Answer to Question #317627 in General Chemistry for ara

Question

how do you calculate enthalpy change using Hess's Law?

Answer 1

Hess's Law says the total enthalpy change does not rely on the path taken from beginning to end. Enthalpy can be calculated in one grand step or multiple smaller steps.

To solve this type of problem, organize the given chemical reactions where the total effect yields the reaction needed. There are a few rules that you must follow when manipulating a reaction.

Enthalpy change, ∆H, can be defined as the amount of heat absorbed or released during a reaction. In each individual step of a multistep reaction, there is a beginning and end enthalpy value- the difference between them being the enthalpy change. This value can be either negative if the heat was absorbed, or positive if the heat was released. If you add up all the enthalpy changes of each reaction step(∆H_r), you have net enthalpy change, which is found by finding the difference between the final product enthalpy and the beginning reactant enthalpy (∆H_net). That is Hess’s Law!