The s-Block Elements — NCERT Class 11 Chemistry

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This page provides complete, step-by-step NCERT solutions for CBSE Class 11 Chemistry Chapter 10 — The s-Block Elements. Every intext question (grouped by page number) and every chapter-end exercise question is answered with clear explanations, chemical equations, and trend analyses. Use the section headings to jump to the question you need help with.

Key Concepts (Quick Recap)

• Alkali Metals (Group 1): Li, Na, K, Rb, Cs, Fr. ns¹ configuration, highly reactive, low ionization enthalpy, form M⁺ ions, stored under kerosene.
• Alkaline Earth Metals (Group 2): Be, Mg, Ca, Sr, Ba, Ra. ns² configuration, less reactive than Group 1, form M²⁺ ions, harder and denser than alkali metals.
• Trends Down the Group: Atomic/ionic radius increases, ionization enthalpy decreases, reactivity increases, melting/boiling points decrease (alkali metals), electropositive character increases.
• Anomalous Behaviour of Li: Small size, high polarizing power, forms covalent compounds, hardest metal in Group 1, highest IE, forms nitride (Li₃N) with N₂.
• Anomalous Behaviour of Be: Small size, high charge/size ratio, amphoteric oxide (BeO), covalent compounds, does not form peroxide or superoxide, does not impart colour to flame.
• Diagonal Relationships: Li–Mg (similar ionic radius, carbonates decompose on heating, form nitrides, hydroxides less soluble). Be–Al (amphoteric oxides, form covalent halides, passivate with HNO₃).
• Important Compounds of Na: NaOH (caustic soda), Na₂CO₃ (washing soda), NaHCO₃ (baking soda), NaCl (common salt).
• Important Compounds of Ca: CaO (quicklime), Ca(OH)₂ (slaked lime), CaCO₃ (limestone), CaSO₄·2H₂O (gypsum), CaSO₄·½H₂O (Plaster of Paris).
• Biological Importance: Na⁺ and K⁺ (nerve impulse, fluid balance), Mg²⁺ (chlorophyll, enzymes), Ca²⁺ (bones, teeth, blood clotting, muscle contraction).
• Cement: Mixture of limestone (CaCO₃) and clay (aluminosilicates). Composition: CaO (60–67%), SiO₂ (17–25%), Al₂O₃ (3–8%), Fe₂O₃ (0.5–6%).

Intext Questions

Page 244–245

• The atomic radius increases (additional shells are added)
• The valence electron is farther from the nucleus
• The shielding effect of inner electrons increases
• The effective nuclear charge on the valence electron decreases

As a result, less energy is required to remove the outermost electron, so ionization enthalpy decreases: Li > Na > K > Rb > Cs.

• Only one electron per atom contributes to the electron sea
• Large atomic radii mean the metallic bonds are longer and weaker
• The bond strength decreases down the group

Weak metallic bonding results in softness and low melting/boiling points. In fact, Cs and Rb are so soft they can be cut with a knife.

• With oxygen: Form different oxides depending on the metal.
• Li → Li₂O (oxide)
• Na → Na₂O₂ (peroxide)
• K, Rb, Cs → MO₂ (superoxide)

2M + 2H₂O → 2MOH + H₂↑

• With moisture: React vigorously with atmospheric moisture.

• With nitrogen (Li only): 6Li + N₂ → 2Li₃N

The reactivity increases down the group: Li < Na < K < Rb < Cs.

Page 252–254

• Anode: Carbon
• Cathode: Mercury (Hg)
• Electrolyte: Brine (concentrated NaCl solution)

The process occurs in three compartments:

At the cathode (Hg): Na⁺ + e⁻ → Na (deposited on Hg as sodium amalgam, Na-Hg)

At the anode: 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas liberated)

The sodium amalgam reacts with water in a separate compartment: 2Na-Hg + 2H₂O → 2NaOH + H₂ + 2Hg

Mercury is recycled. This process gives very pure NaOH.

Q7. Write the chemical formula of: (a) Washing soda (b) Baking soda (c) Quicklime (d) Slaked lime

(a) Washing soda: Na₂CO₃·10H₂O (sodium carbonate decahydrate)

(b) Baking soda: NaHCO₃ (sodium hydrogen carbonate / sodium bicarbonate)

(c) Quicklime: CaO (calcium oxide)

(d) Slaked lime: Ca(OH)₂ (calcium hydroxide)

1. Ammoniation of brine: NaCl + NH₃ + H₂O → NH₄Cl + NaOH (partial)

2. Carbonation: NH₃ + H₂O + CO₂ → NH₄HCO₃ NaCl + NH₄HCO₃ → NaHCO₃↓ + NH₄Cl

3. Heating: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

4. Recovery of NH₃: 2NH₄Cl + Ca(OH)₂ → 2NH₃ + CaCl₂ + H₂O

Raw materials: NaCl, CaCO₃ (lime stone), NH₃. CO₂ is obtained from: CaCO₃ → CaO + CO₂.

