question_answer

Given the following bond dissociation enthalpies \[(kJ\,\,mo{{l}^{-1}})\]. \[P\equiv P-490,\,P-P=209,\,N\equiv N-946,N-N-160\]Compare the enthalpy changes for the process (i) \[2{{P}_{2}}(g)\xrightarrow{{}}{{P}_{4}}(g)\] (ii) \[2{{N}_{2}}(g)\xrightarrow{{}}{{N}_{4}}(g)\] Choose the correct option.

A)  \[{{P}_{4}}\] is feasible but \[{{N}_{4}}\]is hot feasible

B)  \[{{P}_{4}}\] is not feasible but \[{{N}_{4}}\] is feasible

C)  Both \[{{P}_{4}}\] and \[{{N}_{4}}\] are feasible

D)  Both \[{{P}_{4}}\] and \[{{N}_{4}}\]are not feasible

Correct Answer: A

Solution :

The process involves replacement of \[2\,(X\equiv X)\] units by \[6\,X-X\] bonds in the \[{{X}_{4}}\]tetrahedron. Energy is released when bonds are formed and thus species formed is stable. (i) \[2{{P}_{2}}(g)\xrightarrow{{}}{{P}_{4}}(g)\] \[\Delta {{H}_{f}}=-6\times BE\,(P-P)+2BE\,(P\equiv P)\] (BE = Bond energy) \[=-6\times 209+2\times 490\] \[=-274\,\,kJmo{{l}^{-1}}\] Formation of \[{{P}_{4}}\] is exothermic thus is feasible. (ii) \[2{{N}_{2}}(g)\xrightarrow{{}}{{N}_{4}}(g)\]                 \[\Delta {{H}_{f}}=-6\,BE\,(N-N)+2\,BE\,(N\equiv N)\]                 \[=-6\times 160+2\times 946\]                 \[=+932\,\,\,kJ\,\,mo{{l}^{-1}}\] Formation of \[{{N}_{4}}\] is endothermic, thus is not feasible.