JEE Chemistry S Block Elements MCQs Set 16

Answer: (b) \( \ce{SrO} \) > \( \ce{CaO} \) > \( \ce{MgO} \) > \( \ce{BeO} \)

Explanation:
1. Basic strength rises with ionic character, which rises as the cation gets larger.
2. So \( \ce{SrO} \) > \( \ce{CaO} \) > \( \ce{MgO} \) > \( \ce{BeO} \), and BeO is amphoteric.

Teacher's Note:
1. Basicity of oxides and hydroxides increases down group 2.
2. Link basic strength to ionic character.

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

Explanation:
1. \( \ce{KOH} \) and \( \ce{NaOH} \) are strong bases and \( \ce{Ca(OH)2} \) is moderately strong.
2. \( \ce{Zn(OH)2} \) is amphoteric and the weakest base.

Teacher's Note:
1. Amphoteric hydroxides are weaker bases than the alkali and alkaline earth hydroxides.
2. Zinc lies in the d-block.

Answer: (c) alcohol

Explanation:
1. Sodium reacts with alcohols: \( \ce{2Na + 2ROH -> 2RONa + H2} \).
2. It is inert to hydrocarbons such as toluene, kerosene and benzene, so alcohol cannot be used.

Teacher's Note:
1. Store alkali metals under inert hydrocarbons.
2. Alcohol and water react with them.

Answer: (a) \( \ce{KNO3} \)

Explanation:
1. \( \ce{2KNO3 -> 2KNO2 + O2} \): alkali metal nitrates (except Li) give nitrite and oxygen.
2. The nitrates of Ag, Pb and Cu give \( \ce{NO2} \) along with oxygen and the oxide or metal.

Teacher's Note:
1. Nitrates of heavy metals give \( \ce{NO2} \) on heating.
2. Only \( \ce{LiNO3} \) among alkali nitrates behaves like them.

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

Explanation:
1. Lattice energy varies inversely with the sum of the ionic radii.
2. \( \ce{Li+} \) is the smallest cation, so \( \ce{LiF} \) has the highest lattice energy.

Teacher's Note:
1. Small ions pack closely and attract strongly.
2. LiF is sparingly soluble because of this.

Answer: (c) a sulphate

Explanation:
1. \( \ce{BaSO4} \) is the only white barium salt insoluble in dilute HCl.
2. \( \ce{BaCO3} \) dissolves in HCl, and \( \ce{BaCl2} \) is soluble, so A is a sulphate.

Teacher's Note:
1. This is the standard test for sulphate ions.
2. Carbonates give a precipitate that dissolves with effervescence.

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

Explanation:
1. Hydration rises with polarising power, i.e. with falling ionic radius.
2. So the degree of hydration is \( \ce{Cs+} \) < \( \ce{Rb+} \) < \( \ce{K+} \) < \( \ce{Na+} \) < \( \ce{Li+} \).

Teacher's Note:
1. The smallest ion is hydrated the most.
2. Hydration enthalpy follows the same order.

Answer: (b) X is \( \ce{BeH2} \)

Explanation:
1. \( \ce{2BeCl2 + LiAlH4 -> 2BeH2 + LiCl + AlCl3} \).
2. The hydride ion replaces chloride, giving beryllium hydride.

Teacher's Note:
1. \( \ce{LiAlH4} \) is a source of hydride.
2. \( \ce{BeH2} \) is a covalent polymeric hydride.

Answer: (c) \( \ce{Ca} \) and \( \ce{CaH2} \)

Explanation:
1. \( \ce{Ca + 2H2O -> Ca(OH)2 + H2} \) and \( \ce{CaH2 + 2H2O -> Ca(OH)2 + 2H2} \): both give calcium hydroxide and hydrogen.
2. The metal oxides give only hydroxide, with no hydrogen.

Teacher's Note:
1. A metal gives \( \ce{H2} \) with water, and so does its hydride.
2. An oxide gives only the hydroxide.

