The polymerisation of ethylene to linear polyethylene is represented by the reaction
`nCH_(2)=CH_(2)rarr(-CH_(2)-CH_(2)-)_(n)`
When `n` has a large integral value. Given theat the average enthalpies of bond dissociation for `C=C` and `C-C` at `298K`are `+590` and `+331kJ mol^(-1)` respectively, calculate the enthalpy of polymerisation per mole of ethylene at `298K`.

Answer

`-72 kJ "mole"^(-)`

Explanation

The polymersiation reaction is `nCH_(2) = CH_(2) rarr (CH_(2) - CH_(2))_(n)`
In this process, one double bond `(c = C)` breaks and two -`CO_(2)` group are lined with single bonds thus forming three single bonds (two single bonds are formed when each `CH_(2)` group of ethylene `(CH_(2) = CH_(2))` links with another `CH_(2)` - group of another ethylene molecule). Therefore, in polymerisation reaction one `C = C` is replaced by two `C - C` bonds or one mole of `C = C` bonds. Energy Relased = Energy due of formation of 2 single bonds. `= (2 xx 331) kJ = 662 kJ`. Energy needed to dissociate one mole of `C = C` bonds = 590 kJ. `Delta H_("pot")` or enthalpy of polymerisation `= (590 - 662) kJ = 72 kJ` moles