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Hess's Law vs. Rxn Enthalpies

Posted: Sun Feb 03, 2019 1:02 pm
by Sophie Roberts 1E
How do you know when to you use Hess's Law to calculate the overall reaction Enthalpy versus when to use the equation:
Enthalpy of formation for products - the enthalpy of formation for the reactants
For example #4D15 in the 7th edition, I used the equation above but the solutions manual used Hess's Law...

Re: Hess's Law vs. Rxn Enthalpies

Posted: Sun Feb 03, 2019 2:34 pm
by Timothy_Yueh_4L
You use Hess's law when adding the enthalpy of reactions for two or more reactions are given, with certain compounds able to be replaced in between the two reactions. A + B --> C plus C + D --> E: the compound C is used and created in both situations. On the other hand enthalpy of formation is used when individual values of enthalpies are given that describe the amount of energy given off when the compound is formed. You would add up the enthalpy of formations of the reactants and subtract that from the sum of enthalpies of formation of the products.

Re: Hess's Law vs. Rxn Enthalpies

Posted: Sun Feb 03, 2019 5:02 pm
by 505095972
I think they are both ways to find a change in total enthalpy of the system, so it depends on the given information to see which one is easiest to use.