Page 264–265

• The size of the M²⁺ ion increases
• The lattice enthalpy of the hydroxide decreases (larger ion = weaker lattice)
• The hydration enthalpy also decreases, but the decrease in lattice enthalpy is more significant
• The net result is that the dissolution becomes more favourable

Hence, solubility increases: Be(OH)₂ < Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂

• Down the group, the cation size increases (Be²⁺ → Ba²⁺)
• The lattice enthalpy decreases but the hydration enthalpy decreases more rapidly
• Since the anion (CO₃²⁻ or SO₄²⁻) is large, the decrease in hydration enthalpy of the cation dominates
• The overall free energy change becomes less negative

Hence, solubility decreases: BeCO₃ > MgCO₃ > CaCO₃ > SrCO₃ > BaCO₃ (same trend for sulfates).

Preparation: CaCO₃(s) →(900–1000 °C) CaO(s) + CO₂(g) (calcination of limestone)

Uses: 1. Manufacture of cement and mortar 2. Manufacture of Ca(OH)₂ (slaked lime) 3. As a drying agent (desiccant) 4. In steel manufacture (removing impurities) 5. In the manufacture of sugar 6. As a refractory material (lined furnaces) 7. In soil treatment (reducing acidity)

Preparation: CaSO₄·2H₂O (gypsum) →(120–130 °C) CaSO₄·½H₂O + 1½H₂O

When gypsum is heated carefully at 120–130 °C, it loses 1.5 molecules of water to form Plaster of Paris. If heated above 200 °C, it forms dead-burnt plaster (anhydrous CaSO₄), which does not set on mixing with water.

Uses: Surgical casts, casts for statues, decorative work, fire-proofing, dentistry.

Chapter-End Exercise

• Atomic size increases
• Ionization enthalpy decreases (valence electron is farther from nucleus, more shielded)
• The energy required to lose the valence electron decreases
• It becomes easier to form M⁺ ions

Hence, reactivity increases from Li to Cs: Li < Na < K < Rb < Cs.

Q4. Compare the alkali metals and alkaline earth metals with respect to: (a) Ionization enthalpy (b) Basicity of oxides (c) Solubility of hydroxides

(a) Ionization enthalpy: Group 2 metals have higher IE than Group 1 metals because of smaller size and higher nuclear charge. Both groups show decreasing IE down the group.

(b) Basicity of oxides: Group 2 oxides are less basic than Group 1 oxides. Both increase in basicity down the group. BeO is amphoteric; others are basic.

(c) Solubility of hydroxides: Group 1 hydroxides are more soluble than Group 2 hydroxides. For Group 2, solubility increases down the group: Be(OH)₂ < Mg(OH)₂ < Ca(OH)₂ < Ba(OH)₂.

• Extremely small size
• High charge/size ratio (high polarizing power)
• High ionization enthalpy (highest in Group 1)

Examples of anomalous behaviour:

1. Harder and lighter than other alkali metals 2. Forms covalent compounds — LiCl is covalent (partially), others are ionic 3. Reacts with nitrogen — 6Li + N₂ → 2Li₃N (no other alkali metal does this at room temperature) 4. Oxide — Li forms normal oxide (Li₂O), not peroxide or superoxide 5. Less reactive with water — Li reacts steadily, others react violently 6. Carbonate decomposes on heating — Li₂CO₃ → Li₂O + CO₂ (others are stable) 7. Hydroxide decomposes on heating — 2LiOH → Li₂O + H₂O (others are stable)

1. Both form normal oxides (Li₂O, MgO) — not peroxides or superoxides 2. Both form nitrides with N₂: 6Li + N₂ → 2Li₃N; 3Mg + N₂ → Mg₃N₂ 3. Both carbonates decompose on heating: Li₂CO₃ → Li₂O + CO₂; MgCO₃ → MgO + CO₂ 4. Both hydroxides are weak bases and decompose on heating 5. Both fluorides, phosphates, and oxalates are sparingly soluble 6. Both form organometallic compounds (Li is widely used in organolithium chemistry, similar to Grignard reagents of Mg) 7. Both have similar polarizing power due to comparable ionic radii

