The bond energies of `C=C` and `C-C` at `298 K` are `590` and `331 kJ mol^(-1)`, respectively. The enthalpy of polymerisation per mole of ethaylene is

Options

A. `+259 kJ mol^(-1)`
B. `+72 kJ mol^(-1)`
C. `-259 kJ mol^(-1)`
D. `-72 kJ mol^(-1)`

Answer

D

Explanation

In the given polymerisation reaction, one mole of ethylene involves breaking of one `C=C` double bond a nd formation of three `C-C` single bonds. However, in the complete polymer chain, the number of single `C-C` bonds is two per `C=C` double bond broken.
Energy required in breaking of one mole of `C=C` double bond `=590 kJ`
Energy released in the formation of two moles of `C-C` single bonds `=2xx331=662 J`
`:. "Net energy released per mole ethylene"`
`=662-590=72 kJ`
i.e., `DeltaH=-72kJ "mole"^(-1)`