Discussion Forum

The standard molar heat of formnation of ethane , $CO_{2}$ and water (l) are respectively -21.1 ,-94.1 and -68.3 kcal . The standard molar heat of combustion of ethane will be 

Answer:

Explanation:

Given, 

(a)  $2C+3H_{2} \rightarrow  C_{2}H_{6}; \triangle H=-21.1 kcal$

(b)   $C+O_{2} \rightarrow  CO_{2}; \triangle H=-94.1kcal$

(c)   $H_{2}+\frac{1}{2}O_{2} \rightarrow H_{2}O; \triangle H= -68.3 kcal$

Now, eqs, $2 \times (b) +3\times (c)-(a)$

$C_{2}H_{6}+\frac{3}{2}O_{2} \rightarrow 2CO_{2}+3H_{2}O$

Heat  of combustion  of ethane  $\triangle x=2(-94.1)$

+3(-68.3)-(-21.1)

=$(-188.2)+(-204.9)-21.1$

=$-372 kcal$