Question

Given the following standard molar enthalpies:
$\Delta H_f^o$ of ${CH}_{3}CN\left(g\right)=88kJ/mol$, $\Delta H_f^o$ of $C_2{H}_6=-84kJ/mol$, $\Delta H_{sublimation}^o$ of $C\left(gr\right)=717kJ/mol$, bond dissociation energy of $N_2\left(g\right)$ and $H_2\left(g\right)$ are $946$ and $436kJ/mol$ respectively, $B.E(C-H)=410kJ/mol$. Determine $C-C$ and $C\equiv N$ bond energies respectively.

• A. $366kJ/mol$; $1095kJ/mol$
• B. $366kJ/mol$; $877kJ/mol$
• C. $877kJ/mol$; $366kJ/mol$
• D. $877kJ/mol$; $1095kJ/mol$

Answer 1

Answer: (B)

$366kJ/mol$; $877kJ/mol$

Given
The balanced chemical reaction for the formation of acetonitrile is
$2C_{\left(s\right)}+\frac{3}{2}{H}_{2\left(g\right)}+\frac{1}{2}{N}_{2\left(g\right)}⟶{CH}_3{CN}_{\left(g\right)}$ $\Delta H=88$
$\Delta H_{f,C{H}_{3}C\equiv N}=2\times \Delta H_{sublimation,C\left(g\right)}^o+1.5B_{H-H}+0.5B_{N\equiv N}-[3\times B.C-H+B_{C-C}+B_{C\equiv N}]......\left(i\right)$
The balanced chemical reaction for the formation of ethane is
$2C_{\left(s\right)}+3H_{2\left(g\right)}⟶C_2{H}_{6\left(g\right)}$ $\Delta H=-\mathrm{84......}\left(ii\right)$
For the sublimation of C(graphite)
$C_{\left(s\right)}⟶C_{\left(g\right)}$ $\Delta H=717$
For the dissociation of nitrogen,
$N_{2\left(g\right)}⟶2N_{\left(g\right)}$ $\Delta H=946$
For the dissociation of hydrogen
$H_{2\left(g\right)}⟶2H_{\left(g\right)}$ $\Delta H=436$
The bond dissociation energy for C-H bond is
$B_{C-H}=410$
Substitute values in equation $\left(i\right)$
$(2\times 717+1.5\times 436+0.5\times 946)-(3\times 410+B_{C-C}+B_{C\equiv N})=88$
$B_{C-C}+B_{C\equiv N}=\mathrm{1243.........}\left(iii\right)$
from equation $\left(ii\right)$
$(2\times 717+3\times 436)-B_{C-C}+6\times 410)=-84$
$B_{C-C}=366kJ/mole$ from equation $\left(iii\right)$
$B_{C\equiv N}=877kJ/mole$