Thermodynamics and Thermochemistry 2 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 $298 \mathrm{~K}$ are +590 and $+311 \mathrm{~kJ} / \mathrm{mol}$ respectively, calculate the enthalpy of polymerization per mole of ethylene at $298 \mathrm{~K}$.
(1994, 2M)
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Solution:
- Per mole of ethylene polymerized, one $\mathrm{C}=\mathrm{C}$ bond is broken and two $\mathrm{C}-\mathrm{C}$ bonds are formed.
$$ \begin{aligned} \Delta H^{\circ}(\text { Polym. }) & =590-2 \times 311 \ & =-32 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$
