Hess's Law vs. Standard Enthalpies of Formation

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JChen_2I
Posts: 107
Joined: Fri Aug 09, 2019 12:17 am

Hess's Law vs. Standard Enthalpies of Formation

Postby JChen_2I » Thu Jan 30, 2020 5:42 pm

How do we know when to use Hess's Law as opposed to standard enthalpies of formation where we do the enthalpies of product minus enthalpies of reactants?

Zoya Mulji 1K
Posts: 105
Joined: Thu Jul 11, 2019 12:15 am

Re: Hess's Law vs. Standard Enthalpies of Formation

Postby Zoya Mulji 1K » Thu Jan 30, 2020 6:36 pm

I'm pretty sure that we will be given the multiple rxns and their enthalpies for Hess's Law.

Abhi Vempati 2H
Posts: 104
Joined: Fri Aug 09, 2019 12:17 am

Re: Hess's Law vs. Standard Enthalpies of Formation

Postby Abhi Vempati 2H » Thu Jan 30, 2020 9:09 pm

Adding on to what @Zoya Mulji 1K said, if we're given a table of standard enthalpies of formation, the problem will want us to use enthalpies of product minus enthalpies of reactants. Most of the time, it should be pretty clear what method the problem wants you to use. Hope this helps!

Robert Tran 1B
Posts: 118
Joined: Thu Jul 11, 2019 12:15 am

Re: Hess's Law vs. Standard Enthalpies of Formation

Postby Robert Tran 1B » Thu Jan 30, 2020 10:00 pm

If multiple reactions are given, the Hess's Law should be applied. If one reaction and the standard enthalpies of formation are given, then you should subtract the standard enthalpies of formation of the products by that of the reactants.

Anna Heckler 2C
Posts: 102
Joined: Wed Sep 18, 2019 12:18 am

Re: Hess's Law vs. Standard Enthalpies of Formation

Postby Anna Heckler 2C » Thu Jan 30, 2020 11:30 pm

It will be obvious which method to use since each one has different steps and requirements. So the problem will provide multiple equations for Hess' Law, and a table of standard enthalpies of formation for this method.

Morgan Carrington 2H
Posts: 54
Joined: Wed Nov 14, 2018 12:22 am

Re: Hess's Law vs. Standard Enthalpies of Formation

Postby Morgan Carrington 2H » Wed Feb 05, 2020 9:46 pm

I'm sure that the midterm will explicitly say which of these methods to use when solving these kinds of problems if it is not already obvious, much like the textbook problems do. You could probably also utilize the formulas page to try and narrow down the choices if it is unclear.


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