Question

For the combustion reaction at $298\, K$
$2 Ag (s)+\frac{1}{2} O _{2}( g ) \rightarrow 2 Ag _{2} O ( s )$
Which of the following alternatives is correct?

• A. $\Delta H =\Delta U$
• B. $\Delta H >\Delta U$
• C. $\Delta H <\Delta U$
• D. $\Delta H$ and $\Delta U$ has no relation with each other

Answer 1

Answer: (C)

$\Delta H =\Delta U +\Delta nRT$
$\Delta n=n_{P}-n_{R}$
Now, $\Delta n=2-\frac{5}{2}=-\frac{1}{2}$
$\therefore \Delta H=\Delta U-\frac{1}{2} R T$
Thus, $\Delta U=\Delta H+\frac{1}{2} R T$
$\therefore \Delta U>\Delta H$