Question

Calculate the standard molar heat of combustion of ethane.
Given: $\Delta H_{_{f o r m a t i o n}}^{o}$ of $C_{2}H_{6},CO_{2}$ and water $\left(l\right)$ are, $-21.1-94.1$ and $-68.3$ kcal, respectively .

• A. $-372\text{ Kcal}$
• B. $162\text{kcal}$
• C. $-240$ $\text{kcal}$
• D. $183.5\text{kcal}$

Answer 1

Answer: (A)

Given, $2C+3H_{2} \rightarrow C_{2}H_{6}; \, \Delta H=-21.1$
$C+O_{2} \rightarrow CO_{2}; \, \Delta H=-94.1$
$H_{2}+\frac{1}{2}O_{2} \rightarrow H_{2}O; \, \Delta H=-68.3$
Eqs .2(ii) + 3(iii) $-$ (i)
$C_{2}H_{6}+\frac{7}{2}O_{2} \rightarrow 2CO_{2}+3H_{2}O$
$\Delta x=2\left(- 94.1\right)+3\left(- 68.3\right)-\left(\right.-21.1\left.\right)$
$=-372 \, kcal$