Question

Explain the following facts giving appropriate reason in each case.
(i) $N{F_3}$ is an exothermic compound whereas $NC{l_3}$ is not.
(iI) All the bonds in SF4 are not equivalent.

Answer 1

Hint: The bonds are stronger in $N{F_3}$ than in $NC{l_3}$ and breaking these bonds releases energy. The structure of $S{F_4}$ depends on the electron pairs and the geometry of the compound.

Complete answer:
(i) In case of $N{F_3}$ , $N - F$ bond strength is greater than $F - F$ bond strength whereas in $NC{l_3}$ , $N - Cl$ bond strength is lower than $Cl - Cl$ bond strength. Also, $Cl$ atom has a large size when compared to $F$ atom and therefore the bond strength of $NC{l_3}$ is less. Therefore, the formation of $N{F_3}$ is spontaneous but during the formation of $NC{l_3}$ energy needs to be supplied. Hence $N{F_3}$ is an exothermic compound but $NC{l_3}$ is an endothermic compound.

(ii) The five electron pairs around sulphur in $S{F_4}$ attain trigonal bi-pyramidal geometry in which one position is occupied by a lone pair. The lone pair finds a position that minimizes the number of repulsions it has with bonding electron pairs. It occupies an equatorial position with two repulsions. The bonded electrons occupy the axial and equatorial positions. The axial $S - F$ bonds are bent slightly away from the lone pair. Therefore, all the bonds in $S{F_4}$ are not equivalent.

Additional Information:
$N{F_3}$ is also called Nitrogen trifluoride. It is an inorganic compound which is colourless and non-flammable gas with a slightly musty odour. It has an important use as an etchant in microelectronics. It is an extremely strong greenhouse gas.
$NC{l_3}$ is also called Nitrogen trichloride and trichloramine. It is a chemical compound with yellow, oily, pungent smelling and explosive liquid. It is commonly encountered as a by-product of chemical reactions between ammonia derivatives and chlorine.
$S{F_4}$ also known as sulphur tetrafluoride is a chemical compound with colourless corrosive gas that releases dangerous HF upon exposure to water or moisture. This compound is a useful reagent for the preparation of organofluorine compounds.

Note: In $N{F_3}$ , $N - F$ bond is stronger and in $NC{l_3}$ , $N - Cl$ bond is less stronger and therefore more energy is released during breaking this bond and so it is an exothermic reaction. In $S{F_4}$, all the bonds are not equivalent as lone pairs find position to reduce repulsion and therefore bonded electrons occupy the axial and equatorial position.