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Bond Enthalpies

Posted: Thu Jan 28, 2016 2:44 pm
by Michelle Yu 1L
I am still very confused on how do you know what bonds are being broken to what bonds are being formed.

Re: Bond Enthalpies

Posted: Thu Jan 28, 2016 3:14 pm
by Chloe 3D
To determine which bonds are broken and which are formed, it helps me to draw the lewis structures for the reaction. Then you can interpret the result visually.

For example, consider the reaction 2H2(g) + O2(g) --> 2H2O(g). After drawing the lewis structure, you see that the bonds of the reactants (2 H-H bonds and 1 O=O bond) are broken and those of the products (4 O-H bonds) are formed.

Re: Bond Enthalpies

Posted: Thu Jan 28, 2016 3:39 pm
by Michelle Yu 1L
so the reactants always break bonds and products form bonds?

Re: Bond Enthalpies

Posted: Thu Jan 28, 2016 4:00 pm
by Heerali Patel 3A
Yes. Essentially, reactants are the molecules whose bonds are broken, and products are the molecules whose bonds are formed.

Re: Bond Enthalpies

Posted: Thu Jan 28, 2016 4:08 pm
by Nisha Patel 3I
Yes, the bonds that are broken will be in the reactants, and the bonds that are formed will be in the products. Drawing the Lewis structures of each reactant and product will help you see which bonds are broken and which bonds are formed. For example, in homework problem #8.73b, the following reaction is given:

CH4(g)  4 Cl2(g) --> CCl4(g)  4 HCl(g)

Reactants: If you draw the Lewis structures of CH4 and 4Cl2, you will see that 4C-H bonds and 8 Cl-Cl will be broken.
Products: If you draw the Lewis structures of CCl4 and 4HCl, you will see that 4C-Cl bonds and 4H-Cl bonds will be formed.