Question

The heat of combustion of $CH$${}_4$(g), $C$ (graphite) and $H$${}_4$ (g) are respectively $-20k.cal$, $-40k.cal$ and $-10k.cal$. The heat of formation of CH${}_4$ is

• A. $-40K.cal$
• B. $+40K.cal$
• C. $-80K.cal$
• D. $+80K.cal$

Answer 1

The correct option is A $-40K.cal$

(i) $C+O_2\to C{O}_2\Delta H=-40$
(ii) $H_2+\frac{1}{2}{O}_2\to H_{2}O\Delta H=-10$
(iii) $C{H}_4+2O_2\to C{O}_2+2H_{2}O$
(i) + 2 (ii) - (iii)
$C+2H_2\to C{H}_4$
$\Delta H=-40-20+20$
$=-40$ k cal