Is this example of Bond Energy correct?

[esale]

Homework Statement

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(see attachments for example 2.9)

Homework Equations

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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]

Forgot to add example 2.9...

And Table 2.2

 

[mjc123]

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]

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!