Question

The standard molar heat of formation of ethane, $CO_2$ and water are respectively $-21.1, -94.1$ and $-68.3\, kcal$. The standard molar heat of combustion of ethane will be

• A. -372 kcal
• B. 162 kcal
• C. -240 kcal
• D. 183.5 kcal

Answer 1

Answer: (A)

-372 kcal

Answer 2

Given, (i) $2 C +3 H _{2} \longrightarrow C _{2} H _{6} ; \Delta H=-21.1$ (ii) $C + O _{2} \longrightarrow CO _{2} ; \Delta H=-94.1$ (iii) $H _{2}+\frac{1}{2} O _{2} \longrightarrow H _{2} O ; \Delta H=-68.3$ Eqs. 2 (ii) $+3$ (iii) - (i) $C _{2} H _{6}+\frac{3}{2} O _{2} \longrightarrow 2 CO _{2}+3 H _{2} O$ $\Delta x =2(-94.1)+3(-68.3)-(-21.1)$ $=-372\, kcal$