Question

At 300 K, the standard enthalpies of formation of $C_6{H}_{5}COOH\left(s\right)$,$C{O}_2\left(g\right)$ and $H_{2}O\left(s\right)$ are -408, -393 and $-286kJmo{l}^{-1}$, respectively. Calculate the heat of combustion of benzoic acid at constant volume.

• A. $-3201kJmo{l}^{-1}$
• B. $-2300kJmo{l}^{-1}$
• C. $-2301kJmo{l}^{-1}$
• D. $-3603kJmo{l}^{-1}$

Answer 1

Answer: (A)

$-3201kJmo{l}^{-1}$

Solution:- (A) $-3201kJ/mol$

Combustion of ethane-

${C_6{H}_{5}COOH}_{\left(s\right)}+\frac{15}{2}{O_2}_{\left(g\right)}⟶7{C{O}_2}_{\left(g\right)}+3{H_{2}O}_{\left(l\right)}$

$\Delta H^0={\sum \Delta {H^0}_f}_{\left(product\right)}-{\sum \Delta {H^0}_f}_{\left(reactant\right)}$

$\therefore \Delta H^0=(7\times (-393)+3\times (-286))-(-408+0)$

$\Rightarrow \Delta H^0=-3609+408=-3201kcal$

Hence the standard molar heat of combustion will be $-3201kJ/mol$.