Questions & explanations
1. In Group 14 elements, the tendency to show +2 oxidation state increases down the group due to the inert pair effect. Which of the following statements is correct?
- Carbon shows +2 oxidation state more readily than lead.
- Lead(II) compounds are more stable than lead(IV) compounds.
- Silicon exhibits inert pair effect more than tin.
- The inert pair effect is due to poor shielding of d and f electrons, making s electrons more available.
Answer: Lead(II) compounds are more stable than lead(IV) compounds.
The inert pair effect makes the ns² electrons of heavier Group 14 elements reluctant to participate in bonding, so Pb(II) is stable while Pb(IV) compounds are strongly oxidizing. Carbon does not exhibit this effect; its +2 state is less stable. Silicon shows weaker inert pair effect than tin. The inert pair effect arises because the ns² electrons are held more tightly due to poor shielding, making them less available for bonding.
2. Among the following group 13 elements, which one has the highest first ionization enthalpy?
- Boron
- Aluminium
- Gallium
- Indium
Answer: Boron
Ionization enthalpy decreases down the group due to increasing atomic size and shielding effect. Boron, being the smallest atom at the top, has the highest ionization enthalpy. Actually Ga (~579 kJ/mol) is slightly higher than Al (~577 kJ/mol), an NCERT-noted anomaly; the order is B > Tl > Ga > Al > In roughly (not monotonically decreasing from Al to In).
3. Which of the following accounts for the anomalous behaviour of carbon in Group 14?
- Carbon has the largest atomic radius in the group.
- Carbon can form multiple bonds involving pπ–pπ overlap due to its small size and lack of d orbitals.
- Carbon shows inert pair effect.
- Silicon forms stronger π bonds than carbon.
Answer: Carbon can form multiple bonds involving pπ–pπ overlap due to its small size and lack of d orbitals.
Carbon's small size allows effective lateral overlap of p orbitals to form strong π bonds (C=C, C≡C), and the absence of d orbitals prevents expansion of octet. Carbon has the smallest atomic radius in the group. The inert pair effect is observed in heavier elements, not carbon. Silicon cannot form strong pπ–pπ bonds and prefers single bonds.
4. Which of the following statements correctly distinguishes carbon monoxide (CO) from carbon dioxide (CO₂)?
- CO is an acidic oxide while CO₂ is a neutral oxide.
- CO has a triple bond between C and O, whereas CO₂ has double bonds.
- CO₂ is a reducing agent while CO is an oxidizing agent.
- Both CO and CO₂ are linear molecules with zero dipole moment.
Answer: CO has a triple bond between C and O, whereas CO₂ has double bonds.
Carbon monoxide has a C≡O triple bond (one sigma, two pi bonds), while carbon dioxide has two C=O double bonds (each with one sigma and one pi). CO is a neutral oxide; CO₂ is acidic. CO is a reducing agent, not an oxidizing agent. CO has a small dipole moment due to coordinate bonding, whereas CO₂ is nonpolar with zero dipole moment.
5. Which of the following is a correct statement about silicon compounds?
- Silicates are polymers containing Si–O–Si linkages and are used as building materials.
- Zeolites are aluminosilicates used as water softeners and catalysts.
- Silicones are synthetic polymers containing Si–O bonds and organic groups, used as lubricants and sealants.
- All of the above.
Answer: All of the above.
All three statements are true. Silicates form the backbone of many rocks and are used in cement and glass. Zeolites are microporous aluminosilicates employed in water softening and as heterogeneous catalysts. Silicones are organosilicon polymers (with Si–O backbone) widely used as sealants, lubricants, and insulating materials.
6. Which of the following statements about catenation in Group 14 elements is correct?
- Catenation tendency increases down the group.
- Silicon shows stronger catenation than carbon.
- The decreasing trend of catenation down Group 14 is due to decreasing bond enthalpy of M–M bonds.
- Catenation is not shown by any element other than carbon.
Answer: The decreasing trend of catenation down Group 14 is due to decreasing bond enthalpy of M–M bonds.
Catenation decreases down Group 14 because the M–M bond enthalpy decreases as atomic size increases, making chains less stable. Carbon has the highest catenation due to its small size and strong C–C bonds. Silicon shows weaker catenation than carbon. Many elements besides carbon (e.g., silicon, sulfur) also exhibit catenation.
7. Which of the following statements is correct about diborane (B₂H₆)?
- It contains two 3-center-2-electron B–H–B bonds and four 2-center-2-electron B–H bonds.
- It is a planar molecule with all B–H bonds equal.
- It is an ionic compound of boron and hydrogen.
- It is conveniently prepared by heating boron with hydrogen gas.
Answer: It contains two 3-center-2-electron B–H–B bonds and four 2-center-2-electron B–H bonds.
Diborane has a unique structure with two bridging hydrogen atoms involved in three-center-two-electron bonds (B–H–B) and four terminal B–H bonds. It is not planar; the terminal and bridge bonds differ. It is covalent, not ionic, and is prepared by reduction of BF₃ or BCl₃ with hydrides, not by direct combination of elements.
8. The anomalous behaviour of boron, compared to other group 13 elements, is primarily due to its:
- high electronegativity
- small atomic size
- high melting point
- ability to form dπ-pπ bonds
Answer: small atomic size
Boron's small atomic size and high ionization enthalpy lead to covalent bonding, lack of metallic character, and tendency to form electron-deficient compounds. While high electronegativity and dπ-pπ bonding are related consequences, the primary cause is its small size.
9. Which of the following is a correct comparison of diamond, graphite, and fullerene?
- Diamond and graphite are both good conductors of electricity.
- Graphite has a three-dimensional network structure while diamond has a layered structure.
- Fullerenes are molecules of carbon atoms arranged in spherical or tubular shapes.
- In diamond, each carbon atom is sp² hybridized.
Answer: Fullerenes are molecules of carbon atoms arranged in spherical or tubular shapes.
Fullerenes (e.g., C₆₀) are cage-like molecules made of carbon atoms. Diamond has a three-dimensional network of sp³ hybridized carbon and is an insulator. Graphite has layered structure of sp² carbon and conducts electricity. Diamond is sp³ hybridized, not sp².
10. In group 13, which element exhibits a greater stability of the +1 oxidation state compared to the +3 oxidation state due to the inert pair effect?
- Aluminium
- Gallium
- Indium
- Thallium
Answer: Thallium
The inert pair effect becomes significant down the group. For thallium (Tl), the 6s² electrons are reluctant to participate in bonding, making the +1 state more stable than the +3 state. Aluminium, gallium, and indium predominantly show +3 oxidation state.
11. Boric acid (H₃BO₃) behaves as a:
- monobasic acid
- dibasic acid
- tribasic acid
- strong acid
Answer: monobasic acid
Boric acid acts as a weak monobasic acid because it accepts a hydroxyl ion from water to form [B(OH)₄]⁻, releasing one H⁺. It does not donate three protons; thus it is not tribasic or dibasic. It is also not a strong acid.
12. The amphoteric nature of aluminium is best demonstrated by the fact that aluminium hydroxide:
- dissolves only in acids
- dissolves only in bases
- dissolves in both acids and strong bases
- is insoluble in both acids and bases
Answer: dissolves in both acids and strong bases
Aluminium hydroxide (Al(OH)₃) reacts with acids to form aluminium salts (e.g., AlCl₃) and with strong bases to form aluminates (e.g., NaAlO₂). This amphoteric behaviour shows it acts as both a base and an acid.