Methods of Finding Enthalpy

[Kristy Lin 2I]

There were 3 different methods used to calculate enthalpy mentioned in lecture (Hess's Law, bond enthalpies, and standard enthalpies of formation). In what circumstances would certain methods be preferred over the other two?

[Nicole Anisgard Parra 2H]

The method used would vary based on the values you are given. If you are given a table with bond enthalpies, you would utilize that information and use that method; if you are given the delta H value for the formation of one molecule and the delta H value for the formation of another molecule, you can use Hess's Law to add the reactions to obtain the total delta H for the reaction. Finally, if you are given the standard enthalpies of formation for all the substances/molecules involved in a reaction (products and reactants), you can use the standard enthalpy method and then subtract the total standard enthalpy of formation of the reactants from the total standard enthalpy formation of the products.
TLDR the method used depends on the values you are given to work with :)