APSC Legal Metrology: C-H bond energy Paper 2

In the series ethane, ethene and ethyne, the C-H bond energy is
(A) the same in all the compounds
(B) greatest in ethane
(C) greatest in ethene
(D) greatest in ethyne

The correct answer is: (D) greatest in ethyne

Explanation:

In the series ethane (C₂H₆), ethene (C₂H₄), and ethyne (C₂H₂), the C–H bond energy increases as we move from ethane to ethyne.

This is because:

• In ethane, the carbon atom is sp³-hybridized, and the C–H bond has more s-character from hydrogen but less from carbon (25% s-character in sp³).
• In ethene, the carbon is sp²-hybridized (33% s-character), so the C–H bond is slightly stronger due to more s-character from carbon.
• In ethyne, the carbon is sp-hybridized (50% s-character), so the C–H bond is strongest due to maximum s-character, meaning the bonding electrons are closer to the nucleus and the bond is stronger.

Hence, C–H bond energy is greatest in ethyne.