Q62 / 180 · AIIMS 2002 · Chemistry

Assertion : F - F bond has low bond dissociation energy. Reason : The fluorine has lower reactivity.

  • A
    If both the assertion and reason are true and reason is a correct explanation of the assertion.
  • B
    If both assertion and reason are true but assertion is not a correct explanation of the assertion.
  • Correct answer
    C
    If the assertion is true but the reason is false.
  • D
    If both assertion and reason are false.

Solution

Lower value of bond dissociation energy of fluorine is due to the high inter electronic repulsions between non-bonding electrons of the $2 p$-orbital of small sized fluorine. As a result $\mathrm{F}-\mathrm{F}$ bond is weaker as comparison to $\mathrm{Cl}-\mathrm{Cl}$ and $\mathrm{Br}-\mathrm{Br}$ bonds. $$ \begin{array}{lr} & \text { Bond dissociation energy } \\ \mathrm{F}-\mathrm{F} & 38 \mathrm{~K} \mathrm{cal} / \mathrm{mole} \\ \mathrm{Cl}-\mathrm{Cl} & 57 \mathrm{~K} \mathrm{cal} / \mathrm{mole} \\ \mathrm{Br}-\mathrm{Br} & 45.5 \mathrm{~K} \mathrm{cal} / \mathrm{mole} \\ \mathrm{I}-\mathrm{I} & 35.6 \mathrm{~K} \mathrm{cal} / \mathrm{mole} \end{array} $$

Hence, the correct answer is option: 'C'.

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