Question

What is the correct order of enthalpy of dissociation of halogens?

(a) F2 > Cl2 > Br2 > I2

(b) Cl2 > Br2 > F2 > I2

(c) Cl2 > F2 > Br2 > I2

(d) F2 > Br2 > Cl2 > I2

The question was asked in a job interview.

I would like to ask this question from P-Block topic in division P-Block Elements of Chemistry – Class 12

Answer 1

Correct option is (b) Cl2 > Br2 > F2 > I2

Easiest explanation: Enthalpy of dissociation decreases as the bond distance increases from F2 to I2 due to a corresponding increase in the size of the atom as we move down the group from F to I. The F-F bond dissociation enthalpy is, however, smaller than that of Cl-Cl and even smaller than that of Br-Br. This is due to the reason that F atom is very small and hence the electron-electron repulsions between the lone pairs of electrons are very large.