Question

The difference in $\Delta$ H and $\Delta U$ for the combustion of solid benzoic acid at 300 K is equal to:

• A. $-1.247$ kJ
• B. $+1.247$ kJ
• C. $-2.247$ J
• D. $2.247$ J

Answer 1

Answer: (A)

$-1.247$ kJ

Solution:- (A) $-1.247kJ$

Combustion of benzoic acid $\to$

${C_6{H}_5-COOH}_{\left(s\right)}+\frac{15}{2}{O_2}_{\left(g\right)}⟶7{C{O}_2}_{\left(g\right)}+3{H_{2}O}_{\left(l\right)}$

$\Delta n_g=n_P-n_R=7-\frac{15}{2}=-\frac{1}{2}$

$T=300K\left(\text{Given}\right)$

Now, as we know that,

$\Delta H=\Delta U+\Delta n_{g}RT$

$\Rightarrow \Delta H-\Delta U=(-\frac{1}{2})\times 8.314\times 300$

$\Rightarrow \Delta H-\Delta U=-1247.1J=-1.247kJ$

Hence the difference in $\Delta H$ and $\Delta U$ for the combustion of solid benzoic acid at $300K$ is $-1.247kJ$.