Answer: (c) Lithium is softer than the other alkali metals

Explanation:
1. Lithium is HARDER than the other alkali metals, so the statement that it is softer is false and is not a property of lithium at all.
2. The other three statements are genuine properties of lithium.

Teacher's Note:
1. Lithium is the hardest alkali metal.
2. Be careful with 'not' in the stem.

Answer: (b) \( \ce{Mg} \), \( \ce{Sr} \), \( \ce{Ba} \)

Explanation:
1. Elements of the same group have the same valence configuration and similar chemistry.
2. Mg, Sr and Ba all belong to group 2.

Teacher's Note:
1. Choose members of the same group.
2. Be and Al are similar only in a few properties.

Answer: (d) their covalent nature

Explanation:
1. Large halide ions are polarised by the small \( \ce{Mg^2+} \), giving covalent character.
2. Covalent compounds dissolve in less polar organic solvents such as acetone.

Teacher's Note:
1. Like dissolves like.
2. \( \ce{MgCl2} \) is less covalent, so it dissolves less in acetone.

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

Explanation:
1. \( \ce{BeO} \) reacts with both acids and bases.
2. It shows this because of the diagonal relationship of Be with Al.

Teacher's Note:
1. Be is the only amphoteric member of group 2.
2. MgO and \( \ce{Mg(OH)2} \) are basic.

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

Explanation:
1. Lithopone is \( \ce{BaSO4} \) mixed with \( \ce{ZnS} \).
2. So the metal is Ba, and \( \ce{BaCl2} \) is soluble in water.

Teacher's Note:
1. Lithopone is a white paint pigment.
2. \( \ce{BaSO4} \) is the least soluble of the common sulphates.

Answer: (a) \( \ce{Mg(OH)2} \)

Explanation:
1. Milk of magnesia is a suspension of magnesium hydroxide in water.
2. It neutralises excess stomach acid.

Teacher's Note:
1. It is a mild base and safe to ingest.
2. Do not confuse with magnesia, which is MgO.

Answer: (d) \( \ce{K} \), \( \ce{Rb} \)

Explanation:
1. K and Rb form \( \ce{K2O} \), \( \ce{K2O2} \) and \( \ce{KO2} \) (and similarly for Rb) depending on the oxygen supply.
2. Li gives only the normal oxide and Na gives the oxide and peroxide.

Teacher's Note:
1. Superoxide formation needs a large cation.
2. Cs also forms all three, but it is not paired with K or Rb here.

Answer: (b) more than that of \( \ce{Na+} \)

Explanation:
1. Hydration energy rises with charge and falls with size.
2. \( \ce{Mg^2+} \) is more highly charged than \( \ce{Na+} \) and is of about the same size, so its hydration energy is larger.
3. \( \ce{Mg3} \)+ does not exist in aqueous solution, and \( \ce{Al3} \)+ is smaller and more charged.

Teacher's Note:
1. Charge density controls hydration.
2. Compare ions of similar size by charge.

Answer: (b) emission spectrum

Explanation:
1. In the flame the valence electron is excited and returns to the ground state by emitting yellow light.
2. This is an emission spectrum.

Teacher's Note:
1. Flame colour is an emission phenomenon.
2. Low ionisation energy helps excitation but is not the explanation of the colour.

Answer: (a) \( \ce{LiI} \) > \( \ce{LiBr} \) > \( \ce{LiCl} \) > \( \ce{LiF} \)

Explanation:
1. Solubility in non-polar solvents rises with covalent character.
2. The large \( \ce{I-} \) is polarised most by \( \ce{Li+} \), so the order is \( \ce{LiI} \) > \( \ce{LiBr} \) > \( \ce{LiCl} \) > \( \ce{LiF} \).

Teacher's Note:
1. Large anions are more polarisable.
2. Fajans' rule predicts the order.

