Thermodynamics and Thermochemistry 2 Question 30
30. The molar heats of combustion of $\mathrm{C}{2} \mathrm{H}{2}(g), \mathrm{C}$ (graphite) and $\mathrm{H}{2}(\mathrm{~g})$ are $310.62 \mathrm{kcal}, 94.05 \mathrm{kcal}$ and $68.32 \mathrm{kcal}$ respectively. Calculate the standard heat of formation of $\mathrm{C}{2} \mathrm{H}_{2}(g)$.
$(1983,2 \mathrm{M})$
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Solution:
- The standard state formation reaction of $\mathrm{C}{2} \mathrm{H}{2}(g)$ is :
$$ \begin{aligned} & 2 \mathrm{C}(g)+\mathrm{H}{2}(g) \longrightarrow \mathrm{C}{2} \mathrm{H}{2}(g) ; \quad \Delta H{f}^{\circ} \ \Delta H_{r}^{\circ} & =\Sigma \Delta H_{\text {comb }}^{\circ} \text { (reactants) }-\Sigma \Delta H_{\text {comb }}^{\circ} \text { (products) } \ = & -2 \times 94.05-68.32-(-310.62) \ = & 54.2 \mathrm{kcal}=\Delta H_{f}^{\circ}\left(\mathrm{C}{2} \mathrm{H}{2}\right) \end{aligned} $$
