Question

The polymerisation of ethylene to linear polyethylene is represented by the reaction
$nC{H}_2=C{H}_2\to (-C{H}_2-C{H}_2-)n$
where n has a large integral value. Given that average enthalpies of bond dissociation for $C=C$ &
$C-C$ at 298 K are $+$ 590 & $+$ 331 kJ $mo{l}^{-1}$ respectively. Then the enthalpy of polymerisation per mole of ethylene at 298 K is:

• A. $-$ 62 kJ $mo{l}^{-1}$
• B. $-$ 72 kJ $mo{l}^{-1}$
• C. $-$ 82 kJ $mo{l}^{-1}$
• D. $-$ 52 kJ $mo{l}^{-1}$

Answer 1

Answer: (B)

$-$ 72 kJ $mo{l}^{-1}$

During the polymerization of ethylene, one mole of ethylene breaks i.e. one C = C double bond breaks and the two ${CH}_2$- groups are linked with C-C single bonds thus forming three single bonds.

But in the whole unit of polymer, number of single C-C bonds formed/mole of ethylene is 2.

Energy due to formation of 2 C-C single bonds = 2 X 331 = 662 kJ/mol.

Energy due to dissociation of 1 C = C double bond = 590 kJ/mol.

Thus, the enthalpy of polymerisation per mole of ethylene at 298 K = (590-662) = -72 kJ/mol