Why is polymerization bond energy positive not negative?

esale · Sep 2, 2017

Homework Statement

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Textbook worked example 2.10: calculating reaction energy for polymerization of polyvinyl chloride, showing C=C to 2C-C bond energies of 680 and 2(370) kJ/mol giving 60 kJ/mol released, with a note.

(see attachments for example 2.9)

Homework Equations

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Equations: H_reaction = SUM(BE_reactants) - SUM(BE_products), or if you prefer using symbols H_reaction = ΣBE_reactants - ΣBE_products.

The Attempt at a Solution

As far as I have read and understood, most polymerizations (such as the one in this example) are exothermic. If this is the case, shouldn't this produce a negative (-) reaction energy? And the equation they should have used is the one I included in section two (2)?

Thank you in advance for any help.

-E

esale · Sep 2, 2017

Forgot to add example 2.9...

Textbook example 2.9 solution showing vinyl chloride monomer drawn as a Lewis structure with H H over C=C over H Cl, followed by a repeating chain of carbon atoms each bearing H and Cl, labelled mer, illustrating PVC polymerization.

And Table 2.2

Table 2.2 titled Bond Energies and Bond Lengths for Representative Covalent Bonds, listing bond energy in kcal/mol and kJ/mol and bond length in nm for C-C, C=C, C≡C, C-H, C-N, C-O, C=O, C-F, C-Cl, O-H, O-O, O-Si, N-H, N-O, F-F and H-H bonds.

mjc123 · Sep 4, 2017

Unless they are defining "reaction energy" differently from usual, the blue box is wrong. You input the energy to break the C=C bond, and the energy of forming two C-C bonds is given out. So the enthalpy change is +680 - 740 = -60 kJ/mol. They may be confused by the fact that
ΔH_rxn = H_products - H_reactants
But this is not equal to BE_products - BE_reactants
Relative to the state where all bonds are broken, H = -BE. So your white box is right.

esale · Sep 12, 2017

mjc123 said:

Unless they are defining "reaction energy" differently from usual, the blue box is wrong. You input the energy to break the C=C bond, and the energy of forming two C-C bonds is given out. So the enthalpy change is +680 - 740 = -60 kJ/mol. They may be confused by the fact that
ΔH_rxn = H_products - H_reactants
But this is not equal to BE_products - BE_reactants
Relative to the state where all bonds are broken, H = -BE. So your white box is right.

Thank you for your answer!

Why is polymerization bond energy positive not negative?

Homework Statement

Homework Equations

The Attempt at a Solution

Discussion

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