The P Block Elements - Result Question 64
####67. Which one of the following orders is correct for the bond dissociation enthalpy of halogen molecules?
[2016]
(a) $I_2>Br_2>Cl_2>F_2$
(b) $Cl_2>Br_2>F_2>I_2$
(c) $Br_2>I_2>F_2>Cl_2$
(d) $F_2>Cl_2>Br_2>I_2$
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Solution:
- (b) Bond dissociation enthalpy decreases as the bond distance increases from $F_2$ to $I_2$. This is due to increase in the size of the atom, on moving from $F$ to $I$.
$F-F$ bond dissociation enthalpy is smaller then $Cl-Cl$ and even smaller than $Br-Br$. This is because $F$ atom is very small and hence the three lone pairs of electrons on each $F$ atom repel the bond pair holding the $F$-atoms in $F_2$ molecules. The increasing order of bond dissociation enthalphy is
$ I_2<F_2<Br_2<Cl_2 $
