## Method 3

Andrew F 2L
Posts: 103
Joined: Sat Aug 17, 2019 12:17 am

### Method 3

What does Method 3 mean/ how can it be used? In lecture Professor Lavelle said to only use it when bond enthalpies aren't available but the example he used was a bit confusing for me to understand now that I look back. Can someone please explain this to me? Thank you!

Clarice Chui 2C
Posts: 101
Joined: Fri Aug 30, 2019 12:16 am

### Re: Method 3

Andrew F 2L wrote:What does Method 3 mean/ how can it be used? In lecture Professor Lavelle said to only use it when bond enthalpies aren't available but the example he used was a bit confusing for me to understand now that I look back. Can someone please explain this to me? Thank you!

Yes please and can someone break down the equation?

Chris Charton 1B
Posts: 69
Joined: Mon Jun 17, 2019 7:23 am

### Re: Method 3

Method 3 is more accurate than Method 2, you can use it as long as you know the standard enthalpy of formation of all the products and reactants. Something like O2 in gaseous form will not have a listed standard enthalpy of formation because by definition the standard enthalpy of formation for an element in its most stable form is 0.

Jeremy_Guiman2E
Posts: 82
Joined: Fri Sep 28, 2018 12:29 am

### Re: Method 3

Method 3 uses the standard enthalpies of formation for both products and reactants. It is different than Method 2 which looks specifically at the bonds of the reactants and products.

You can take the standard enthalpy of formation for the products and subtract the standard enthalpy of formation for all reactants from it (total products' enthalpy - total reactants' enthalpy). This will give us delta H of the overall reaction. As Dr. Lavelle noted, you would use delta H of formation tables to not only find the standard enthalpies of the reactants/products but also ensure that everything is in standard state and that the reaction is balanced.

To generalize the example given in class:

Delta H of the reaction = the sum of delta H of formation of products - the sum of delta H of formation of reactants

Also make sure to multiply your delta H of formations by the amount of moles in the reaction (the coefficients).

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

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