The standard molar heat of formation of ethane `CO_2` and water (l) are respectively -21.1 ,-94.1 and -68.3 kcal .the standard molar heat of combustion of ethane will be

Options

A. `-372` kcal
B. 162 kcal
C. `- 240` kcal
D. 183.5 kcal

Answer

A

Explanation

(i) `2C(s) + 3H_(2)(g) rarr C_(2)H_(6) (g) , Delta H= -21.1`
(ii) `C(s) + O_(2)(g) rarr CO_(2)(g), Delta H= -94.1`
(iii) `H_(2)(g) + (1)/(2) O_(2)(g) rarr H_(2)O(l), Delta H= -68.3`
`3 xx` Eq (iii) `+2 xx` Eq (ii)- Eq (i)
`C_(2)H_(6) = (7)/(2) O_(2) rarr 2CO_(2) + 3H_(2)O`
`Delta H= 3(-68.3) +2 (-94.1) - (-21.1)` kcal
`Delta H= -372` kcal