Enthalpies  [ENDORSED]

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Tiffany Cao 1D
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Postby Tiffany Cao 1D » Fri Jan 19, 2018 10:27 pm

When finding enthalpies of reactions, what are the indicators that will show you when to use Hess's Law, Bond Enthalpies, or Standard Enthalpies of Formation?

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Re: Enthalpies  [ENDORSED]

Postby Chem_Mod » Fri Jan 19, 2018 10:46 pm

As I mentioned in class it will depend on the information available.

Or the question could ask: Using bond enthalpies calculate the reaction enthalpy.

Also, Standard Enthalpies of Formation for many compounds are not yet known.

Naomi Jennings 2H
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Re: Enthalpies

Postby Naomi Jennings 2H » Fri Jan 19, 2018 10:50 pm

I've been working based on the following and it seems to be working! If they give you multiple other reactions and their enthalpies, then you probably need to use Hess's law. If they refer you to Appendix 2A, you should use the enthalpies of formation. If they tell you to use bond enthalpies, then use those. If they don't give you any additional reactions or specify using enthalpies of formation or bond enthalpies, then I would use enthalpies of formation, because there are no other equations to use the Hess's law method with, and bond enthalpies are less accurate than the other two methods for non-diatomic molecules. Since bond enthalpies for non-diatomic molecules are an average of a bunch of different enthalpies which differ depending on the other things the atoms in question are bonded to, they will be less accurate than using the enthalpies of formation. Hope this helps!

Sean Monji 2B
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Re: Enthalpies

Postby Sean Monji 2B » Fri Jan 19, 2018 10:57 pm

For Hess's Law: you must know the total energies produced by at least two different reactions which can be combined due to similar reactants or products.
For Bond Enthalpies: you need the enthalpies of each bond in all the reactants and products
For Standard Enthalpies of Formtation: is kinda between Hess's law and bond enthalpies except you need the enthalpies of a molecule, not just the bonds that make up the molecule
Hope that helps

edit: a little late, the above cover it

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