Thermodynamics And Thermochemistry Result Question 23
23. Determine the enthalpy of the reaction,
$\mathrm{C} _3 \mathrm{H} _8(g)+\mathrm{H} _2(g) \longrightarrow \mathrm{C} _2 \mathrm{H} _6(g)+\mathrm{CH} _4(g)$, at $25^ {\circ}\mathrm{C}$, using the given heat of combustion values under standard conditions.
Compound: $ \quad \quad\quad \mathrm{H}_2(g) \quad \quad\mathrm{CH}_4(g) \quad \quad\mathrm{C}_2 \mathrm{H}_6(g) \quad \mathrm{C}$ (graphite)
$\Delta H^{\circ}(\mathrm{kJ} / \mathrm{mol}): \quad-285.8 \quad-890.0 \quad-1560.0 \quad-393.0$
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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Answer:
Correct Answer: 23. $(-55 \mathrm{~kJ})$
Solution:
- First we need to determine heat of combustion of $\mathrm{C}_3 \mathrm{H}_8$.
$3 \mathrm{C}(\mathrm{gr})+4 \mathrm{H}_2(\mathrm{~g}) \longrightarrow \mathrm{C}_3 \mathrm{H}_8(\mathrm{~g}) \quad \Delta H_f^{\circ}=-103 \mathrm{~kJ}$
$\Rightarrow -103 \mathrm{~kJ}=-3 \times 393-4 \times 285.80-\Delta H_{\mathrm{comb}}^{\circ}\left(\mathrm{C}_3 \mathrm{H}_8\right)$
$\begin{aligned} & \Rightarrow \Delta H_{\mathrm{comb}}^{\circ}\left(\mathrm{C}3 \mathrm{H}8\right)=-2219.20 \mathrm{~kJ} \\ & \Rightarrow \Delta H_r^{\circ}=\Sigma \Delta H{\text {comb }}^{\circ} \text { (reactants) }-\Sigma \Delta H{\text {comb }}^{\circ} \text { (products) } \\ & =-2219.20-285.80+1560+890 \\ & =-55 \mathrm{~kJ} \\ & \end{aligned}$
BETA
