Question

Consider the reaction at $300K$
$H_2\left(g\right)+{Cl}_2\left(g\right)\to 2HCl\left(g\right)$
$\Delta H^o=-185kJ/mol$
If $3$ mole of $H_2$ completely react with $3$ mole of ${Cl}_2$ to form $HCl$. What is $\Delta U^o$ for this process?

• A. $0kJ$
• B. $185kJ$
• C. $555kJ$
• D. $-553.8kJ$

Answer 1

The correct option is D $-553.8kJ$

$\Delta H=-184.6KJ/mol$

As we know

$\Delta H=\Delta U+p\times \Delta V$

There is no volume change in this reaction. At constant $T,P$ we have $\Delta V=\left(\Delta n\right)\frac{RT}{P}$ from the ideal gas law, and in this reaction $\Delta n=0$.

So, $\Delta H=\Delta U$

for 3 mole,

$\Delta U=3\times (-184.6)$

$\Delta U=-553.8KJ$