Question

Evaluate $\Delta U$ at $500K$ for the process
$Mg\left(s\right)+2HCl\left(g\right)=Mg{Cl}_2\left(s\right)+H_2\left(g\right)$
$\Delta H=-109kcal$
The volume change of solids to be ignored:

• A. $-204kcal$
• B. $-108kcal$
• C. $-9kcal$
• D. $+9kcal$

Answer 1

The correct option is B $-108kcal$

$△n=1-2=-1$

$R=2$ cal

$T=300K$

$△H=△U+△nRT$

$△=△H-△nRT$

$=-109$ kcal $-(-1)\left(2\right)\left(500\right)$ cal

$=-109$ kcal $+1000$ cal

$=1$ kcal $-109$ kcal

$=-108$ kcal.