Question

The polymerisation of propene to linear polypropene is represented by the reaction:
where n has a large integral value. The average enthalpies of bond dissociation for $(C=C)$ and $(C-C)$ at 298 K are +590 and $+331{\text{kJmol}}^{-1}$, respectively. The enthalpy of polymerisation is
$-360{\text{kJmol}}^{-1}$. Find the value of n.

Answer 1

During the polymerization of ethylene, one mole of ethylene breaks i.e. one $C=C$ double bond breaks and the two $C{H}_2$ - groups are linked with C-C single bonds thus forming three single bonds.
so enthalpy of polymersisation for `n` units:
$-360=n(590-2\times 331)$
$n=5$