Question

The change in internal energy $\left({\Delta }_{f}U\right)$ during the formation of $C{H}_4\left(g\right)$ at ${27}^{\circ }$ is $-393{\text{kJ mol}}^{-1}$. The value of change in enthalpy during the formation of $C{H}_4\left(g\right)$ at that temperature is

• A. $-395.5{\text{kJ mol}}^{-1}$
• B. $-390.5{\text{kJ mol}}^{-1}$
• C. $-398{\text{kJ mol}}^{-1}$
• D. $-388{\text{kJ mol}}^{-1}$

Answer 1

The correct option is A $-395.5{\text{kJ mol}}^{-1}$

$C\left(s\right)+2H_2\left(g\right)\to C{H}_4\left(g\right)$
$\therefore \Delta n_g=1-2=-1$
So, ${\Delta }_{f}H={\Delta }_{f}U-RT\Rightarrow \Delta H=-393{\text{kJ mol}}^{-1}-8.314\times {10}^{-3}\times 300{\text{kJ mol}}^{-1}\Rightarrow \Delta H=-395.5{\text{kJ mol}}^{-1}$