s-Block Elements (Alkali and Alkaline Earth Metals)NEET Chemistry · Class 11 · NCERT Chapter 11

Solution

Li forms a normal oxide (Li₂O). Na forms a peroxide (Na₂O₂). K, Rb, and Cs form superoxides (KO₂, RbO₂, CsO₂). The larger the alkali metal, the higher the oxide formed: Li → oxide; Na → peroxide; K, Rb, Cs → superoxide. Be is not an alkali metal (it is Group 2).

Solution

The solubility of Group 2 sulphates decreases down the group: MgSO₄ is soluble in water, CaSO₄ is sparingly soluble, SrSO₄ is slightly soluble, and BaSO₄ is practically insoluble (Ksp ≈ 1.1 × 10⁻¹⁰). BaSO₄ precipitates as the test for Ba²⁺ and SO₄²⁻.

Solution

Lithium (Li) is the only alkali metal that reacts with N₂ at room temperature to form lithium nitride: 6Li + N₂ → 2Li₃N. This is an anomalous property due to Li's small size and high charge density. Other alkali metals do not react with N₂ directly.

Solution

Plaster of Paris (CaSO₄·½H₂O) sets by absorbing water (1½ mol per formula unit) to form gypsum (CaSO₄·2H₂O): CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. This process involves a slight expansion in volume and is slightly exothermic.

Solution

LiOH decomposes on heating: 2LiOH → Li₂O + H₂O. This is an anomalous property of lithium due to its small size and high charge density. The other alkali metal hydroxides (NaOH, KOH, RbOH, CsOH) are thermally stable and do not decompose on heating.

Solution

Be(OH)₂ is amphoteric (dissolves in both acid and alkali). Solubility of Group 2 hydroxides increases down the group: Mg(OH)₂ ≈ slightly soluble → Ca(OH)₂ slightly soluble → Sr(OH)₂ moderately soluble → Ba(OH)₂ soluble (strongly basic). So Ba(OH)₂ is the most soluble and most strongly basic Group 2 hydroxide.

Solution

Sodium reacts vigorously with water (forming NaOH and H₂) and with oxygen (forming Na₂O₂). To protect it from both atmospheric moisture and oxygen, sodium is stored under kerosene oil, which is a non-polar, non-reactive liquid that does not react with sodium.

Solution

Li (Group 1, Period 2) has a diagonal relationship with Mg (Group 2, Period 3). Both have similar charge-to-radius ratios (polarising power). Common similarities: both react with N₂ (Li₃N and Mg₃N₂), both form normal oxides on burning, both decompose their carbonates on mild heating.

Solution

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The CO₂ released is a non-flammable gas heavier than air; it smothers the fire by cutting off the oxygen supply. Dry powder fire extinguishers (for oil and electrical fires) use NaHCO₃ for this reason.

Solution

Beryllium is amphoteric — it dissolves in both dilute acids (Be + 2HCl → BeCl₂ + H₂) and strong alkalis (Be + 2NaOH + 2H₂O → Na₂[Be(OH)₄] + H₂). No other Group 2 element shows this behaviour. BeCl₂ is covalent, not ionic. Be does not react with cold water.

Solution

Sodium imparts a characteristic golden-yellow (sometimes called brilliant yellow) colour to a flame. This is due to the 3s → 3p transition of the sodium D-line doublet at 589 nm. Potassium gives violet/lilac, barium gives apple green, calcium gives brick red.

Solution

BaCl₂ is used to test for sulphate ions (SO₄²⁻). Ba²⁺ + SO₄²⁻ → BaSO₄↓ (white precipitate, insoluble in dilute HCl). This is because BaSO₄ has an extremely low Ksp. CaSO₄ is slightly soluble, and Mg/Na sulphates are soluble.

Solution

Hydration energy (enthalpy of hydration) depends on the charge density of the ion (charge/radius ratio). Li⁺ is the smallest Group 1 ion (ionic radius ≈ 76 pm), so it has the highest charge density and attracts the dipole of water molecules very strongly. Hydration enthalpy: Li⁺ (−520 kJ/mol) >> Cs⁺ (−264 kJ/mol).

Solution

When CO₂ is passed into lime water: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O (milky, due to insoluble CaCO₃). On passing excess CO₂: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Calcium bicarbonate is soluble, so the milkiness disappears and the solution becomes clear again.