1.

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

Answer»

Solution :Given that,
(i) `2C(s)+3H_(2)(g) to C_(2)H_(2)(g),DELTAH^(0)=-21.1"kcal"`
(ii) `C(s)+O_(2)(g) to CO_(2)(g),DeltaH^(0)=-94.1"kcal"`
(III) `H_(2)(g)+(1)/(2)O_(2)(g) to H_(2)O(g),DeltaH^(0)=-68.3"kcal"`
We have to calculate `DeltaH^(0)` of the EQUATION
`C_(2)H_(6)(g)+3(1)/(2)O_(2)(g) to 2CO_(2)(g)+3H_(2)O(g),DeltaH^(0)=?`
Applying the INSPECTION method,
[-Eqn. (i)+2xxEqn. (ii) +3xxEqn. (iii)], we GET,
`-2C(s)-3H_(2)(g)+2C(s)+2O_(2)(g)+3H_(2)(g)+1(1)/(2)O_(2)(g) to -C_(2)H_(6)(g)+2CO_(2)(g)+3H_(2)O(g),DeltaH^(0)=-(-21.1)+2xx(-94.1)+3xx(-68.3)`
`C_(2)H_(6)(g)+3(1)/(2)O_(2)(g) to 2CO_(2)(g)+3H_(2)O(g),DeltaH^(0)=-372.0"kcal"`


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