Question

Heat of combustion $ \Delta H^{\circ}$ for $ C ( s ), H _{2}( g )$ and $CH _{4}( g ) $ are -94,-68 and $-213 \,Kcal / mol $ then $\Delta H ^{\circ}$ for $C ( s )+2 H _{2}( g ) \to CH _{4}( g )$ is:-

• A. - 17 Kcal
• B. - 111 Kcal
• C. - 170 Kcal
• D. - 85 Kcal

Answer 1

Answer: (A)

For reaction ,
$C (s) + 2H_2 (g) \to CH_4(g), \Delta H^\circ = ? $
$\Delta H^\circ = - [(H^\circ of \,\, combustion\,\,of C + 2\times \Delta H of\,\,combustion \,\, of H_2)] $
$C + O_2 \to CC_2; \Delta H = - 94\,\, kcal $
$ 2H_2 +O_2 \to 2H_2O; \Delta H = - 68 \times 2\,\, kcal? $
$ CH_4 +2O_2 \to CO_2+ 2H_2O; \Delta = - 213 \,\,kcal$
$= - [(-213) - (-94+2 \times - 68)] Kcal/mol $
$= - [-213+230] = - 17 kcal/mol $