Question

The difference between $\Delta H$ and $\Delta U$ for the combustion reaction of benzene at ${127}^{\circ }C$ is:

• A. $-200R$
• B. $-600R$
• C. $200R$
• D. $600R$

Answer 1

The correct option is B $-600R$

Combustion of benzene:

$C_6{H}_6\left(l\right)+\frac{15}{2}{O}_2\left(g\right)⟶6C{O}_2\left(g\right)+3H_{2}O\left(l\right)$

$\Delta H=\Delta U+P\Delta V;$ $\Delta n=6-\frac{15}{2}=-1.5$

$\Delta H-\Delta U=P\Delta V$

$\Delta H-\Delta U=\Delta nRT$

$\Delta H-\Delta U=-1.5\times R\times (127+273)$

$\Delta H-\Delta U=-600R$ .