### 3 Methods

Posted:

**Thu Jan 18, 2018 6:05 pm**I know we use three methods, but what exactly are they used for? What are we trying to find?

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=76&t=25772

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Posted: **Thu Jan 18, 2018 6:05 pm**

I know we use three methods, but what exactly are they used for? What are we trying to find?

Posted: **Thu Jan 18, 2018 6:07 pm**

Enthalpy of the total reaction.

Posted: **Thu Jan 18, 2018 6:42 pm**

You are calculating the ∆H (change in enthalpy). There are three methods to get this.

Method 1: Hess's Law

Enthalpy changes are additive. Enthalpy change at each step of a multi-step reaction can be added to give the total enthalpy change.

Method 2: Bond Enthalpies

Add up all new bonds formed (energy released), and all bonds broken (energy required) to get ∆H.

Method 3: Standard Reaction Enthalpies

If bond enthalpies are not given, one can calculate the standard reaction enthalpies by calculating the standard enthalpy of formation of all products and subtracting the standard enthalpy of formation of all reactants. (Use of tables of standard enthalpy of formation to calculate.)

Method 1: Hess's Law

Enthalpy changes are additive. Enthalpy change at each step of a multi-step reaction can be added to give the total enthalpy change.

Method 2: Bond Enthalpies

Add up all new bonds formed (energy released), and all bonds broken (energy required) to get ∆H.

Method 3: Standard Reaction Enthalpies

If bond enthalpies are not given, one can calculate the standard reaction enthalpies by calculating the standard enthalpy of formation of all products and subtracting the standard enthalpy of formation of all reactants. (Use of tables of standard enthalpy of formation to calculate.)