• Very small size
• High charge/size ratio
• High ionization enthalpy (highest in Group 2)

Anomalous properties:

1. Amphoteric oxide — BeO is amphoteric (reacts with both acid and base); other Group 2 oxides are basic 2. Covalent compounds — Be forms predominantly covalent compounds (BeCl₂ is covalent) 3. No peroxide or superoxide — Be does not form peroxide or superoxide 4. Does not impart colour to flame — Be has no low-energy excited states for flame test 5. Does not form hydrated salts easily — due to very high hydration energy but small size limits coordination 6. Coordination number limited to 4 — Be compounds have max coordination 4 7. Does not react with water — even at red heat

1. Both have amphoteric oxides — BeO and Al₂O₃ dissolve in both acids and bases 2. Both form covalent halides — BeCl₂ and AlCl₃ are covalent 3. Both resist acid attack — passivated by concentrated HNO₃ 4. Both hydroxides are amphoteric — dissolve in both acid and base 5. Both carbonates are unstable — decompose on heating 6. Both form complexes — Be forms [BeF₄]²⁻; Al forms [AlF₆]³⁻

• The ionization enthalpy is very high (899 kJ/mol)
• The energy required to excite the electron is very high
• The emitted radiation lies in the UV region, not the visible region

Hence, Be does not impart any colour to the flame.

Q11. What happens when: (a) Sodium metal is dropped on water (b) Sodium metal is heated in free supply of air (c) Sodium peroxide dissolves in water

(a) 2Na + 2H₂O → 2NaOH + H₂↑ (vigorous, exothermic; H₂ may catch fire)

(b) 2Na + O₂ → Na₂O₂ (sodium peroxide, as sodium forms peroxide, not normal oxide)

(c) Na₂O₂ + 2H₂O → 2NaOH + H₂O₂ (sodium peroxide dissolves to give NaOH and H₂O₂)

Q12. What happens when: (a) Magnesium is burnt in air (b) Calcium is burnt in air

(a) 2Mg + O₂ → 2MgO (brilliant white flame, forms magnesium oxide) 3Mg + N₂ → Mg₃N₂ (also forms some magnesium nitride)

(b) 2Ca + O₂ → 2CaO (forms calcium oxide) Ca + H₂O (moisture in air) → CaO + H₂O → Ca(OH)₂ (also reacts with moisture)

Carbonates: Solubility decreases down the group. BeCO₃ > MgCO₃ > CaCO₃ > SrCO₃ > BaCO₃

Reason: Lattice enthalpy decreases, but hydration enthalpy decreases more rapidly. The large CO₃²⁻ anion means hydration enthalpy dominates the trend.

Hydroxides: Solubility increases down the group. Be(OH)₂ < Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂

Reason: Lattice enthalpy decreases more rapidly than hydration enthalpy for the smaller OH⁻ anion. The decrease in lattice enthalpy favours dissolution.

Q14. Describe the preparation and uses of: (a) Sodium hydroxide (b) Sodium carbonate (c) Quicklime

(a) Sodium hydroxide (NaOH):

2NaCl + 2H₂O → 2NaOH + H₂ + Cl₂

• Preparation: Electrolysis of brine (Castner-Kellner / membrane cell)
• Uses: Soap manufacture, paper industry, textile industry, petroleum refining, drain cleaner

(b) Sodium carbonate (Na₂CO₃):

• Preparation: Solvay process (from NaCl, CaCO₃, NH₃)
• Uses: Glass manufacture, soap/detergent, water softening, laundry

(c) Quicklime (CaO):

• Preparation: CaCO₃ →(heat) CaO + CO₂ (calcination of limestone)
• Uses: Cement manufacture, steel industry, soil treatment, drying agent, water treatment

Manufacture: 1. Mixing: Limestone (CaCO₃) and clay (aluminosilicate) are mixed in correct proportions 2. Heating: The mixture is heated in a rotary kiln at 1500–1600 °C

3. Clinker formation: 3CaO·SiO₂, 2CaO·SiO₂, 3CaO·Al₂O₃, 4CaO·Al₂O₃·Fe₂O₃ 4. Grinding: Clinker is ground with 2–3% gypsum (CaSO₄·2H₂O) to retard setting time

• CaCO₃ → CaO + CO₂
• CaO reacts with SiO₂, Al₂O₃, Fe₂O₃ to form clinker compounds

