Question

Heat of reaction for, $CO\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\to C{O}_2\left(g\right)$ at constant $V$ is $-67.71kCal$ at ${17}^{\circ }C$. The heat of reaction at constant $P$ at ${17}^{\circ }C$ is

• A. $-68.0Kcal$
• B. $+68.0Kcal$
• C. $-67.42Kcal$
• D. None of the above

Answer 1

Answer: (A)

$-68.0Kcal$

Solution:- (A) $-68.0Kcal$

Heat change at constant volume for the combustion of carbon monoxide $=-67.71Kcal$

${CO}_{\left(g\right)}+\frac{1}{2}{O_2}_{\left(g\right)}⟶{C{O}_2}_{\left(g\right)}$

From the above reaction,

$\Delta n_g=n_P-n_R=1-(\frac{1}{2}+1)=-\frac{1}{2}$

Temperature $\left(T\right)=17℃=(17+273)K=290K\left(\text{Given}\right)$

Now from first law of thermodynamics,

$\Delta H=\Delta E+\Delta n_{g}RT$

$\Delta H=-67.71+(-\frac{1}{2})\times 2\times {10}^{-3}\times 290$

$\Rightarrow \Delta H=-67.71-0.29=-68Kcal$

Hence the heat of reaction at constant pressure will be $-68Kcal$.