s block elements- Solve Exercise (Punjab Boards)

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Q4 (a):
Names, electronic configurations, and occurrence of s-block elements:

  • Elements: The s-block elements include Group 1 (alkali metals: Li, Na, K, Rb, Cs, Fr) and Group 2 (alkaline earth metals: Be, Mg, Ca, Sr, Ba, Ra).
  • Electronic configurations:
  • Alkali metals: General configuration is (ns^1).
    • Li: (1s2 2s1), Na: (1s^2 2s^2 2p^6 3s^1), etc.
  • Alkaline earth metals: General configuration is (ns^2).
    • Be: (1s^2 2s^2), Mg: (1s^2 2s^2 2p^6 3s^2), etc.
  • Occurrence: These elements are found in the Earth’s crust in minerals, salts, and ores (e.g., NaCl, CaCO₃, MgCO₃).

(b):
Peculiar behavior of lithium with respect to other alkali metals:

  • Lithium has smaller atomic and ionic size compared to other alkali metals.
  • It forms a stable oxide (Li₂O) and reacts with nitrogen to form lithium nitride (Li₃N).
  • Lithium salts have more covalent character due to its higher polarization ability.
  • It is less reactive with water than other alkali metals.
  • Shows diagonal relationship with magnesium, resembling its properties more.

Q5:
Trends in the chemical properties of oxides, hydroxides, carbonates, nitrates, and sulfates of Group IA (alkali metals) and Group IIA (alkaline earth metals):

  • Oxides: Alkali metal oxides (e.g., Li₂O) are basic, while alkaline earth metal oxides (e.g., MgO, CaO) are also basic but less soluble.
  • Hydroxides: Alkali metal hydroxides (e.g., NaOH) are strong bases, while alkaline earth metal hydroxides (e.g., Ca(OH)₂) are less soluble and weaker bases.
  • Carbonates: Alkali metal carbonates (e.g., Na₂CO₃) are soluble in water, while alkaline earth metal carbonates (e.g., CaCO₃) are insoluble.
  • Nitrates: Both alkali and alkaline earth nitrates (e.g., NaNO₃, Ca(NO₃)₂) decompose on heating to give oxides, but alkaline earth nitrates decompose more readily.
  • Sulfates: Alkali metal sulfates (e.g., Na₂SO₄) are soluble in water, while alkaline earth metal sulfates (e.g., BaSO₄) are less soluble.

Q6:
Comparison of chemical behavior of lithium with magnesium:

  • Both lithium and magnesium form oxides (Li₂O and MgO) and hydroxides (LiOH and Mg(OH)₂).
  • They form nitrides when reacted with nitrogen (Li₃N, Mg₃N₂).
  • Lithium and magnesium carbonates decompose on heating to form oxides and CO₂.
  • Both do not form peroxides or superoxides, unlike other members of their groups.
  • Both show covalent bonding due to their smaller size and higher ionization energies.

Q7 (a):
Properties of beryllium that make it different from its family members:

  • Beryllium does not react with water, unlike other alkaline earth metals.
  • Its salts are generally covalent rather than ionic.
  • Beryllium forms complexes, e.g., [BeF₄]²⁻.
  • It has a high melting point and does not show characteristic flame coloration.
  • Beryllium oxide (BeO) is amphoteric, unlike other Group II metal oxides, which are basic.

Q8 (b):
Why is an aqueous solution of Na₂CO₃ alkaline?

  • Na₂CO₃ is a salt of a strong base (NaOH) and a weak acid (H₂CO₃). In water, it hydrolyzes to produce OH⁻ ions, making the solution alkaline:
    [ Na₂CO₃ + H₂O → 2Na⁺ + CO₃^{2-} ]
    [ CO₃^{2-} + H₂O → HCO₃^- + OH⁻ ]

Q9 (a):
Describe with a diagram the manufacture of sodium by Down’s cell:

  • Down’s cell is an electrolytic process used to manufacture sodium by the electrolysis of molten NaCl.
  • The cathode is made of iron and the anode is made of graphite.
  • Na⁺ ions are reduced at the cathode to form sodium metal, and Cl⁻ ions are oxidized at the anode to form chlorine gas:
    [ 2NaCl → 2Na + Cl₂ ]

(b):
Three advantages of the Down’s process:

  1. High-purity sodium is obtained.
  2. It is an economical and continuous process.
  3. Chlorine gas, a useful byproduct, is obtained.

