How to: Hess's Law

[AnnaYan_1l]

Would someone be able to explain how they do Hess's Law problems? I'm not able to follow the notes from class as clearly, and the problems in discussion confused me. How do you know when you need to reverse the reaction? Are you trying to get the same compound on both the reaction and products sides to cancel them out?

A step-by-step method would be really helpful for me, if possible.

Thank you in advance for your help!

[Noah Cook 3O]

When working on a problem using Hess's Law, you first need to make sure you have a balanced equation for the reaction. Then, you want to find the reaction enthalpies of reactions involving the molecules in the original reaction (probably in some table in the book or given on a test). Then, look to see if these reactions (the ones with reaction enthalpies) result in the molecule acting as a reactant or a product. If it acts as the same as it does in your original reaction, the you use the reaction enthalpy given. If it is the opposite, you would need to change the sign of the reaction enthalpy. Also, make sure you consider the moles produced/consumed by each of the reactions, as you have to match those up to your original equation (you would need to multiply the reaction enthalpy by 2 if the original reaction has 2 times as many moles as the reaction from the table for example). Then, by adding the reaction enthalpies of the constituent reactions you will have the overall reaction enthalpy