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Question
Arrange the following in the order of property indicated for each set:
F2, Cl2, Br2, I2 - increasing bond dissociation enthalpy.
Solution 1
Bond dissociation energy usually decreases on moving down a group as the atomic size increases. However, the bond dissociation energy of F2 is lower than that of Cl2 and Br2. This is due to the small atomic size of fluorine. Thus, the increasing order for bond dissociation energy among halogens is as follows:
I2 < F2 < Br2 < Cl2
Solution 2
Bond dissociation enthalpy decreases as the bond distance increases from F2 to I2 due to increase in the size of the atom, on moving from F to I.
F – F bond dissociation enthalpy is smaller then the Cl – Cl and even smaller than Br – Br. This is because F atom is very small and have large electron-electron repulsion among the lone pairs of electrons in F2molecule where they are much closer to each other than in case of Cl2. The increasing order of bond dissociation enthalphy is I, < F2 < Br2< Cl2
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