Question

Given ;
(i) $C_{\left(s\right)}+O_{2\left(g\right)} \to CO_{2\left(g\right)} ; \Delta H=-94.0\, kcal$
$\left(ii\right) H_{2\left(g\right)}+\frac{1}{2}O_{2\left(g\right)} \to H_{2}O_{\left(l\right)}; \Delta H=-68.4\, kcal$
$\left(iii\right) CH_{4\left(g\right)}+O_{2\left(g\right)}\to CO_{2\left(g\right)}+2H_{2}O_{\left(l\right)}; \Delta H=-212.4\,kcal$
The heat of formation of $CH_{4}$ is

• A. $-1 8 .4\, kcal$
• B. $+18.4 \, kcal$
• C. $-443.2 \, kcal$
• D. $+443.2 \, kcal$

Answer 1

Answer: (A)

$(i)$ $\ce{C +O2 -> CO2}$ ; $\Delta H=-94.0\,kcal$
$(ii)$ $\ce{H2 +\frac{1}{2} O2 -> H2O}$ ; $\Delta H=-68.4\,kcal] \times 2$
$(iii)$ $\ce{CH4 +2O2 -> CO2 +2H2O}$ ; $\Delta H=-212.4\,kcal$
On multiplying equation $(ii)$ with $2$, add equation $(i)$ and subtract equation $(iii)$, we get
$C+2H_{2} +2O_{2} \to CO_{2} +2H_{2}O$ ; $\Delta H=-230.8\,kcal$
$CH_{4}+2O_{2} \to CO_{2}+2H_{2}O$ ; $\Delta H=-212.4\,kcal$
$C +2H_{2} \to CH_{4} ; \Delta H=-18.4\,kcal$