JEE Chemistry S Block Elements MCQs Set 25

Answer: (d) Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

Explanation:
1. Hydration enthalpy depends on the charge density of the ion.
2. The ion size increases down the group, so the charge density and the hydration enthalpy fall steadily: Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺.

Teacher's Note:
1. Smaller ions attract water molecules more strongly, so they have higher hydration enthalpy.

Answer: (a) MgO and Mg₃N₂

Explanation:
1. Magnesium burns in oxygen to give MgO.
2. It also reacts with the nitrogen of air at high temperature to give magnesium nitride: 3Mg + N₂ → Mg₃N₂.

Teacher's Note:
1. Magnesium burning in air gives both the oxide and the nitride.

Answer: (d) chain and dimeric

Explanation:
1. In the solid state BeCl₂ has a polymeric chain structure with chlorine bridges.
2. In the vapour phase (below about 1200 K) it exists as a chlorine-bridged dimer, which dissociates to the linear monomer at higher temperature.

Teacher's Note:
1. BeCl₂ is electron-deficient and completes its octet through chlorine bridging.

Answer: (b) MgCO₃ < CaCO₃ < SrCO₃ < BaCO₃

Explanation:
1. The large carbonate ion is stabilised by large cations and polarised by small ones.
2. Down the group the cation size increases and the polarising power falls, so thermal stability increases: MgCO₃ < CaCO₃ < SrCO₃ < BaCO₃.

Teacher's Note:
1. Thermal stability of alkaline earth carbonates increases down the group.

Answer: (c) LiNO₃ decomposes on heating to give LiNO₂ and O₂

Explanation:
1. Li has the most negative E° (due to its very high hydration enthalpy), so it is the strongest reducing agent, yet it reacts the least vigorously with water.
2. LiCl is highly hydrated and crystallises as LiCl·2H₂O.
3. LiNO₃ decomposes to Li₂O, NO₂ and O₂ (4LiNO₃ → 2Li₂O + 4NO₂ + O₂), not to the nitrite, so statement (c) is incorrect.

Teacher's Note:
1. Lithium nitrate behaves like magnesium nitrate on heating, giving the oxide, not the nitrite.

Answer: (d) Li

Explanation:
1. Among the alkali metals only lithium combines directly with nitrogen: 6Li + N₂ → 2Li₃N.
2. The small Li⁺ ion stabilises the large nitride ion through strong lattice energy.

Teacher's Note:
1. Lithium is the only alkali metal that forms a nitride directly.

Answer: (b) Be

Explanation:
1. Be has the highest ionisation enthalpy of these metals because of its very small size and 2s² configuration.
2. It has a very low atomic number and a low absorption of X-rays, so Be is used for the window.

Teacher's Note:
1. Beryllium is transparent to X-rays because of its low atomic number.

Answer: (d) ammoniated electrons

Explanation:
1. Na + (x + y)NH₃ → [Na(NH₃)ₓ]⁺ + [e(NH₃)ᵧ]⁻.
2. The ammoniated electrons absorb visible light and give the deep blue colour; they also make the solution paramagnetic, strongly reducing and conducting.

Teacher's Note:
1. The blue colour of alkali metals in liquid ammonia is due to solvated electrons.

Answer: (a) Be(OH)₂

Explanation:
1. The basicity of group 2 hydroxides increases down the group.
2. Be(OH)₂ is amphoteric: Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O and Be(OH)₂ + 2NaOH → Na₂[Be(OH)₄].

Teacher's Note:
1. Beryllium and its compounds are amphoteric because of the small size and high charge density of Be²⁺.

Answer: (c) Rb

Explanation:
1. Rubidium forms the superoxide RbO₂ in excess air, because large cations stabilise the large O₂⁻ ion.
2. The superoxide hydrolyses as 2RbO₂ + 2H₂O → 2RbOH + H₂O₂ + O₂.

Teacher's Note:
1. K, Rb and Cs form superoxides; Na forms the peroxide and Li the oxide.

