Bond Enthalpies

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Kennedi2J
Posts: 101
Joined: Wed Sep 18, 2019 12:20 am

Bond Enthalpies

Postby Kennedi2J » Sun Feb 09, 2020 8:00 pm

When calculating bond enthalpies, how can you tell which bonds are broken and which ones are formed?

Jeremy_Guiman2E
Posts: 82
Joined: Fri Sep 28, 2018 12:29 am

Re: Bond Enthalpies

Postby Jeremy_Guiman2E » Sun Feb 09, 2020 8:10 pm

Basically you compare the molecule(s) before and after (it would thus be very helpful to draw them out) and identify (based off of your drawing) which bonds are present before that are not present after AND which bonds are present after that were not there before.

Andrew Pfeiffer 2E
Posts: 101
Joined: Sat Sep 28, 2019 12:16 am

Re: Bond Enthalpies

Postby Andrew Pfeiffer 2E » Sun Feb 09, 2020 8:12 pm

Try drawing out the Lewis structures for the molecules that are the reactants. From there, cross out bonds that need to be broken and draw in bonds needed to get your Lewis structures to look like the Lewis structures of the products. Hope this helped!

J_CHEN 4I
Posts: 54
Joined: Tue Nov 14, 2017 3:01 am

Re: Bond Enthalpies

Postby J_CHEN 4I » Sun Feb 09, 2020 8:20 pm

You can list all bonds on the reactant's side and the product's side, and then see if there are bonds on the product side that are not featured in the reactant side (the bonds formed) and then see if there are bonds that appeared in the reactant side but no longer appear in the product side (the bonds broken).

Cavalli_1H
Posts: 92
Joined: Thu Sep 19, 2019 12:15 am

Re: Bond Enthalpies

Postby Cavalli_1H » Sun Feb 09, 2020 8:23 pm

i think when comparing structures before and after, its easiest to use colors and determine which bonds changed by either being added or formed. you should ask yourself, which bonds were there before but aren't there now? or which bonds look different?

Charysa Santos 4G
Posts: 107
Joined: Wed Sep 18, 2019 12:21 am

Re: Bond Enthalpies

Postby Charysa Santos 4G » Sun Feb 09, 2020 8:24 pm

Bond enthalpy questions usually give you the reaction (A + B --> C) and then you just look at each molecule that is in the reaction and look up their bond enthalpy values. Bonds broken are usually correlated to those of the reactants and bonds formed correlate to those of the products.

Sydney Myers 4I
Posts: 100
Joined: Fri Aug 09, 2019 12:17 am

Re: Bond Enthalpies

Postby Sydney Myers 4I » Sun Feb 09, 2020 9:23 pm

If it's a confusing problem with bonds, you can count up all of the bonds on either side and write them out. For example, 3 carbon-carbon bonds, 2 oxygen-carbon bonds. Do this for the reactants and the products. Then you can easily figure out which bonds are present on both sides and ignore them, since they did not contribute to the enthalpy change. From there, simply subtract the bonds that were formed from the bonds that were broken.


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