Question

For the reaction $N_2+3X_2\to 2N{X}_{3}whereX=F,Cl$ (the average bond energies are $F-F=155kJmo{l}^{-1},N-F=272kJmo{l}^{-1},Cl-ClkJmo{l}^{-1},N-ClkJmo{l}^{-1}andN\equiv N=941kJmo{l}^{-1})$.
The heats of formation of $N{F}_{3}andNC{l}_{3}inkJmo{l}^{-1}$, respectively, are closest to

• A. – 226 and + 467
• B. + 226 and –467
• C. –151 and + 311
• D. + 151 and -311

Answer 1

The correct option is A

– 226 and + 467

– 226 kJ and 467 kJ are heat of reaction not heat of formation.

$\Delta H_r=ϵN\equiv N+3\times {ϵ}_{x-x}-2[3\times {ϵ}_{N-x}]$

$\Delta H_r={ϵ}_{N\equiv N}+3\times {ϵ}_{F-F}-2\times 3\times {ϵ}_{N-F}$

$=941+3\times 155-6\times 272\Delta H_r=-226kJ$

$\Delta H_r={ϵ}_{N\equiv N}+3{ϵ}_{Cl-Cl}-6\times {ϵ}_{N-Cl}=941+3\times 242-6\times 200=467kJ$