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Question: Which of the following order(s) is/are CORRECT regarding bond strengths:...

Which of the following order(s) is/are CORRECT regarding bond strengths:

A

F₂ > Cl₂ > Br₂ > I₂

B

C≡C > C=C > C-C

C

-C-C- > -Si-Si-

D

-N-N- > -P-P- > -As-As-

Answer

B, C

Explanation

Solution

The problem asks to identify the correct order(s) of bond strengths among the given options. We will analyze each option based on known chemical principles and experimental bond dissociation energies.

Analysis of Options:

(A) F₂ > Cl₂ > Br₂ > I₂ This order pertains to the bond dissociation energies of dihalogen molecules. The general trend for bond strength down a group is a decrease due to increasing atomic size and bond length. However, there is an exception for F₂. The bond dissociation energies are:

  • Cl₂: ~242 kJ/mol
  • Br₂: ~193 kJ/mol
  • F₂: ~159 kJ/mol
  • I₂: ~151 kJ/mol The correct order of bond strengths is Cl₂ > Br₂ > F₂ > I₂. The F-F bond is weaker than expected due to significant lone pair-lone pair repulsion between the small fluorine atoms. Therefore, option (A) is incorrect.

(B) C≡C > C=C > C-C This order compares the bond strengths of carbon-carbon single, double, and triple bonds. Multiple bonds are generally stronger than single bonds due to the presence of additional pi bonds.

  • C≡C (triple bond): ~839 kJ/mol
  • C=C (double bond): ~614 kJ/mol
  • C-C (single bond): ~348 kJ/mol This order correctly reflects that a triple bond is stronger than a double bond, which is stronger than a single bond. Therefore, option (B) is correct.

(C) -C-C- > -Si-Si- This option compares the bond strengths of C-C and Si-Si single bonds.

  • C-C bond: ~348 kJ/mol
  • Si-Si bond: ~226 kJ/mol Carbon atoms are smaller than silicon atoms, leading to better orbital overlap and stronger bonds. This is why carbon exhibits extensive catenation compared to silicon. Therefore, option (C) is correct.

(D) -N-N- > -P-P- > -As-As- This order compares the bond strengths of single bonds between Group 15 elements. Similar to the F-F and O-O bonds, the N-N bond is anomalously weak.

  • P-P bond: ~209 kJ/mol
  • N-N bond: ~163 kJ/mol
  • As-As bond: ~146 kJ/mol The correct order of bond strengths is P-P > N-N > As-As. The N-N bond is weaker than the P-P bond due to significant lone pair-lone pair repulsion between the small nitrogen atoms. Therefore, option (D) is incorrect.

Conclusion: Options (B) and (C) present correct orders of bond strengths.