Question

Give reasons for the following:
Bond enthalpy of${{F}_{2}}$ is lower than that of $C{{l}_{2}}$

Answer 1

The bond enthalpy is also known as the bond dissociation energy. The bond dissociation energy is defined as the energy which is required in the endothermic process for breaking the chemical bond for the formation of the two molecules or the atomic structure with alone or the shared pair of the electron. From this it means that a large amount of the dissociation energy is required for breaking the stable chemical bond.

Complete step-by-step answer: The bond enthalpy tends to increase with the increase in the difference of the electronegativities of the atoms which are bonded.
Fluorine atom is considered as the smallest molecule which has the small radii and according to theoretical concept it has the greatest force of attraction between its nuclei and the shared pair of its electrons which tends to make its covalent bond very hard to break. Whereas in the chlorine atom it is big in size as compared to fluorine and in chlorine the attraction between the nuclei and the shared pair of electrons is not much as compared to fluorine.
In fluorin the nuclear attraction is strong to the electrons because it has the high effective nuclear charge which tends to cause less shielding of the valence electrons and causes the interelectronic repulsions between the F – F molecules. The molecules tend to repel each other because of the valence electrons to be closer to each other which tends to make the F – F bond weak and so the less bond enthalpy is required. In chlorine though the nuclear attraction is less but the electron electron repulsion between the electrons is less so due to less interelectronic repulsion the bond enthalpy of chlorine is more in comparison to fluorine.

Note: The bond enthalpy helps in calculating the standard enthalpy change when the bond is been cleaved between that of the reactant and the products of the reaction at 0K. The fluorine as a molecule has high electronegative property. Despite its low size it needs low bond enthalpy for breaking the bond.