### Hess's Law

Posted:

**Sun Feb 03, 2019 10:08 pm**How do you use Hess's law to find the reaction enthalpy? 8.61, 4D.19?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=76&t=41301

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Posted: **Sun Feb 03, 2019 10:08 pm**

How do you use Hess's law to find the reaction enthalpy? 8.61, 4D.19?

Posted: **Sun Feb 03, 2019 10:54 pm**

For Hess' law, think of it as manipulating equations in order to add up their delta H's. Here is a link to a video that explains it way better than me: https://www.youtube.com/watch?v=2q2u5sj4V00.

Posted: **Sun Feb 03, 2019 11:23 pm**

Hess's law basically states that enthalpy changes are additive. Enthalpy change at each step of a multi-step reaction can be added to give Total Enthalpy Change.

Posted: **Sun Feb 03, 2019 11:32 pm**

Hess's Law allows us to manipulate equations where one constituent appears in one equation's reactants while the same constituent appears in another equation's products. As such, you can cancel out the constituent by adding the equations together. In doing so, you must also add up the enthalpies. In adding the equations together, you are trying to get an equation that matches the equation of a reaction where you are looking for its enthalpy.