Heat of combustion of ethyl alcohol is 325 kcal and that of acetic acid is 209.5 kcal, calculate the heat evolved in the following reaction.
`C_(2)H_(5)OH + O_(2) to CH_(3)COOH + H_(2)O`

Explanation

Given that,
(i) `C_(2)H_(5)OH(l)+3O_(2)(g) to 2CO_(2)(g)+3H_(2)O(g),DeltaH=-325kcal.`
(ii) `CH_(3)COOH(l)+2O_(2)(g) to 2CO_(2)(g)+2H_(2)O(g),`
`DeltaH=-209.5kcal`
`C_(2)H_(5)OH` in Eqn. (i) and in the required equation is on the same sides, wheras `CH_(3)COOH` in Eqn. (ii) and in the required equation is on the opposite sides. Hence, applying
`[Eqn. (i)- Eqn. (ii)]`, we get,
`C_(2)H_(5)OH(l)+3O_(2)(g)-CH_(3)COOH(l)-2O_(2)(g) to `
`2CO_(2)(g)+3H_(2)O(l)-2CO_(2)(g)-2H_(2)O(l),`
`DeltaH=-325-(-209.5)`
or `C_(2)H_(5)OH(l)+O_(2)(g) to CH_(3)COOH(l)+H_(2)O(l),`
DeltaH=-115.5kcal`