Anomalous Behaviour of Carbon - UNSOLVED PRACTICE SET
Chapter: p-Block Elements Groups 13 and 14 | Topic: Anomalous Behaviour of Carbon
ANOMALOUS BEHAVIOUR OF CARBON - UNSOLVED PRACTICE SET
Topic: Anomalous Behaviour of Carbon
Multiple Choice Questions
Q1. Carbon shows anomalous behaviour compared to other Group 14 elements mainly because of:
- Its high atomic mass
- Its small size, high electronegativity, and ability to form pπ-pπ bonds
- Its metallic character
- Its availability of d-orbitals
Q2. Carbon can form stable multiple bonds (C=C, C≡C), but silicon cannot because:
- Silicon is more electronegative
- Silicon atoms are too large for effective sideways overlap of p-orbitals
- Silicon has fewer valence electrons
- Silicon is less reactive
Q3. Carbon dioxide (CO₂) is a gas, while silicon dioxide (SiO₂) is a solid with a very high melting point because:
- CO₂ has a higher molecular mass
- CO₂ exists as discrete molecules with weak van der Waals forces, while SiO₂ forms a giant covalent network
- CO₂ has stronger covalent bonds
- CO₂ is more polar than SiO₂
Q4. The maximum covalency of carbon is 4, while other Group 14 elements can show a covalency of 6 because:
- Carbon has fewer electrons
- Carbon does not have vacant d-orbitals in its valence shell
- Carbon is more electronegative
- Carbon forms stronger bonds
Q5. Carbon forms a vast number of compounds (millions of organic compounds) because of:
- Its ability to form only single bonds
- Its property of catenation and formation of multiple bonds
- Its high reactivity with all elements
- Its metallic nature
Q6. Which of the following is NOT a reason for carbon's anomalous behaviour?
- Small atomic size
- High ionisation enthalpy
- Availability of vacant d-orbitals
- High electronegativity
Short Answer Questions
Q7. Why does carbon form strong pπ-pπ bonds, while silicon and other Group 14 elements cannot? Explain with reference to atomic size and orbital overlap.
Q8. Carbon tetrachloride (CCl₄) does not hydrolyse, while silicon tetrachloride (SiCl₄) hydrolyses readily in water. Explain this difference.
Q9. Why does carbon form a large number of stable hydrides (hydrocarbons), while silicon forms only a limited number of silanes that are less stable?
Q10. Explain why carbon monoxide (CO) is stable, while silicon monoxide (SiO) is not known as a stable compound under normal conditions.
Q11. Your teacher explains that while both carbon and silicon are in Group 14, carbon-based life dominates Earth, but silicon-based life is a popular science fiction concept. Explain two chemical reasons why carbon is better suited as the basis of life than silicon.
Q12. Why is the C–C bond energy (347 kJ/mol) much higher than the Si–Si bond energy (226 kJ/mol)? What does this imply about the stability of carbon chains vs. silicon chains?
Long Answer Questions
Q13. Discuss the anomalous behaviour of carbon compared to other Group 14 elements. Explain at least five properties or characteristics where carbon differs significantly from silicon, germanium, tin, and lead. Relate these anomalies to carbon's small size, high electronegativity, and absence of d-orbitals.
Q14. Compare the chemistry of carbon and silicon under the following headings, highlighting carbon's anomalous nature:
(a) Nature of oxides — CO₂ (molecular, gas) vs. SiO₂ (network, solid)
(b) Nature of halides — CCl₄ (non-hydrolysable) vs. SiCl₄ (readily hydrolysed)
(c) Catenation — stability of carbon chains vs. silicon chains
(d) Multiple bond formation — C=C, C≡C vs. inability of Si to form similar bonds
Give reasons for each difference.
Q15. Carbon's unique properties have made it the foundation of life and technology. Discuss:
(a) Why carbon is the "element of life" and the basis of organic chemistry and biochemistry
(b) The concept of "silicon-based life" in science fiction and why it remains fiction
(c) Why carbon nanomaterials (fullerenes, nanotubes, graphene) are revolutionising technology while silicon remains the workhorse of electronics
(d) The emerging field of "carbene" and "graphene" research in Indian institutes like IISc and IITs
Numerical / Application-Based Problems
Q16. The bond energies of various bonds are given below:
C–C = 347 kJ/mol, Si–Si = 226 kJ/mol
C–H = 413 kJ/mol, Si–H = 318 kJ/mol
C–O = 358 kJ/mol, Si–O = 452 kJ/mol
(a) Calculate the total bond energy for a C₂H₆ molecule and a Si₂H₆ molecule.
(b) Which hydride is more stable based on these values? Give reason.
(c) Despite Si–O bonds being stronger than C–O bonds, why does carbon (not silicon) form the basis of biological macromolecules?
Q17. In organic chemistry, carbon forms chains of varying lengths. Consider a straight-chain alkane with formula CₙH₂ₙ₊₂.
(a) If the total bond energy of a C₅H₁₂ molecule is calculated using the given bond energies (C–C = 347, C–H = 413 kJ/mol), calculate this total energy.
(b) Compare this with the total bond energy of a hypothetical straight-chain silane Si₅H₁₂ (Si–Si = 226, Si–H = 318 kJ/mol).
(c) Based on these calculations, explain why carbon chains are more stable and can grow longer than silicon chains.
Q18. Carbon's ability to form pπ-pπ bonds is quantified by considering the overlap integral. The effective overlap of p-orbitals decreases with distance.
(a) If the C=C bond length is 134 pm and the Si=Si bond length (if it existed) would be about 214 pm, calculate the ratio of overlap for C=C to Si=Si assuming overlap is inversely proportional to distance.
(b) Explain why this reduced overlap makes Si=Si bonds unstable.
(c) If carbon's electronegativity is 2.5 and silicon's is 1.8, calculate the difference and explain how this affects the polarity and stability of C–H vs. Si–H bonds.