The heat of combustion of `CH_(4)(g)`, C (graphite), `H_(2)(g)` are 20kcal, -40 kcal-10 kcal respectively. The heat of formation of methane is

Options

A. `-40kcal`
B. `+40kcal`
C. `-80.0kcal`
D. `+80kcal`

Answer

A

Explanation

(a) `CH_(4)+2O_(2)toCO_(2)+2H_(2)O,DeltaH=20kcal` . . . (1)
`C+O_(2)toCO_(2),DeltaH=40kcal` . . (2)
`2H_(2)+O_(2) to 2H_(2)O,DeltaH=-20kcal` . . (3)
`(2)+(3)-(1)` we have, `C+2H_(2) to CH_(4)`.
or `DeltaH=-20+(-40)-(-20)=-40kcal`