JEE Chemistry S Block Elements MCQs Set 12

Answer: (a) A - III, B - I, C - II, D - IV

Explanation:
1. Chlorophyll contains Mg2+ at its centre (III).
2. Soda ash is anhydrous Na2CO3 (I).
3. Plaster of Paris, CaSO4.1/2 H2O, is used in dentistry and ornamental work (II).
4. Slaked lime, Ca(OH)2, is used in white washing (IV).

Teacher's Note:
1. Chlorophyll is a Mg complex and haemoglobin an Fe complex.
2. Plaster of Paris is made from gypsum.

Answer: (a) (NH4)2BeF4

Explanation:
1. BeO + 4HF + 2NH3 gives (NH4)2BeF4 + H2O.
2. On heating, (NH4)2BeF4 gives BeF2 + 2NH4F, which matches the question.

Teacher's Note:
1. This is how BeF2 is prepared industrially.
2. Balance charges: 2 NH4+ and BeF4 2-.

Answer: (c) A and E only

Explanation:
1. A is correct: E values are Na -2.71 V, Rb -2.98 V and Li -3.04 V, so Na > Rb > Li.
2. B is wrong because LiF has a very high lattice energy and is sparingly soluble.
3. C is wrong because Li2CO3 decomposes easily to Li2O and CO2.
4. D is wrong because the blue paramagnetic solution is dilute. Concentrated solutions are bronze and diamagnetic.
5. E is correct because all alkali metal hydrides are ionic (saline) solids.
6. Correct: A and E only.

Teacher's Note:
1. Li has the most negative E value because of its very high hydration enthalpy.
2. Dilute metal-ammonia solutions are blue, concentrated ones are bronze.

Answer: (c) A - II, B - I, C - IV, D - III

Explanation:
1. Potassium gives a violet (lilac) flame.
2. Calcium gives brick red, strontium crimson red and barium apple green.

Teacher's Note:
1. Flame colours: Li crimson, Na golden yellow, K violet, Rb red-violet, Cs blue.
2. Ca brick red, Sr crimson, Ba apple green.

Answer: (c) Ca(OH)2, NH3, NH4HCO3

Explanation:
1. In the Solvay process, Ca(OH)2 (A) heated with NH4Cl releases NH3 (B).
2. NH3 with H2O and excess CO2 forms NH4HCO3 (C).
3. NH4HCO3 reacts with NaCl to give NaHCO3 and NH4Cl.

Teacher's Note:
1. Solvay process: NH3, CO2 and brine give NaHCO3.
2. Excess CO2 gives the hydrogen carbonate, not the carbonate.

Answer: (c) A is not correct but R is correct

Explanation:
1. The flame test uses the non-luminous (oxidising) Bunsen flame, in which the heated salt emits characteristic colours. A reducing flame is not used for this, so statement A is incorrect.
2. Alkali metals can be detected by flame tests, so statement R is correct.

Teacher's Note:
1. The characteristic colour arises from excitation and return of valence electrons.
2. The test is done in a non-luminous flame.

Answer: (c) Na

Explanation:
1. A weaker reducing agent has a less negative reduction potential.
2. E values: Li -3.04 V, Rb -2.98 V, K -2.93 V and Na -2.71 V.
3. Na has the least negative value, so it is the weakest reducing agent of these.

Teacher's Note:
1. Li is the strongest reducing agent in water because of its high hydration enthalpy.
2. Compare the standard electrode potentials, not the ionisation energies.

Answer: (d) C > E > A > B > D

Explanation:
1. Hydration enthalpy rises with charge and falls with ionic size.
2. Divalent ions hydrate more strongly, and Mg2+ is smaller than Ca2+, so Mg2+ > Ca2+.
3. For the monovalent ions the size order is K+ < Rb+ < Cs+, so K+ > Rb+ > Cs+.
4. Order: C > E > A > B > D.

Teacher's Note:
1. Charge density controls the hydration enthalpy.
2. Smaller and more highly charged ions are more hydrated.

Answer: (a) 1

Explanation:
1. Lithium nitrate decomposes as 4LiNO3 gives 2Li2O + 4NO2 + O2.
2. Of the species listed, only NO2 is a compound formed. O2 is an element, and Li2, N2 and LiNO2 are not formed.
3. Number of compounds = 1.

Teacher's Note:
1. Lithium is anomalous and its nitrate gives the oxide, as 2nd group nitrates do.
2. Other alkali metal nitrates give nitrites and oxygen.

Answer: (c) A and B

Explanation:
1. Sulphate solubility decreases down group 2 because the lattice energy falls less than the hydration energy.
2. BeSO4 and MgSO4 are readily soluble, CaSO4 is sparingly soluble, SrSO4 and BaSO4 are insoluble.

Teacher's Note:
1. Sulphate solubility decreases down group 2.
2. Hydroxide solubility increases down the group.

Answer: (c) \( 4LiNO_3 \xrightarrow{\Delta} 2Li_2O + 4NO_2 + O_2 \)

Explanation:
1. Lithium nitrate behaves like the nitrates of group 2 metals and gives the oxide, NO2 and O2.
2. 4LiNO3 gives 2Li2O + 4NO2 + O2, and this equation is balanced (4 Li, 4 N, 12 O on each side).

Teacher's Note:
1. Only Li among alkali metals gives the oxide.
2. Always check the balance of atoms.

Answer: (d) Be

Explanation:
1. BeCl2 is largely covalent because the small, highly charged Be2+ polarises the chloride ion strongly (Fajans' rules).
2. Covalent chlorides dissolve in organic solvents, whereas CaCl2, MgCl2 and KCl are ionic.

Teacher's Note:
1. Be shows diagonal similarity with Al.
2. Covalent halides dissolve in organic solvents.

Answer: (b) Ca

Explanation:
1. Calcium ions are needed for nerve impulse transmission, muscle contraction and blood clotting.
2. Magnesium works with enzymes and ATP, and Be and Li have no such role.

Teacher's Note:
1. Ca is also present in bones, teeth and in blood clotting.
2. Na and K work with the nerve impulse as well.

Answer: (b) 2

Explanation:
1. First ionisation enthalpies (kJ/mol): Li 520, Na 496, K 419, Rb 403 and Cs 376. So Li, Na, K and Rb have values above 400.
2. Stable superoxides are formed by K, Rb and Cs.
3. Alkali metals satisfying both: K and Rb, which is 2.

Teacher's Note:
1. Larger cations stabilise the large superoxide ion.
2. Li gives the oxide and Na gives the peroxide.

Answer: (c) A - II, B - IV, C - I, D - III

Explanation:
1. Slaked lime is Ca(OH)2 (II).
2. Dead burnt plaster is anhydrous CaSO4 (IV).
3. Caustic soda is NaOH (I).
4. Washing soda is Na2CO3.10H2O (III).

Teacher's Note:
1. Plaster of Paris is CaSO4.1/2 H2O, and heating it strongly gives dead burnt plaster.
2. Learn the common names of sodium and calcium compounds.

Answer: (b) B, C and D only

Explanation:
1. BeO is amphoteric, so A is wrong.
2. BeCO3 is unstable and decomposes easily, so it is kept in an atmosphere of CO2, so B is correct.
3. BeSO4 is readily soluble in water, so C is correct.
4. Be is anomalous because of its small size and high charge density, so D is correct.
5. Correct: B, C and D.

Teacher's Note:
1. Be and Al oxides are both amphoteric.
2. The stability of carbonates increases down group 2.