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### Bond Enthalpies

Posted: Thu Jan 12, 2017 6:46 pm
To calculate what delta H is for a particular reaction using bond enthalpies (method 2), we have to know the structure with which the bonds break in the reactants and the new bonds are formed in the products. But, how do we know which bonds require energy and are broken, and which bonds release energy and are formed?

### Re: Bond Enthalpies

Posted: Thu Jan 12, 2017 7:14 pm
By drawing out the reactants and products with their correct Lewis structures.

### Re: Bond Enthalpies

Posted: Thu Jan 12, 2017 10:16 pm
Draw the lewis structures for all the products and reactants and compare the bonds. If a bond exists in one of the reactants but doesn't in the products then that bond is broken and if a bond does not exist in one of the reactants but exists in the products then that bond is formed. It's basically a matter of comparing the lewis structures.

### Re: Bond Enthalpies

Posted: Fri Jan 13, 2017 10:59 am
In using this method, when a bond is formed do we write it as negative energy, and when a bond is broken, we write it as positive energy to add them up?

### Re: Bond Enthalpies

Posted: Fri Jan 13, 2017 12:08 pm
Dianna Grigorian 1C wrote:To calculate what delta H is for a particular reaction using bond enthalpies (method 2), we have to know the structure with which the bonds break in the reactants and the new bonds are formed in the products. But, how do we know which bonds require energy and are broken, and which bonds release energy and are formed?

For method 2, we can just list all the bonds of products and reactants and then compare those bonds. The bonds that are different in reactants and products are the ones that will be used in calculation.

### Re: Bond Enthalpies

Posted: Fri Jan 13, 2017 12:32 pm
In the course reader when we are looking at method two, all the bond enthalpies are in kJ.mol^-1. However after we add them all up, the units are kJ so I am curious on what happened to the mol^-1?

### Re: Bond Enthalpies

Posted: Fri Jan 13, 2017 12:51 pm
Jenny_Luu_2A wrote:In using this method, when a bond is formed do we write it as negative energy, and when a bond is broken, we write it as positive energy to add them up?

When a bond a formed, energy is released so that will be positive energy. When the bond is broken, energy is used, making it negative. To find the total, just add but make sure your signs are constant in order to get the correct total.

### Re: Bond Enthalpies

Posted: Sat Jan 14, 2017 1:01 am
For these types of problems, will we always be given the reaction?