Hess's Law vs. Using Bond Enthalpies

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Abigail Urbina 1K
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Hess's Law vs. Using Bond Enthalpies

Postby Abigail Urbina 1K » Thu Jan 11, 2018 2:11 pm

What are the ultimate differences between the two methods we were taught in lecture yesterday to calculate change in enthalpies? The first was Hess's Law and the other was using the bond enthalpies.

I was trying to watch videos online to clarify this concept but was a little confused. Where does the heat of formation come into play in either of the methods?

Jared Smith 1E
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Re: Hess's Law vs. Using Bond Enthalpies

Postby Jared Smith 1E » Thu Jan 11, 2018 2:33 pm

Hess's law and using bond enthalpies are different ways of accomplishing the same task. Both will give the change in enthalpy. I think that we are supposed to select either one method or the other based on the information provided in the problem. In the examples we did in class, we were given the change in enthalpies for chemical reactions for the Hess's law example. In the bond enthalpy example, we were not given the change in enthalpy but the bond enthalpies instead.

As for the heat of formation, I'm not quite sure.

Wenjie Dong 2E
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Re: Hess's Law vs. Using Bond Enthalpies

Postby Wenjie Dong 2E » Thu Jan 11, 2018 3:03 pm

Hess's Law is more macroscopic while bond enthalpies method is more microscopic. Hess's Law focuses merely on the heat changes during reactions and presents the conclusion that "the total enthalpy change for the reaction is the sum of all changes." Bond enthalpies method concentrates on the atomic process of reactions, which involves bond breaks and formations. The heat of formation is needed in bond enthalpies.

Guangyu Li 2J
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Re: Hess's Law vs. Using Bond Enthalpies

Postby Guangyu Li 2J » Thu Jan 11, 2018 11:59 pm

For both Hess' Law and Bond Enthalpies, we are supposed to figure out the enthalpy of certain reaction through the changes of enthalpies (Ex. The NO2 example in the lecture). However, according to different circumstances, we have to choose using the bond enthalpies (when given the Lewis structure or bond energy) or Hess' Law(when given the reaction enthalpy).

Hope this helps!

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