Q10:
Compare the physical and chemical properties of alkali metals with those of alkaline earth metals:

  • Physical properties:
  • Alkali metals are softer and have lower melting and boiling points than alkaline earth metals.
  • Alkali metals are more reactive than alkaline earth metals.
  • Alkali metals have lower densities compared to alkaline earth metals.
  • Chemical properties:
  • Alkali metals form monovalent cations (M⁺), whereas alkaline earth metals form divalent cations (M²⁺).
  • Alkali metals react more vigorously with water than alkaline earth metals.
  • Alkali metal oxides are more basic than alkaline earth metal oxides.

Here are the answers to the given questions based on the image:

Q9 (b):
What happens when:

  1. Lithium carbonate is heated?
  • Lithium carbonate (Li₂CO₃) decomposes upon heating to form lithium oxide (Li₂O) and carbon dioxide (CO₂):
    [ Li₂CO₃ → heat Li₂O + CO₂ ]
  1. Lithium hydroxide is heated to red heat?
  • Lithium hydroxide (LiOH) decomposes to form lithium oxide (Li₂O) and water (H₂O):
    [ 2LiOH → Li₂O + H₂O ]
  1. Beryllium is treated with sodium hydroxide?
  • Beryllium reacts with sodium hydroxide (NaOH) to form sodium beryllate (Na₂BeO₂) and hydrogen gas (H₂):
    [ Be + 2NaOH + 2H₂O → Na₂BeO₂ + H₂ ]
  1. Lithium hydride is treated with water?
  • Lithium hydride (LiH) reacts with water to form lithium hydroxide (LiOH) and hydrogen gas (H₂):
    [ LiH + H₂O→ LiOH + H₂ ]

Q10:
Give the formulas of the following minerals:

  1. Dolomite:
    [ \text{CaMg(CO₃)₂} ]
  2. Asbestos:
    [ \text{Mg₃Si₂O₅(OH)₄} ] (Chrysotile, the most common form)
  3. Halite:
    [ \text{NaCl} ] (Sodium chloride)
  4. Natron:
    [ \text{Na₂CO₃·10H₂O} ] (Sodium carbonate decahydrate)
  5. Beryl:
    [ \text{Be₃Al₂Si₆O₁₈} ] (Beryllium aluminum cyclosilicate)
  6. Sylvite:
    [ \text{KCl} ] (Potassium chloride)
  7. Phosphorite:
    [ \text{Ca₅(PO₄)₃F} ] or [ \text{Ca₃(PO₄)₂} ] (Fluorapatite or phosphate rock)
  8. Chile saltpeter:
    [ \text{NaNO₃} ] (Sodium nitrate)

Q11:
Answer the following questions briefly:

(a) Why are alkali and alkaline earth metals among the most reactive elements of the periodic table?

  • Alkali metals (Group 1) and alkaline earth metals (Group 2) have low ionization energies and readily lose electrons to form cations. Their reactivity increases down the group due to the decreasing ionization energy.

(b) Why does lime water turn milky with CO₂ but becomes clear with excess CO₂?

  • When CO₂ is bubbled through lime water (Ca(OH)₂), it reacts to form calcium carbonate (CaCO₃), which is insoluble and causes the milky appearance:
    [ Ca(OH)₂ + CO₂ &→ CaCO₃ + H₂O ]
    On passing excess CO₂, calcium carbonate reacts with water and CO₂ to form soluble calcium bicarbonate (Ca(HCO₃)₂), causing the solution to clear:
    [ CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ ]

(c) How is gypsum converted into plaster of Paris?

  • Gypsum (CaSO₄·2H₂O) is heated at around 150°C to 180°C, losing water and converting into plaster of Paris (CaSO₄·½H₂O):
    [ CaSO₄·2H₂O →{\text{150°C-180°C}} CaSO₄·½H₂O + 1½H₂O ]

(d) Why is 2% gypsum added in cement?

  • Gypsum is added to cement to slow down the setting process by controlling the hydration reaction of the cement. It prevents the cement from setting too quickly during the mixing and application phase.

(e) Why is lime added to acidic soil?

  • Lime (CaO or Ca(OH)₂) neutralizes the excess acidity in soil by reacting with the acids, thus increasing the pH and improving soil fertility.

(f) How are lime and sand used to make glass?

  • Lime (CaO) and sand (SiO₂) are melted together at high temperatures to form calcium silicate (CaSiO₃), a component of glass. Lime acts as a stabilizer, preventing the glass from being too soluble in water.

(g) How is lime mortar prepared?

  • Lime mortar is prepared by mixing slaked lime (Ca(OH)₂) with sand and water. When exposed to air, the lime reacts with carbon dioxide (CO₂) to form calcium carbonate, hardening the mortar:
    [ Ca(OH)₂ + CO₂ → CaCO₃ + H₂O ]

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