Question

The heats of combustion of carbon, hydrogen, and ethane are $-94,-68.3$ and $-373kcal/mol$ respectively. The heat of formation of ethane in $kcal/mol$ is:

• A. $+19.9$
• B. $-19.9$
• C. $+39.8$
• D. $-39.8$

Answer 1

The correct option is B $-19.9$

(i) $C+O_2\to C{O}_2\left(g\right),$ $\Delta H=-94$ kcal/mol

(ii) $H_2+\frac{1}{2}{O}_2\to H_{2}O\left(l\right),$ $\Delta H=-68.3$ kcal/mol

(iii) $C_2{H}_6+\frac{7}{2}{O}_2\to 2C{O}_2+3H_{2}O,$ $\Delta H=-373$ kcal/mol

Now,

$2\times \left(i\right)+3\times \left(ii\right)-\left(iii\right)$, will give,

$2C+3H_2\to C_2{H}_6$

$\Delta H=2\times (-94)+3\times (-68.3)-(-373)$

$\Delta H=-19.9kcalmo{l}^{-1}$

Option B is correct.