JEE Chemistry S Block Elements MCQs Set 07

Answer: (d) \(\ce{MgCO3}\)

Explanation:
1. Thermal stability of carbonates of group 2 increases down the group.
2. Mg2+ is the smallest here and polarises the carbonate ion most.
3. So MgCO3 decomposes at the lowest temperature.

Teacher's Note:
1. Order of stability: BeCO3 < MgCO3 < CaCO3 < SrCO3 < BaCO3.

Answer: (d) chlorophyll

Explanation:
1. Chlorophyll has Mg at the centre of a porphyrin-type ring.
2. Haemoglobin has Fe and vitamin B12 has Co.

Teacher's Note:
1. Mg - chlorophyll, Fe - haemoglobin, Co - vitamin B12.

Answer: (a) \(\ce{BeH2}\)

Explanation:
1. Hydrides of Ca, Sr and Ba are ionic (saline).
2. \(\ce{BeH2}\) is covalent and polymeric (like \(\ce{MgH2}\), which is largely covalent).

Teacher's Note:
1. Be is the anomalous member of group 2.

Answer: (d) \(\ce{CaSO4.1/2H2O}\)

Explanation:
1. Plaster of Paris is calcium sulphate hemihydrate, \(\ce{2CaSO4.H2O}\).
2. It is made by heating gypsum at about 120 C.

Teacher's Note:
1. The hemihydrate sets back into gypsum on adding water.

Answer: (b) anhydrous \(\ce{CaCl2}\)

Explanation:
1. Anhydrous \(\ce{CaCl2}\) absorbs water strongly and forms hydrates.
2. The other salts are not efficient drying agents.

Teacher's Note:
1. Anhydrous CaCl2 cannot dry alcohol or ammonia because it forms adducts.

Answer: (a) \(\ce{KCl.MgCl2.6H2O}\)

Explanation:
1. Carnallite is the double chloride of potassium and magnesium with six molecules of water.

Teacher's Note:
1. Electrolysis of fused carnallite is a source of magnesium.

Answer: (a) \(\ce{BaSO4}\)

Explanation:
1. Lattice energy falls slowly down the group but hydration energy falls faster.
2. So solubility of sulphates decreases down the group.
3. BaSO4 is practically insoluble.

Teacher's Note:
1. Sulphates: solubility falls down the group; hydroxides: solubility rises.

Answer: (c) \(\ce{NaHCO3}\) and \(\ce{Mg(OH)2}\)

Explanation:
1. Sodium hydrogencarbonate and milk of magnesia are mild bases used to neutralise excess stomach acid.
2. Ca(OH)2 is too strongly caustic and Na2CO3 is too alkaline for this purpose.

Teacher's Note:
1. A good antacid is a weak base that does not harm the stomach lining.

Answer: (a) IV < II < III < I

Explanation:
1. Stability of group 2 carbonates increases down the group.
2. So BeCO3 < MgCO3 < CaCO3 < BaCO3, that is IV < II < III < I.

Teacher's Note:
1. Write the group order first, then replace the names by the roman numbers.

Answer: (b) superoxide

Explanation:
1. \(\ce{KO2}\) contains the superoxide ion \(\ce{O2-}\).
2. Normal oxides contain \(\ce{O^2-}\) and peroxides contain \(\ce{O2^2-}\).

Teacher's Note:
1. Superoxide ion has one unpaired electron, so KO2 is paramagnetic.

Answer: (c) making an amalgam of sodium

Explanation:
1. Sodium amalgam reacts only slowly with water.
2. Heating or adding acid speeds up the reaction.
3. Adding more sodium only increases the total heat.

Teacher's Note:
1. Sodium amalgam is used as a milder reducing agent in the laboratory.

Answer: (c) paramagnetic to diamagnetic

Explanation:
1. Dilute solutions are paramagnetic because of free ammoniated electrons.
2. In concentrated solutions the electrons pair up and the solution becomes diamagnetic and bronze with metallic conduction.

Teacher's Note:
1. Free electrons give paramagnetism; paired electrons give diamagnetism.

Answer: (c) ammonia

Explanation:
1. \(\ce{3Mg + N2 -> Mg3N2}\).
2. \(\ce{Mg3N2 + 6H2O -> 3Mg(OH)2 + 2NH3}\).

Teacher's Note:
1. Metal nitrides of group 2 hydrolyse to give ammonia.

