Question

Heats of combustion of methane, carbon and hydrogen are 212, 94, 68 kcal mol${}^{-1}$.

The heat of formation of ${CH}_4$ is:

• A. $-18$ kcal mol ${}^{-1}$
• B. $+18$ kcal mol ${}^{-1}$
• C. $-9$ kcal mol ${}^{-1}$
• D. None of these

Answer 1

Answer: (A)

$-18$ kcal mol ${}^{-1}$

According to given data,

$C{H}_4+2O_2\to C{O}_2+2H_{2}O$ $\Delta {H^0}_c=212$ kcal mol ${}^{-1}$

$C+O_2\to C{O}_2$ $\Delta {H^0}_c=94$ kcal mol ${}^{-1}$

$2H_2+O_2\to 2H_{2}O$ $\Delta {H^0}_c=68$ kcal mol ${}^{-1}$

and required is

$C+2H_2\to C{H}_4$

Heat of formation of CH${}_4=$ $\Delta {{H^0}_c}_{\left(C{H}_4\right)}-\Delta {{H^0}_c}_{\left(C\right)}-2\times \Delta {{H^0}_c}_{\left(H_2\right)}$

$=212-94-68\times 2=-18$ kcal mol ${}^{-1}$

Thus, the heat of formation of CH${}_4=$-18 kcal mol ${}^{-1}$

Hence, the correct option is $A$