Chemical Thermodynamics Question 472

Question: The polymerization of ethylene to linear polyethylene is represented by the reaction $ nCH_2=CH_2\xrightarrow{,},{{(-CH_2-CH_2-)}_{n}} $ when ’n’ has a large integral value. Given that average enthalpies of bond dissociation for $ C=C $ and $ C-C $ at 298 K are + 590 and $ +331kJmo{{l}^{-1}} $ respectively. Then the enthalpy of polymerization/mol of ethylene at298 K is

Options:

A) - 132 kJ

B) - 72 kJ

C) + 80 kJ

D) + 162 kJ

Show Answer

Answer:

Correct Answer: B

Solution:

  • $ n(CH_2=CH_2)\xrightarrow{\ },{{(-CH_2-CH_2-)}_{n}}, $

$ \Delta H=- $

Thus, ’n’ double bonds are dissociated to form a molecule with 2n single bonds. $ \Delta H=( \frac{n}{n} )({\Sigma_{C=C}})-( \frac{2n}{n} )({\Sigma_{C=C}}) $

$ =+,590-2\times 331=-72,kJ,mo{{l}^{-1}} $



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