Answer: (c) their high lattice energies

Explanation:
1. The M2+ ion carries a double charge and is smaller than the M+ ion of the same period.
2. So the lattice energy of the salts is much higher.
3. The hydration energy cannot make up for this, so solubility is lower.

Teacher's Note:
1. Lattice energy goes with charge squared in simple terms.

Answer: (b) greater hydration energy of the \(\ce{Be^2+}\) ion

Explanation:
1. \(\ce{Be^2+}\) is very small, so its hydration energy is very large.
2. This exceeds the lattice energy of \(\ce{BeF2}\), so it dissolves.

Teacher's Note:
1. Solubility = hydration energy against lattice energy.

Answer: (d) \(\ce{Cs+ < Rb+ < K+ < Na+ < Li+}\)

Explanation:
1. Smaller ions are more heavily hydrated.
2. So the hydrated radius decreases from Li+ to Cs+, giving Cs+ < Rb+ < K+ < Na+ < Li+.

Teacher's Note:
1. The order of hydrated radii is the reverse of the order of bare radii.

Answer: (b) \(\ce{CaH2}\)

Explanation:
1. \(\ce{CaH2 + 2H2O -> Ca(OH)2 + 2H2}\) gives two moles of hydrogen per mole.
2. Lime water turns milky with \(\ce{CO2}\).
3. NaH would give only one mole of H2 per mole.

Teacher's Note:
1. Check the mole ratio of gas to substance.

Answer: (c) \(\ce{Be(OH)2}\), \(\ce{Zn(OH)2}\)

Explanation:
1. \(\ce{Be(OH)2}\) and \(\ce{Zn(OH)2}\) dissolve in both acids and bases.
2. LiOH and Mg(OH)2 are basic, and B(OH)3 is acidic.

Teacher's Note:
1. Amphoteric hydroxides: Be, Al, Zn, Sn, Pb.

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

Explanation:
1. Hardness is caused by dissolved calcium and magnesium salts.
2. CaSO4, MgCl2 and Ca(HCO3)2 all contain these ions.
3. NaCl does not form scum with soap, so it is not hardness imparting.

Teacher's Note:
1. Hardness = presence of Ca2+ or Mg2+ ions.
2. Sodium and potassium salts do not cause it.

Answer: (b) \(\ce{CaCO3}\)

Explanation:
1. Isomorphous substances have the same crystal structure and similar formula type.
2. NaNO3 and CaCO3 (calcite) both have the same rhombohedral structure with ions of similar size.
3. KNO3 and KClO3 do not share it.

Teacher's Note:
1. Isomorphism needs the same type of formula and the same crystal form.

Answer: (b) \(\ce{KO2}\) only

Explanation:
1. \(\ce{KO2}\) contains the superoxide ion \(\ce{O2-}\), which has one unpaired electron.
2. \(\ce{BaO2}\) has the peroxide ion \(\ce{O2^2-}\) (all paired).
3. \(\ce{AlO2-}\) and \(\ce{NO2+}\) have all electrons paired.

Teacher's Note:
1. Superoxides are paramagnetic and peroxides are diamagnetic.

Answer: (d) all of these are correct properties of \(\ce{Na2O2}\)

Explanation:
1. The peroxide ion \(\ce{O2^2-}\) has no unpaired electrons, so it is diamagnetic.
2. It is the dianion of the dibasic acid H2O2.
3. In alkaline medium it oxidises Cr(III) to chromate.

Teacher's Note:
1. Sodium peroxide is a typical oxidising agent in alkaline medium.

Answer: (a) I and IV

Explanation:
1. I is correct: sulphate stability rises down the group.
2. II is wrong: the basic nature is CaO > MgO > BeO > ZnO.
3. III is wrong: the solubility of alkali hydroxides increases down the group.
4. IV is correct: NaCl 801 C, KCl 770 C, RbCl 718 C, LiCl 605 C.

Teacher's Note:
1. Check each statement separately before choosing the combination.

Answer: (b) the action of \(\ce{BeCl2}\) with \(\ce{LiAlH4}\)

Explanation:
1. Be does not react directly with hydrogen.
2. \(\ce{2BeCl2 + LiAlH4 -> 2BeH2 + LiCl + AlCl3}\).

Teacher's Note:
1. Hydrides of Be and Mg are made indirectly.