JEE Chemistry S Block Elements MCQs Set 03

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Chapter-wise Objective Questions: S Block Elements

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Multiple Choice Questions

Question 1: The correct order of density is:
(a) Li > Na
(b) K > Na
(c) Mg > Ca
(d) Cs < Rb
Show Answer & Explanation

Answer: (c) Mg > Ca

Explanation:
1. Densities (g/cm3): Mg 1.74 and Ca 1.55, so Mg > Ca
2. Li 0.53 < Na 0.97, K 0.86 < Na, Cs 1.90 > Rb 1.53

Teacher's Note:
1. Density generally rises down a group, with K lower than Na as an exception.

Question 2: Which has the highest melting point?
(a) Be
(b) Mg
(c) Ca
(d) Sr
Show Answer & Explanation

Answer: (a) Be

Explanation:
1. Be has the strongest metallic bonding (small size, two electrons)
2. m.p. about 1560 K, decreasing down the group

Teacher's Note:
1. Be is much harder than the others.

Question 3: The metal used in a photoelectric cell is:
(a) Na
(b) Cs
(c) Mg
(d) Ca
Show Answer & Explanation

Answer: (b) Cs

Explanation:
1. Cs has the lowest ionisation energy
2. It loses electrons readily on exposure to light

Teacher's Note:
1. The photoelectric effect is easiest for Cs.

Question 4: More stable hydride is:
(a) Cs-H
(b) Rb-H
(c) K-H
(d) Li-H
Show Answer & Explanation

Answer: (d) Li-H

Explanation:
1. Stability of alkali metal hydrides decreases down the group
2. Li-H has the strongest bond because of small size

Teacher's Note:
1. LiH decomposes only at about 1000 K.

Question 5: In which compound is hydrogen electronegative (negative oxidation state)?
(a) CaH2
(b) CH4
(c) HCl
(d) all
Show Answer & Explanation

Answer: (a) CaH2

Explanation:
1. In CaH2, Ca is more electropositive, so H is H-
2. In CH4 and HCl, H is partly positive

Teacher's Note:
1. Saline hydrides contain H-.

Question 6: Which of the following metals will give an apple green colour to the Bunsen flame?
(a) Ba
(b) Sr
(c) Ca
(d) K
Show Answer & Explanation

Answer: (a) Ba

Explanation:
1. Ba gives apple green, Sr crimson, Ca brick red, K violet

Teacher's Note:
1. Flame colour arises from excitation of outer electrons.

Question 7: The density of Na compared with K is:
(a) Na > K
(b) Na = K
(c) K > Na
(d) Li > K
Show Answer & Explanation

Answer: (a) Na > K

Explanation:
1. Na is 0.97 g/cm3 and K is 0.86 g/cm3
2. Potassium is less dense because of its larger atomic volume

Teacher's Note:
1. This is an exception to the trend of density.

Question 8: Alkali metal salts are:
(a) diamagnetic and coloured
(b) diamagnetic and colourless
(c) paramagnetic and coloured
(d) paramagnetic and colourless
Show Answer & Explanation

Answer: (b) diamagnetic and colourless

Explanation:
1. M+ ions have noble gas configuration, with no unpaired electrons
2. So the salts are diamagnetic and colourless

Teacher's Note:
1. Colour needs unpaired d electrons or charge transfer.

Question 9: The order of ionic conductances of hydrated M+ ions in water is:
(a) Li+ > Na+ > K+ > Rb+ > Cs+
(b) Li+ < Na+ > K+ < Rb+ > Cs+
(c) Li+ > Na+ < K+ > Rb+ < Cs+
(d) Li+ < Na+ < K+ < Rb+ < Cs+
Show Answer & Explanation

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

Explanation:
1. Li+ is the most heavily hydrated, so it is the largest in water and moves slowest
2. Conductance rises from Li+ to Cs+

Teacher's Note:
1. Smaller ions have larger hydrated radii.

Question 10: Which halide has the highest melting point?
(a) NaCl
(b) KCl
(c) NaBr
(d) NaF
Show Answer & Explanation

Answer: (d) NaF

Explanation:
1. NaF has the highest lattice energy because of the small F-
2. Its m.p. is about 1266 K

Teacher's Note:
1. High lattice energy gives high melting point.

Question 11: Which of the following does not give an oxide on heating?
(a) MgCO3
(b) Li2CO3
(c) ZnCO3
(d) K2CO3
Show Answer & Explanation

Answer: (d) K2CO3

Explanation:
1. K2CO3 is thermally stable
2. The others decompose to the oxide and CO2

Teacher's Note:
1. Stability of carbonates increases down group 1.

Question 12: When strongly heated in steam, Mg burns brilliantly producing:
(a) Mg(OH)2
(b) MgO and H2
(c) MgO and O2
(d) MgO and O3
Show Answer & Explanation

Answer: (b) MgO and H2

Explanation:
1. Mg + H2O (steam) gives MgO + H2
2. The hydrogen burns too

Teacher's Note:
1. Magnesium reduces steam.

Question 13: When magnesium ribbon is heated to redness in an atmosphere of nitrogen and subsequently cooled with water, the gas evolved is:
(a) N2
(b) NH3
(c) O2
(d) CO2
Show Answer & Explanation

Answer: (b) NH3

Explanation:
1. Mg burns in nitrogen to give Mg3N2
2. Mg3N2 + 6H2O gives 3Mg(OH)2 + 2NH3

Teacher's Note:
1. Nitrides hydrolyse to ammonia.

