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. `-4.0` kcal
B. `+40` kcal
C. `-80` kcal
D. `+80` kcal

Answer

C

Explanation

Given,
(i) `CH_(4)+2O_(2)rarrCO_(2)+2H_(2)O, DeltaH=+20 kcal`
(ii) `C+O_(2)rarrCO_(2), DeltaH=-40 kcal`
`(iii) H_(2)+(1)/(2)O_(2)rarrH_(2)O, DeltaH=-10 kcal`
Aim: `C+2H_(2)rarrCH_(4)`
`(ii) +2xx"(iii)-(i) gives"`
`DeltaH=-40+2(-10)-(+20)=-80 kcal`.