Question

The molar heats of combustion of $C _{2} H _{2}(g), C$ (graphite) and $H _{2}(g)$ are $310.62\, kcal , 94.05 \,kcal$ and $68.32\, kcal$ respectively. Calculate the standard heat of formation of $C _{2} H _{2}(g)$

Answer 1

The standard state formation reaction of $C _{2} H _{2}(g)$ is :
$2 C (g)+ H _{2}(g) \longrightarrow C _{2} H _{2}(g) ; \Delta H_{f}^{\circ}$
$\Delta H_{r}^{\circ}=\Sigma \Delta H_{\text {comb }}^{\circ}$ (reactants) $-\Sigma \Delta H_{\text {comb }}^{\circ}$ (products)
$=-2 \times 94.05-68.32-(-310.62)$
$=54.2 \,kcal =\Delta H_{f}^{\circ}\left( C _{2} H _{2}\right)$