The standard molar heat for formation of ethane, carbondioxide and water are respectively, -21.1,-94.1 and -68.3 Kacal `"mol"^(-1)`. What is the standard molar heat of combustion of ethane ?

Explanation

`2C_("(graphite)") + 3H_(2(s)) to C_2H_(6(g)) , DeltaH_1 = -21.1 k cal`
`C_(("graphite")) + O_(2(g)) to CO_(2) , DeltaH_2 = -94.1 kcal`
`H_(2(g)) + 1/2O_(2) to H_2O_((l)) , DeltaH_2 = -68.3 k cal`
The required equations is
`C_(2)H_(6(g)) + 7/2 O_(2(g)) to 2CO_(2(g))  +3H_(2)O_((l)) , DeltaH = ?`
`DeltaH  = 2DeltaH_(2) + 3DeltaH_(3) - DeltaH_1`
`DeltaH = 2(-94.1) + 3(-68.3) - (-21.1) = -372 kcal`