Question 14: Molten potassium chloride conducts electricity due to the presence of:
(a) free electrons
(b) free ions
(c) free molecules
(d) atoms of potassium and chloride
Show Answer & Explanation

Answer: (b) free ions

Explanation:
1. KCl is ionic and melting frees K+ and Cl-
2. These ions carry the current

Teacher's Note:
1. Metals conduct by electrons, ionic melts by ions.

Question 15: Which of the following elements has the maximum tendency to form complex compounds?
(a) Be
(b) Ba
(c) Ca
(d) Mg
Show Answer & Explanation

Answer: (a) Be

Explanation:
1. Be2+ has a very small size and high charge density
2. It forms complexes such as [BeF4]2-

Teacher's Note:
1. Complex formation falls down the group.

Question 16: On heating sodium in a current of dry ammonia, which gas is formed?
(a) NaNH2
(b) NaN3
(c) NH3
(d) H2
Show Answer & Explanation

Answer: (d) H2

Explanation:
1. 2Na + 2NH3 gives 2NaNH2 + H2
2. The gas evolved is H2

Teacher's Note:
1. Sodamide is the solid product.

Question 17: Sodium reacts with water more vigorously than lithium because it:
(a) has higher atomic weight
(b) is more electronegative
(c) is more electropositive
(d) is a metal
Show Answer & Explanation

Answer: (c) is more electropositive

Explanation:
1. Na has a lower ionisation energy and is more electropositive
2. It reacts faster, although Li has a higher hydration energy

Teacher's Note:
1. Reactivity increases down the group.

Question 18: Which alkali metal has the biggest tendency for the half reaction M(g) gives M+(aq) + e-?
(a) sodium
(b) lithium
(c) potassium
(d) caesium
Show Answer & Explanation

Answer: (b) lithium

Explanation:
1. Lithium has the largest hydration energy, so the overall change is most favourable
2. That is why it has the most negative E0

Teacher's Note:
1. Hydration energy outweighs ionisation energy.

Question 19: The strongest reducing agent among the following is:
(a) Be
(b) Mg
(c) Sr
(d) Ba
Show Answer & Explanation

Answer: (d) Ba

Explanation:
1. Reducing power increases down the group
2. Ba is the most easily oxidised

Teacher's Note:
1. Ionisation energy falls down the group.

Question 20: Both Be and Al become passive with conc. nitric acid due to:
(a) the non-reactive nature of the metal
(b) the non-reactive nature of the acid
(c) formation of an inert oxide layer on the surface of the metals
(d) none of these
Show Answer & Explanation

Answer: (c) formation of an inert oxide layer on the surface of the metals

Explanation:
1. A thin protective oxide film forms
2. It stops further attack

Teacher's Note:
1. Cr and Fe also become passive.

Question 21: Sodium loses its lustre on exposure to moist air due to the formation of:
(a) Na2O, NaOH and Na2CO3
(b) Na2O and NaOH
(c) Na2O and Na2CO3
(d) NaOH and Na2CO3
Show Answer & Explanation

Answer: (a) Na2O, NaOH and Na2CO3

Explanation:
1. Na first forms Na2O, then NaOH with moisture and finally Na2CO3 with CO2

Teacher's Note:
1. The dull crust is a mixture.

Question 22: Potassium carbonate when heated to high temperature:
(a) gives CO2
(b) gives O2
(c) gives CO
(d) gives no gas at all
Show Answer & Explanation

Answer: (d) gives no gas at all

Explanation:
1. K2CO3 is stable to heat
2. It does not decompose at ordinary temperatures

Teacher's Note:
1. Only Li2CO3 decomposes among group 1.

Question 23: On a flame test, K gives a ____ colour:
(a) golden yellow
(b) crimson red
(c) violet
(d) apple green
Show Answer & Explanation

Answer: (c) violet

Explanation:
1. Potassium gives a lilac or violet flame

Teacher's Note:
1. Sodium gives golden yellow.

Question 24: An element having the electronic configuration 1s2 2s2 2p6 3s2 3p6 4s1 will form:
(a) an acidic oxide
(b) a basic oxide
(c) an amphoteric oxide
(d) a neutral oxide
Show Answer & Explanation

Answer: (b) a basic oxide

Explanation:
1. The element is K, an alkali metal
2. Its oxides are strongly basic

Teacher's Note:
1. Metal oxides of group 1 are basic.

Question 25: Which decomposes on heating?
(a) NaOH
(b) KOH
(c) LiOH
(d) RbOH
Show Answer & Explanation

Answer: (c) LiOH

Explanation:
1. LiOH decomposes to Li2O and H2O
2. The other hydroxides are stable

Teacher's Note:
1. Li shows anomalous behaviour.

Question 26: Which metal does not form an ionic hydride?
(a) Na
(b) Rb
(c) Ca
(d) Be
Show Answer & Explanation

Answer: (d) Be

Explanation:
1. BeH2 is covalent and polymeric
2. The others form ionic hydrides

Teacher's Note:
1. Be has high ionisation and small size.

Question 27: The group 1 element that combines directly with nitrogen is:
(a) Li
(b) Na
(c) K
(d) Cs
Show Answer & Explanation

Answer: (a) Li

Explanation:
1. Li forms Li3N with N2 at room temperature
2. Other alkali metals do not

Teacher's Note:
1. This is an anomalous property of Li.

Question 28: Which of the following releases 0.2 mole of hydrogen on hydrolysis?
(a) 0.1 mole of LiH
(b) 0.2 mole of LiH
(c) 0.3 mole of LiH
(d) 0.4 mole of LiH
Show Answer & Explanation

Answer: (b) 0.2 mole of LiH

Explanation:
1. LiH + H2O gives LiOH + H2, one mole of H2 per mole of LiH
2. So 0.2 mol H2 needs 0.2 mol LiH

Teacher's Note:
1. One to one stoichiometry.

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