Question

$\Delta U^°$ of combustion of $CH_{4(g)}$ at certain temperature is $- 393 \,kJ\, mol^{-1}$. The value of $\Delta H^°$ is

• A. zero
• B. $< \Delta U^°$
• C. $> \Delta U^°$
• D. equal to $\Delta U^°$

Answer 1

Answer: (B)

The balanced equation for combustion of methane is
$C H _{4\left(g\right)} + 2 O_{2\left(g\right)} \to CO_{2\left(g\right)} + 2H_{2}O_{\left(l\right)}$
Here, $\Delta n_{g} = 1 - 3 = - 2$
$\Delta H^{\circ}= \Delta U^{\circ}+\Delta n_{g}RT$
$\Delta H^{\circ} = -393 - 2RT$
$\therefore \Delta H^{\circ} < \Delta U^{\circ}$