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Enthalpy of formation of (C=C) and (C-C) in $C_2H_4$ is -590 kJ/mol and -331 kJ/mol, respectively. What is the enthalpy change when ethylene polymerises to form polythene?

(a) +259 kJ/mol
(b) +72 kJ/mol
(c) -259 kJ/mol
(d) -72 kJ/mol
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Answer: -72 kJ/mol
In the given polymerisation reaction, one mole of ethylene involves breaking of one C=C double bond and 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 = $2\times 331 = 662$ J
$\therefore$ Net energy releases per mole ethylene = 662 - 590 = 72 kJ
i.e., $\Delta H = -72 $ kJ/mol
answered Feb 27, 2014 by mosymeow_1

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