Answer: (b) I, II and III

Explanation:
1. K⁺ is the main intracellular cation and activates many enzymes.
2. It takes part in glycolysis and the Krebs cycle that produce ATP from glucose.
3. Together with Na⁺ (the sodium-potassium pump) it carries nerve signals, so all three statements are correct.

Teacher's Note:
1. Na⁺ and K⁺ are essential for nerve impulses and enzyme activity.

Answer: (c) Li₂O, Na₂O₂ and KO₂

Explanation:
1. Li forms the normal oxide Li₂O, because the small Li⁺ stabilises the small O²⁻ ion.
2. Na forms the peroxide Na₂O₂, and K forms the superoxide KO₂, because larger cations stabilise larger anions.

Teacher's Note:
1. The oxygen-rich anion that forms grows with the cation size: oxide, peroxide, superoxide.

Answer: (b) BeSO₄

Explanation:
1. The small Be²⁺ ion has a very high hydration enthalpy, while the large sulphate ion keeps the lattice enthalpy relatively low.
2. For the other sulphates the larger cations give a high lattice enthalpy compared with the hydration enthalpy, so BeSO₄ is the one that dissolves readily.

Teacher's Note:
1. Solubility of the sulphates decreases down group 2 because hydration enthalpy falls faster than lattice enthalpy.

Answer: (b) Li₂O + NO₂ + O₂

Explanation:
1. Lithium nitrate behaves differently from the other alkali metal nitrates.
2. It decomposes like the nitrates of group 2: 4LiNO₃ → 2Li₂O + 4NO₂ + O₂.

Teacher's Note:
1. Other alkali metal nitrates give the nitrite and O₂, but LiNO₃ gives the oxide, NO₂ and O₂.

Answer: (c) Oxide ion donates a pair of electrons

Explanation:
1. Na₂O + H₂O → 2NaOH is not a redox reaction, so the oxidation numbers of Na and O do not change.
2. The oxide ion O²⁻ donates a pair of electrons to H⁺ of water and is converted into OH⁻ (it acts as a base).

Teacher's Note:
1. Na₂O is a basic oxide; the oxide ion takes a proton from water.

Answer: (c) two moles of ammonia

Explanation:
1. Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃.
2. So 1 mol of Mg₃N₂ gives 2 mol of NH₃.

Teacher's Note:
1. Metal nitrides hydrolyse to the metal hydroxide and ammonia.

Answer: (a) exhibiting maximum covalency in compounds

Explanation:
1. Be and Al both form polymeric hydrides, covalent halides and amphoteric oxides.
2. Be (Z = 4) has no d orbitals and has a maximum covalency of 4, while Al can reach a covalency of 6, as in [AlF₆]³⁻.

Teacher's Note:
1. The second period element cannot expand its octet beyond four.

Answer: (b) calcined gypsum

Explanation:
1. Marble, sea shells and dolomite (CaCO₃·MgCO₃) all contain calcium carbonate.
2. Calcined gypsum is plaster of Paris, CaSO₄·½H₂O, a sulphate.

Teacher's Note:
1. Gypsum and its derivatives are sulphates, not carbonates.

Answer: (a) CaC₂ + Ca₃P₂

Explanation:
1. Holmes signals use calcium carbide and calcium phosphide put in a perforated pipe and floated on water.
2. Water produces acetylene and phosphine; phosphine ignites spontaneously and lights the acetylene.

Teacher's Note:
1. CaC₂ gives C₂H₂ and Ca₃P₂ gives spontaneously inflammable PH₃ with water.

Answer: (a) Be

Explanation:
1. Be forms soluble BeSO₄, and BeO is very inert.
2. Be(OH)₂ is amphoteric and dissolves in NaOH to give [Be(OH)₄]²⁻, whereas Mg(OH)₂, Ca(OH)₂ and Sr(OH)₂ do not dissolve in alkali (Ca(OH)₂ and Sr(OH)₂ are also fairly soluble in water).

Teacher's Note:
1. Only beryllium hydroxide is amphoteric among the group 2 hydroxides.