Question

$\Delta _{f}U^\circ $ of formation of $CH_{4}$ (g) at certain temperature is -393 kJ $mol^{- 1}$ . The value of $\Delta _{f}H^\circ $ is

• A. zero
• B. $ < \Delta _{f}U^\circ $
• C. $>\Delta _{f}U^\circ $
• D. equal to $\Delta _{f}U^\circ $

Answer 1

Answer: (B)

$\underset{\left(\right. s \left.\right)}{C}+\underset{\left(\right. g \left.\right)}{2 H_{2}} \rightarrow \underset{\left(\right. g \left.\right)}{C H_{4}}$
$\Delta n_{g}=-1$
$\Delta H=\Delta U+\Delta n_{g}RT$
$\Delta n_{g}=-ve$
$\Delta H < \Delta U$