Thermodynamics and Thermochemistry 2 Question 30

30. The molar heats of combustion of $\mathrm{C}{2} \mathrm{H}{2}(g), \mathrm{C}(graphite)and\mathrm{H}{2}(\mathrm{~g})are310.62 \mathrm{kcal}, 94.05 \mathrm{kcal}and68.32 \mathrm{kcal}respectively.Calculatethestandardheatofformationof\mathrm{C}{2} \mathrm{H}_{2}(g)$.

(1983,2M)

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Solution:

  1. The standard state formation reaction of $\mathrm{C}{2} \mathrm{H}{2}(g)$ is :

$$ \begin{aligned} & 2 \mathrm{C}(g)+\mathrm{H}{2}(g) \longrightarrow \mathrm{C}{2} \mathrm{H}{2}(g) ; \quad \Delta H{f}^{\circ} \ \Delta H_{r}^{\circ} & =\Sigma \Delta H_{\text {comb }}^{\circ} \text { (reactants) }-\Sigma \Delta H_{\text {comb }}^{\circ} \text { (products) } \ = & -2 \times 94.05-68.32-(-310.62) \ = & 54.2 \mathrm{kcal}=\Delta H_{f}^{\circ}\left(\mathrm{C}{2} \mathrm{H}{2}\right) \end{aligned} $$