The standard molar heat of formation of ethane, carbon dioxide and liquid water are `-21.1, -94.1 and -68.3` kcal respectively. Calculate the standard molar heat of combustion of ethane.

Explanation

`2C("graphite")+3H_(2)(g) to C_(2)H_(6)(g), Delta H^(0)= - 21.1` kcal
`C("graphite")+O_(2)(g) to CO_(2)(g), Delta H^(0)= -94.1 ` kcal
`H_(2)(g)+(1)/(2)O_(2)(g) to H_(2)O(l) , Delta H^(0)=-68.3` kcal
Multiplying equation (2) by 2 and equation (3) by 3 and then adding them together, we have
`2 C("graphite")+(7)/(2)O_(2)(g)+3H_(2)(g) to 2CO_(2)(g)+3H_(2)O(l), Delta H^(0)= [2(-94.1)+3 (-68.3)]` kcal
Subtracting equation (1) from equation (4), we have `C_(2)H_(6)(g)+(7)/(2)O_(2)(g) to 2CO_(2)(g)+3H_(2)O(l)`
`Delta H^(0)= [2 (-94.1)+3(-68.3)-(-21.1)]` kcal = -372 kcal
Equation (5) indicates the combustin reaction for `C_(2)H_(6)(g)`. So, standard heat of combustion for `C_(2)H_(6)(g)` is `-372 " kcal mol"^(-1)`.