Question

The heat of reaction for $C_{10}{H}_8\left(s\right)+12O_2\left(g\right)\to 10C{O}_2\left(g\right)+4H_{2}O\left(l\right)$ at constant volume is $-1228.2\text{Kcal}$ at ${25}^{o}C$. Calculate the heat of reaction at constant pressure and at ${25}^{o}C$.
(Given : $R=2\times {10}^{-3}{\text{Kcal K}}^{-1}{\text{mol}}^{-1}$)

• A. $-1229.4\text{Kcal}$
• B. $+1229.4\text{Kcal}$
• C. $-1227.0\text{Kcal}$
• D. $+1227.0\text{Kcal}$

Answer 1

The correct option is A $-1229.4\text{Kcal}$

$\Delta n_g=10-12=-2$
$\Delta H=\Delta E+\Delta n_{g}RT$
$=-1228.2+(-2)\times \left(2\right)\times {10}^{-3}\times \left(298\right)$
$=-1228.2-1.192\text{cal}=-1229.392\text{kcal}$
$\Rightarrow \Delta H\approx -1229.4\text{Kcal}$