Due to smaller size of F as compared to Cl, the N-F bond is much stronger `(272k kJ mol^(-1))` than N-Cl bond `(201 kJ mol^(-1))` while bond dissociation energy of `F_(2)(158.8 kJ mol^(-1))` is much lower than that of `Cl_(2)(242.6 kJ mol^(-1))`. Therefore, energy released during the formation of `NF_(3)` molecule is more than the energy needed to break `N_(2)(941.4 kJ mol^(-1))` and `F_(2)` molecules into individual atoms. In other words formation of `NF_(3)` is an exothermic reaction or `NF_(3)` is an exothermic compoud.
`{:(N_(2)(g)+3F_(2)(g)to2NF_(3)(g),,DeltaH=-214.2Jmol^(-)),(N_(2)(g)+3Cl_(2)(g)to2NCl_(3)(g),,DeltaH=-463.2kJmol^(-1)):}`
In contrast, energy released during the formation of `NCl_(3)` molecule is les than the energy needed to break `N_(2)andCl_(2)` molecules into individual atoms. In other words, formation of `NCl_(3)` is an endothermic reaction or `NCl_(3)` is an endothermic compound.
