Standard Enthalpies of Formation

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Imani Johnson 1H
Posts: 14
Joined: Sat Jul 23, 2016 3:00 am

Standard Enthalpies of Formation

Postby Imani Johnson 1H » Sat Jan 14, 2017 10:17 am

Do we use the formula of the sum of products minus the sum of reactants only for standard enthalpies of formation? Why do we not just add all enthalpies, similar to how we solve problems using Hess's law?

Brianna Wummer 2L
Posts: 19
Joined: Wed Sep 21, 2016 2:56 pm

Re: Standard Enthalpies of Formation

Postby Brianna Wummer 2L » Sat Jan 14, 2017 7:56 pm

Hi there! As far as I know, that formula is only for standard enthalpies of formation. You can't just add the standard enthalpies of formation because unlike Hess's law, we are using enthalpies of each molecule in the chemical equation. In Hess's law, the enthalpies of two separate reactions are added, not standard enthalpies of molecules. Therefore, when using standard enthalpies of formation, you subtract the sum of reactants from the sum of products. Hope that helps!

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