question_answer

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

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

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

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

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

Correct Answer: A

Solution :

The process involves replacement of \[\text{2}\left( \text{X}\equiv \text{X} \right)\] 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) \[2{{P}^{2}}(g)\to {{P}_{4}}(g)\]                 \[\Delta {{H}_{t}}=-6\times BE(P-P)+2BE(P\equiv P)\]                                                                 (BE =Bonds energy )                 \[=-6\times 209+2\times 490=-2\text{74 kJ mo}{{\text{l}}^{\text{-}}}\] Formation of \[{{P}_{4}}\] is exothermic thus is feasible. (ii) \[2{{N}_{2}}(g)\to {{N}_{4}}(g)\] \[\Delta {{H}_{t}}=-6BE(N-N)+2BE(N\equiv N)\] \[=-6\times 160+2\times 946=+9\text{32 kJ mo}{{\text{l}}^{\text{-1}}}\]                 Formation of \[{{N}_{4}}\] is endothermic, thus is not feasible.