Thermodynamics And Thermochemistry Result Question 21
21. The polymerisation of ethylene to linear polyethylene is represented by the reaction,
$n\left[\mathrm{CH}_2=\mathrm{CH}_2\right] \longrightarrow-\left[\mathrm{CH}_2-\mathrm{CH}_2\right]_n$
where, $n$ has large integral value. Given that the average enthalpies of bond dissociation for $\mathrm{C}=\mathrm{C}$ and $\mathrm{C}-\mathrm{C}$ at $298$ K are $+590$ and $+311 \mathrm{~kJ} / \mathrm{mol}$ respectively, calculate the enthalpy of polymerization per mole of ethylene at $298$ K .
(1994, 2M)
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Answer:
Correct Answer: 21. $(-32 \mathrm{~kJ} / \mathrm{mol})$
Solution:
- Per mole of ethylene polymerized, one $\mathrm{C}=\mathrm{C}$ bond is broken and two $\mathrm{C}-\mathrm{C}$ bonds are formed.
$\begin{aligned} \Delta H^{\circ}(\text { Polym. }) & =590-2 \times 311 \\ & =-32 \mathrm{~kJ} / \mathrm{mol} \end{aligned}$
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