Question

For the combustion reaction at $298K$
$2Ag\left(s\right)+1/2O_2\left(g\right)⟶2A{g}_{2}O\left(s\right)$ Which of the following alternative is correct?

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

Answer 1

Answer: (C)

$\Delta H<\Delta U$

As we know,
$\Delta H=\Delta U+\Delta nRT$
$\Delta n=n_P-n_R$
$\therefore \Delta n=2-\frac{5}{2}=-\frac{1}{2}$
$\therefore \Delta H=\Delta U-\frac{1}{2}RT$
$\therefore \Delta U=\Delta H+\frac{1}{2}RT$
or $\therefore \Delta U>\Delta H$