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Standard Enthalpies of Formation

Posted: Sat Jan 14, 2017 10:17 am
by Imani Johnson 1H
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?

Re: Standard Enthalpies of Formation

Posted: Sat Jan 14, 2017 7:56 pm
by Brianna Wummer 2L
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!