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.

Options

A. `372 kcal`
B. `162 kcal`
C. `340 kcal`
D. `183.5 kcal`

Answer

A

Explanation

Standard molar heat of combustion of ethane is defined by the following thermochemical equation:
`C_(2) H_(6) (g) + (7)/(2) O_(2) (g) rarr 2 CO_(2) (g) + 2 H_(2) O (1)`
According to Eq. , we have
`Delta_(C ) H^(@) = [(2 mol) (Delta_(f) H_(CO_(2))^(@)) + (3 mol) (Delta_(f) H_(H_(2)O)^(@))]`
`- [(1 mol) (Delta_(f) H_(C_(2) H_(6))^(@)) + ((7)/(2) mol) (Delta_(f) H_(O_(2))^(@))]`
`[(2)(-94.1) + (3)(-68.3)] - [(-21.0) + 0]`
`= [(-188.2) + (-204.9)] + (-204.9)] + 21`
`= - 372.1 kJ`