Question

Heat of formation, ${{\Delta H}}_{\text{f}}^{\text{o}}$ of an explosive compound like ${\text{NC}}{{\text{l}}_{\text{3}}}$ is
(A) Positive
(B) Negative
(C) Zero
(D) Positive or negative

Answer 1

Hint: We know that the change in heat during the formation of one mole of a substance from its constituent elements is known as the heat of formation or the enthalpy of formation. All the elements must be in their standard states.

Complete step by step solution:
We know that nitrogen trichloride $\left( {{\text{NC}}{{\text{l}}_{\text{3}}}} \right)$ is an explosive liquid. it is yellow in colour and has a pungent smell. Nitrogen trichloride is also known as trichloramine.
Nitrogen trichloride is prepared by the treatment of ammonium nitrate with chlorine. Nitrogen trichloride is an explosive and explodes to give nitrogen gas and chlorine gas.
Nitrogen trichloride is highly unstable in nature. This is because the sizes of nitrogen and chlorine are quite different. The bonding in nitrogen trichloride occurs between the 2p orbital of nitrogen and 3p orbital of chlorine and the bonding is not very strong.
The lone pairs on the nitrogen and chlorine atoms cause lone pair-lone pair repulsion making nitrogen trichloride unstable.
When nitrogen trichloride is formed, it is formed by absorption of heat. This is because it is explosive in nature.
As heat is absorbed the value of heat of formation is positive.
Thus, the heat of formation, ${{\Delta H}}_{\text{f}}^{\text{o}}$ of an explosive compound like ${\text{NC}}{{\text{l}}_{\text{3}}}$ is positive.

Thus, the correct option is (A) positive.

Note: The heat of formation of a substance depends on the physical state of the reactants and the products, the pressure of gases present and the temperature at which the reaction occurs. Nitrogen trichloride is sensitive to light, heat, moderate shock and even organic compounds.