Composition: CaO (60–67%), SiO₂ (17–25%), Al₂O₃ (3–8%), Fe₂O₃ (0.5–6%)

• Na⁺: Major extracellular cation; maintains osmotic pressure, fluid balance, and acid-base balance; essential for nerve impulse transmission and muscle function
• K⁺: Major intracellular cation; crucial for nerve impulse conduction, heart function, and muscle contraction; activates enzymes; regulates cell membrane potential
• The Na⁺/K⁺ ATPase pump maintains the concentration gradient: 3 Na⁺ out and 2 K⁺ in per ATP hydrolysed
• Daily requirement: Na ≈ 2–5 g; K ≈ 2–6 g

Magnesium (Mg²⁺):

• Central atom in chlorophyll (essential for photosynthesis)
• Cofactor for many enzymes (especially ATP-utilizing enzymes)
• Required for DNA and RNA synthesis
• Helps in neuromuscular transmission
• Daily requirement: ~300 mg

Calcium (Ca²⁺):

• Major component of bones and teeth (as hydroxyapatite, Ca₅(PO₄)₃OH)
• Essential for blood clotting (factor IV)
• Required for muscle contraction (troponin-tropomyosin system)
• Important for nerve impulse transmission
• Activates enzymes (lipase, amylase)
• Regulates heart beat
• Daily requirement: ~800–1000 mg

The ammoniated electrons are responsible for the characteristic deep blue colour (dilute solutions) and bronze colour (concentrated solutions). The conductivity is due to the movement of both the ammoniated cations and electrons.

NaCl obtained from sea water contains small amounts of MgCl₂, CaSO₄, and other impurities. For pure NaCl, it is dissolved in water, filtered, and recrystallized. HCl gas is passed through the saturated solution to precipitate pure NaCl (common ion effect).

• They react with oxygen to form oxides/peroxides/superoxides
• They react with moisture to form hydroxides and release H₂ (which can ignite)
• Na and K can catch fire spontaneously in moist air

Kerosene provides an oxygen-free and moisture-free environment, preventing these reactions. Kerosene is chosen because alkali metals do not react with it (they are denser than kerosene and sink below its surface).

Q21. What is the effect of heat on: (a) Li₂CO₃ (b) Na₂CO₃ (c) MgCO₃ (d) CaCO₃

(a) Li₂CO₃ → Li₂O + CO₂↑ (decomposes on heating — anomalous, other Group 1 carbonates are stable)

(b) Na₂CO₃ — does not decompose on heating (stable up to its melting point)

(c) MgCO₃ → MgO + CO₂↑ (decomposes on heating)

(d) CaCO₃ → CaO + CO₂↑ (decomposes on heating above 900 °C)

Note: All Group 2 carbonates decompose on heating. Group 1 carbonates are stable except Li₂CO₃.

Q23. What happens when: (a) CaCO₃ is heated (b) CaO reacts with water (c) Ca(OH)₂ reacts with CO₂

(a) CaCO₃ →(heat) CaO + CO₂↑ (thermal decomposition — used in cement manufacture)

(b) CaO + H₂O → Ca(OH)₂ (slaking of lime, highly exothermic reaction)

(c) Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O (formation of insoluble calcium carbonate — this is why limewater turns milky when CO₂ is passed through it)

With excess CO₂: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (soluble — milky solution clears)

Deficiency causes: rickets (children), osteomalacia (adults), tetany, muscle cramps.

Q25. Compare the properties of Group 1 and Group 2 elements with respect to: (a) Electronic configuration (b) Nature of oxides (c) Flame colour

(a) Electronic configuration: Group 1 — ns¹; Group 2 — ns². Group 2 elements have one more proton and one more electron than the corresponding Group 1 element.

(b) Nature of oxides: Group 1 — all form basic oxides (Li₂O is the least basic); Group 2 — BeO is amphoteric, others form basic oxides (basicity increases down the group).

(c) Flame colour: Group 1 — Li (crimson red), Na (yellow), K (violet), Rb (red violet), Cs (blue); Group 2 — Be (no colour), Mg (no colour), Ca (brick red), Sr (crimson red), Ba (apple green).

Internal Links

• [Chapter 10 Guide — The s-Block Elements](/school/cbse/class/11/chemistry/cbse-s-block-elements)
• [All Class 11 Chemistry Solutions](/school/cbse/class/11/chemistry/solutions)
• [Class 11 Chemistry Subject Guide](/school/cbse/class/11/chemistry)