Answer: (b) \( \ce{H2SO4} \) and \( \ce{NaHCO3} \) solutions

Explanation:
1. The two solutions are mixed when the extinguisher is operated: \( \ce{2NaHCO3 + H2SO4 -> Na2SO4 + 2H2O + 2CO2} \).
2. The \( \ce{CO2} \) produced smothers the fire.

Teacher's Note:
1. Soda-acid extinguishers use acid with bicarbonate.
2. \( \ce{CO2} \) is denser than air and excludes oxygen.

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

Explanation:
1. At high temperature \( \ce{6Na + Al2O3 -> 3Na2O + 2Al} \), so X is \( \ce{Na2O} \).
2. \( \ce{Na2O + CO2 -> Na2CO3} \), so Y is \( \ce{Na2CO3} \).

Teacher's Note:
1. Sodium is a stronger reducing agent than aluminium at high temperature.
2. Basic oxide plus \( \ce{CO2} \) gives carbonate.

Answer: (a) \( \ce{K} \) > \( \ce{Ca} \) > \( \ce{Ba} \)

Explanation:
1. K loses its second electron from the stable argon core, so its second ionisation energy is the largest.
2. For Ca and Ba the second electron comes from the ns subshell, and it is lower for the larger Ba.

Teacher's Note:
1. Group 1 second ionisation energies are very high.
2. Within a group, ionisation energy falls with size.

Answer: (a) \( \ce{NaH2PO2} \)

Explanation:
1. \( \ce{P4 + 3NaOH + 3H2O -> PH3 + 3NaH2PO2} \).
2. This is a disproportionation, and sodium hypophosphite is X.

Teacher's Note:
1. White phosphorus disproportionates in alkali.
2. Phosphine and hypophosphite are the products.

Answer: (d) \( \ce{CaCO3} \) < \( \ce{NaHCO3} \) < \( \ce{KHCO3} \)

Explanation:
1. \( \ce{CaCO3} \) is almost insoluble.
2. \( \ce{NaHCO3} \) is sparingly soluble, whereas \( \ce{KHCO3} \) is more soluble, so the order is \( \ce{CaCO3} \) < \( \ce{NaHCO3} \) < \( \ce{KHCO3} \).

Teacher's Note:
1. Solubility of alkali bicarbonates rises down the group.
2. Carbonates of group 2 are insoluble.

Answer: (a) plaster of Paris, dead burnt plaster

Explanation:
1. At about 393 K gypsum gives plaster of Paris (X).
2. At about 478 K it loses all water to give anhydrous \( \ce{CaSO4} \), called dead burnt plaster (Y).

Teacher's Note:
1. Higher temperature removes all the water.
2. Dead burnt plaster no longer sets with water.

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

Explanation:
1. \( \ce{BeSO4} \) is soluble and \( \ce{Be(OH)2} \) is insoluble, and BeO is amphoteric.
2. Only Be fits these properties.

Teacher's Note:
1. Amphoteric oxide in group 2 means Be.
2. Be is the hardest metal of the group.

Answer: (c) \( \ce{OH-} \)

Explanation:
1. \( \ce{K2O + H2O -> 2KOH} \), which is a strong base and fully ionised.
2. The basicity comes from \( \ce{OH-} \) ions.

Teacher's Note:
1. Oxide ions do not exist in water; they are protonated at once.
2. The solution is strongly alkaline.

Answer: (b) \( \ce{Ca(OH)2} \) and \( \ce{NaOH} \)

Explanation:
1. A with \( \ce{CO2} \) gives a milky precipitate C, so A is lime water \( \ce{Ca(OH)2} \) and C is \( \ce{CaCO3} \).
2. \( \ce{Ca(OH)2 + Na2CO3 -> CaCO3 + 2NaOH} \), so B is \( \ce{NaOH} \).

Teacher's Note:
1. Milkiness with \( \ce{CO2} \) is the test for lime water.
2. This reaction is used to make caustic soda by the lime-soda method.