`NCl_(3)` is an endothermic compound while `NF_(3)` is an exothermic compound. Explain.

Explanation

Due to the smaller size of F as compared to Cl, the N - F bond is much stronger (272 kJ-`mol^(- 1 ))` than the 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) (242.6kJ.mol^(- 1))` . Hence, the energy released during formation of `NF_(3)` molecule is more than the energy required to cleave the `N equiv N (941.4kJ .mol^(-1))` and F-F bonds. In other words, formation of `NF_(3)` is an exothermic reactin of `NF_(3)` is an exothermic compound.
`N_(2)(g) +3F_(2)(g)to 2NF_(3)(g), DeltaH^(0)=- 224.2kJ.mol^(-1)`
On the other hand, the energy released during formatiop. of `NCl_(3)` molecule is less than the energy required to cleave `NequivN` and Cl-Cl bonds.
`N_(2)(g) + 3Cl_(2)(g)to2NCl_(3) (g), DeltaH^(0) = +512.2kJ.mol^(-)`
Hence, formation of `NCl_(3)~ is an endothermic reaction or `NCl_(3)` is an endothermic compound.