Thermodynamics and Thermochemistry 2 Question 23
23. Determine the enthalpy of the reaction,
$\mathrm{C}{3} \mathrm{H}{8}(g)+\mathrm{H}{2}(g) \rightarrow \mathrm{C}{2} \mathrm{H}{6}(g)+\mathrm{CH}{4}(g)$, at $25^{\circ} \mathrm{C}$, using the given heat of combustion values under standard conditions.
Compound: $\quad \mathrm{H}{2}(g) \quad \mathrm{CH}{4}(g) \quad \mathrm{C}{2} \mathrm{H}{6}(g) \quad \mathrm{C}$ (graphite)
The standard heat of formation of $\mathrm{C}{3} \mathrm{H}{8}(g)$ is $-103 \mathrm{~kJ} / \mathrm{mol}$.
(1992, 3M)
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Solution:
- First we need to determine heat of combustion of $\mathrm{C}{3} \mathrm{H}{8}$.
$$ \begin{aligned} & 3 \mathrm{C}(\mathrm{gr})+4 \mathrm{H}{2}(g) \longrightarrow \mathrm{C}{3} \mathrm{H}{8}(g) \quad \Delta H{f}^{\circ}=-103 \mathrm{~kJ} \ \Rightarrow & -103 \mathrm{~kJ}=-3 \times 393-4 \times 285.80-\Delta H_{\mathrm{comb}}^{\circ}\left(\mathrm{C}{3} \mathrm{H}{8}\right) \end{aligned} $$
$$ \begin{gathered} \Rightarrow \Delta H_{\text {comb }}^{\circ}\left(\mathrm{C}{3} \mathrm{H}{8}\right)=-2219.20 \mathrm{~kJ} \ \Rightarrow \Delta H_{r}^{\circ}=\Sigma \Delta H_{\text {comb }}^{\circ} \text { (reactants) }-\Sigma \Delta H_{\text {comb }}^{\circ} \text { (products) } \ \quad=-2219.20-285.80+1560+890 \ \quad=-55 \mathrm{~kJ} \end{gathered} $$
