JEE Chemistry S Block Elements MCQs Set 20

Answer: (c) \( \ce{Mg^{2+}} \)

Explanation:
1. Ba2+ gives apple green, Li+ crimson and Ca2+ brick red. Mg2+ (like Be2+) is not excited by a Bunsen flame because its electrons are tightly held, and MgCl2 is water soluble.

Teacher's Note:
1. Only Be and Mg give no flame colour.
2. Their small size binds the outer electrons too strongly for flame excitation.

Answer: (a) Sr

Explanation:
1. Sr salts give crimson red, Ca brick red, Ba apple green and Mg no colour.

Teacher's Note:
1. Strontium salts are used in red flares.
2. Lithium also gives crimson red but is not offered here.

Answer: (d) the greater nuclear charge of the alkaline earth metal

Explanation:
1. Across a period the nuclear charge rises and size falls, so the outer electron is held more strongly.

Teacher's Note:
1. Be, Mg, Ca have one more proton than Li, Na, K respectively.
2. The second electron of an alkaline earth metal also goes into the same s-orbital.

Answer: (b) RbCl and BeCl\(_2\)

Explanation:
1. Ionic character is highest for the large, singly charged Rb+ (low polarising power) and lowest for the small, doubly charged Be2+ (highest polarising power, Fajans' rules).

Teacher's Note:
1. BeCl2 is essentially covalent.
2. Polarising power rises with charge and falls with size.

Answer: (b) \( \ce{Rb2CO3} \)

Explanation:
1. Thermal stability of carbonates rises down the group as the larger cation stabilises the large carbonate ion better. Li2CO3 decomposes to Li2O and CO2 easily.

Teacher's Note:
1. Among the choices Rb2CO3 is at the bottom of the group.
2. The small Li+ polarises the carbonate ion and breaks it.

Answer: (d) \( \ce{Ba(OH)2} \)

Explanation:
1. Basic strength of the hydroxides of group 2 rises down the group as the M-OH bond weakens with increasing cation size.

Teacher's Note:
1. Ba(OH)2 is the most soluble and most basic of these.
2. Mg(OH)2 is only sparingly soluble.

Answer: (a) \( \ce{BeO} \)

Explanation:
1. BeO dissolves in acids and in alkalis (forming beryllate), whereas the other oxides are strongly basic.

Teacher's Note:
1. Be shows diagonal similarity with aluminium.
2. Basic character of oxides grows down the group.

Answer: (c) Be

Explanation:
1. Be is protected by an oxide film and has a less negative reduction potential, so it is not attacked by water even at high temperature.

Teacher's Note:
1. Mg reacts with boiling water but Be does not.
2. Ca, Na and K react readily.

Answer: (d) \( \ce{Na2[Be(OH)4]} \)

Explanation:
1. Be is amphoteric: \( \ce{Be + 2NaOH + 2H2O -> Na2[Be(OH)4] + H2} \).

Teacher's Note:
1. Aluminium behaves similarly.
2. The beryllate ion is soluble.

Answer: (b) \( \ce{Mg3N2} \)

Explanation:
1. The hot metal also combines with nitrogen: \( \ce{3Mg + N2 -> Mg3N2} \).

Teacher's Note:
1. Magnesium nitride gives ammonia with water.
2. No carbonates or nitrates are formed directly.

Answer: (a) \( \ce{KO3} \)

Explanation:
1. Dry KOH reacts with O3 to form the orange potassium ozonide: \( \ce{2KOH + 5O3 -> 2KO3 + 5O2 + H2O} \).

Teacher's Note:
1. KO3 contains the ozonide ion O3-.
2. KO2 is the superoxide formed in air.

Answer: (c) \( \ce{BaO2} \)

Explanation:
1. BaO2 has \( \ce{O2^{2-}} \). KO2 has the superoxide ion, CaO and Li2O have the oxide ion.

Teacher's Note:
1. Barium is the only alkaline earth metal here that forms a peroxide readily.
2. Peroxides are diamagnetic and superoxides are paramagnetic.

Answer: (c) \( \ce{Na2O2} \)

Explanation:
1. \( \ce{Na2O2 + H2SO4 -> Na2SO4 + H2O2} \).

Teacher's Note:
1. Na2O2 is a salt of H2O2.
2. PbO2, MnO2 and SnO2 are dioxides.

Answer: (a) \( \ce{NaIO3 and NaI} \)

Explanation:
1. Halogens disproportionate in hot concentrated alkali: \( \ce{3I2 + 6NaOH -> NaIO3 + 5NaI + 3H2O} \).

Teacher's Note:
1. Hot conditions give the iodate, cold ones the hypoiodite.
2. Disproportionation: oxidation state 0 goes to +5 and -1.

Answer: (d) \( \ce{Na2S, Na2S2O3 and H2O} \)

Explanation:
1. Sulphur disproportionates: \( \ce{4S + 6NaOH -> 2Na2S + Na2S2O3 + 3H2O} \).

Teacher's Note:
1. Sulphur goes to -2 and +2.
2. Similar to the reaction of white phosphorus with NaOH.

Answer: (b) \( \ce{Na2CO3} \)

Explanation:
1. Na2CO3 is thermally stable, whereas Li2CO3, BeCO3 give oxide + CO2 and KHCO3 gives K2CO3, H2O and CO2.

Teacher's Note:
1. Alkali metal carbonates other than Li2CO3 do not decompose at a Bunsen flame.
2. Bicarbonates always decompose to carbonates.

Answer: (b) KHCO3 is more soluble than NaHCO3 and does not precipitate

Explanation:
1. The process relies on precipitation of NaHCO3 from the cold ammoniacal brine. KHCO3 is far more soluble, so it stays in solution.

Teacher's Note:
1. The solubility of NaHCO3 is low enough to crystallise.
2. Hence K2CO3 is made by other methods.

Answer: (d) \( \ce{NaHSO3} \)

Explanation:
1. SO2 is acidic: \( \ce{Na2CO3 + 2SO2 + H2O -> 2NaHSO3 + CO2} \).

Teacher's Note:
1. No oxidation is involved; sulphur stays at +4.
2. The product is the bisulphite.

Answer: (a) acidic

Explanation:
1. The strongly polarising Be2+ hydrates and hydrolyses: \( \ce{[Be(H2O)4]^{2+} + H2O <=> [Be(H2O)3(OH)]^+ + H3O^+} \).

Teacher's Note:
1. NaCl solution is neutral.
2. Hydrolysis of Be salts produces H3O+.

Answer: (c) \( \ce{CH4} \)

Explanation:
1. Be2C is a methanide: \( \ce{Be2C + 4H2O -> 2Be(OH)2 + CH4} \).

Teacher's Note:
1. CaC2 gives acetylene, Mg2C3 gives propyne.
2. The carbide carbon is C4-.

Answer: (d) calcium cyanamide

Explanation:
1. \( \ce{CaC2 + N2 -> CaNCN + C} \) (nitrolim).

Teacher's Note:
1. Calcium cyanamide is a fertiliser.
2. With steam it gives NH3 and CaCO3.

Answer: (b) high lattice energy

Explanation:
1. The doubly charged ions give a much larger lattice energy; the hydration energy compensates this less.

Teacher's Note:
1. Solubility depends on lattice energy versus hydration energy.
2. High charge density raises both, but lattice energy rises more.

Answer: (a) a basic oxide

Explanation:
1. The element is potassium. K2O (and the other alkali metal oxides) are strongly basic.

Teacher's Note:
1. Alkali metals form basic oxides.
2. Count electrons: 19, so potassium.