Bond enthalpies

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JulieAljamal1E
Posts: 71
Joined: Fri Sep 28, 2018 12:24 am

Bond enthalpies

Postby JulieAljamal1E » Sat Feb 02, 2019 5:10 pm

On problem 4E.5 part a, it asks to calculate the reaction enthalpy using bond enthalpies for the reaction 3C2H2->C6H6
The table in the book gives what I’m assuming is a resonance bond enthalpy for benzene's Carbon single and double bonds. Before noticing that, I did 3(348)+3(612) which is for the single and double bonds of carbon, however this does not give me the same result as doing 6(518). Can someone please explain this?

Elaine Pham 2E
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Joined: Fri Sep 28, 2018 12:28 am
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Re: Bond enthalpies

Postby Elaine Pham 2E » Sat Feb 02, 2019 7:05 pm

For this problem, I think you just have to know (by drawing its lewis structure) that benzene has resonance structures. That means all of its bonds are the same length, which is between the length of a single and double bond between carbons. So since benzene has 6 carbon-carbon bonds of the same length and not 3 carbon single bonds and 3 carbon double bonds, the bond enthalpy is different and you would use the bond enthalpy of 518 kJ/mol that's listed in the table.

Eruchi Okpara 2E
Posts: 53
Joined: Fri Sep 28, 2018 12:28 am

Re: Bond enthalpies

Postby Eruchi Okpara 2E » Sat Feb 02, 2019 7:17 pm

Are we expected to automatically know all the lewis structures/resonance structures in order to solve bond enthalpy problems?

Sarah Bui 2L
Posts: 61
Joined: Fri Sep 28, 2018 12:29 am

Re: Bond enthalpies

Postby Sarah Bui 2L » Thu Feb 07, 2019 10:47 pm

Eruchi Okpara 2E wrote:Are we expected to automatically know all the lewis structures/resonance structures in order to solve bond enthalpy problems?


I believe so since this was covered in Chem 14a and they wouldn't give the lewis structures to us on the test either.


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