Thermodynamics and Thermochemistry 2 Question 18
18. From the following data, calculate the enthalpy change for the combustion of cyclopropane at $298 \mathrm{~K}$. The enthalpy of formation of $\mathrm{CO}{2}(g), \mathrm{H}{2} \mathrm{O}(l)$ and propane $(g)$ are -393.5 , -285.8 and $20.42 \mathrm{~kJ} \mathrm{~mol}^{-1}$ respectively. The enthalpy of isomerisation of cyclopropane to propene is $-33.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$.
(1998, 5M)
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Solution:
- Given : Cyclopropane $\longrightarrow$ Propene $\left(\mathrm{C}{3} \mathrm{H}{6}\right) ; \Delta H=-33 \mathrm{~kJ}$ Propene $\left(\mathrm{C}{3} \mathrm{H}{6}\right)+\frac{9}{2} \mathrm{O}{2} \longrightarrow 3 \mathrm{CO}{2}(g)+3 \mathrm{H}_{2} \mathrm{O}(l)$;
$\Delta H=-3(393.5+285.8)-20.42=-2058.32 \mathrm{~kJ}$
Adding :
Cyclopropane $+\frac{9}{2} \mathrm{O}{2}(g) \longrightarrow 3 \mathrm{CO}{2}(g)+3 \mathrm{H}_{2}(g) ;$
$$ \begin{aligned} \Delta H & =H_{1}+H_{2} \ & =-33+(-2058.32) \mathrm{kJ} \ \Delta H & =-2091.32 \mathrm{~kJ} \end{aligned} $$
