁺
    (b) K⁺
    (c) Mg²⁺
    (d) Ca²⁺

Answer Key:

1. (c)
2. (a)
3. (a) (Reaction: 4KO₂(s) + 2CO₂(g) → 2K₂CO₃(s) + 3O₂(g))
4. (c)
5. (c) (Often written as CaSO₄·½H₂O, but 2CaSO₄·H₂O represents the hemihydrate stoichiometry more accurately)
6. (a) (Brine (NaCl), Ammonia (NH₃), Limestone (CaCO₃) which provides CO₂ and Ca(OH)₂ for ammonia recovery)
7. (c)
8. (d)
9. (b)
10. (c)

Study these notes thoroughly, focusing on the trends, exceptions, and specific reactions mentioned. Practice more questions from the Exemplar book itself to solidify your understanding. Good luck with your preparation!