1.

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

Answer»

`-373kcal`
`162 kcal`
`-240kcal`
`183.5 kcal`

Solution :`DeltaH_("reaction")=SUM DeltaH` of product `-sum DeltaH` of REACTANTS
`DeltaH_(f)C_(2)H_(6)=-21.1kcal,""DeltaH_(f)""CO_(2)=-94.1kcal`,
`DeltaH_(f)""H_(2)O=68.3kcal`
`C_(2)H_(6)+(7)/(2)O_(2) to 2CO_(2)+3H_(2)O`
`DeltaH_("combustion")=[2DeltaH_(f)CO_(2)+3DeltaH_(f)" "H_(2)O]-[DeltaH_(f)" "C_(2)H_(6)+(7)/(2)DeltaH_(f)O_(2)]`
`=[2xx(-94.1)+3(-68.3)]-[-21.1+(7)/(2)XX(0)]`
`=(-188.2-204.9)+21.1`
`=-393.1+21.1=372kcal`


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