The polymerization of ethylene to polyethene is −720KJmol−1 given bond dissociation enthalpy of C=C is 590KJmol−1 and bond formation enthalpy of C-C is 331KJmol−1 at 298K then calculate no.of ethylene molecules  undergoes polymerisation

Let us consider the polymerization reaction of ethylene nCH2=CH2→CH2−CH2n‘n’ double bonds forms molecule with ‘n’ single bondsΔHpoly=Bond dissociation enthalpy x n    +  Bond formation enthalpy x 2n( ∵formation enthalpy –Ve; dissociation enthalpy +Ve)ΔHpoly =n×(590)-2n(331)=-720-72×n=-720∴n=10 molecules