The polymerization of ethylene to linear polythene is represented by the reaction

${\mathrm{nCH}}_2={\mathrm{CH}}_2\to -({\mathrm{CH}}_2-{\mathrm{CH}}_2)-{ }_n$

Where n has a large integral value. Given that the average enthalpies of bond dissociation for c=c and c-c at 298 K are +590 and +331 kJ/mol respectively. Calculate the enthalpy of polymerisation per mole of ethylene at 298 K (neglect sign) __________

During polymerization of ethylene c=c double bond breaks and two $C-H$ single bonds are formed. In the whole polymer unit the number of single bonds formed per mole of ethylene are two.

          Energy absorbed due to dissociation of one c=c bond = 590 kJ/mol

          Energy released due to formation of two c-c single bonds = 2x331 = 662 kJ/mol

                    $\mathrm{\Delta H}$ polymerization = 590-662 = -72 kJ/mol