Question
Which one of the following arrangement does not give the correct picture of the trends indicated against it?
-
F2 > Cl2 > Br2 > I2 : Oxidising power
-
F2 > Cl2 > Br2 > I2 : Electron gain enthalpy
-
F2 > Cl2 > Br2 > I2 : Bond dissociation energy
-
F2 > Cl2 > Br2 > I2 : Electronegativity
Solution
C.
F2 > Cl2 > Br2 > I2 : Bond dissociation energy
Generally, bond dissociation energies decrease in a group. Bond dissociation energy also decreases with repulsion.
| X-X Bond | F-F | Cl-Cl | Br- Br | I-I |
| Bond length (A) | 1.42 | 1.99 | 2.28 | 2.67 |
| Bond dissociation energy (kcal/mol) | 38 | 57 | 45.5 | 35.6 |
In general, the bond dissociation energy decreases as the bond length increases, but the bond dissociation energy of F2 is less than that of Cl2. It is due to greater interelectronic repulsion between the lone pair of electrons on the two bonded fluorine atoms. Hence, the order of bond dissociation energy is as:
Cl2 > Br2 > F2 > I2
