Bond Enthalpies

[Michelle Yu 1L]

I am still very confused on how do you know what bonds are being broken to what bonds are being formed.

[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.

[Michelle Yu 1L]

so the reactants always break bonds and products form bonds?

[Heerali Patel 3A]

Yes. Essentially, reactants are the molecules whose bonds are broken, and products are the molecules whose bonds are formed.

[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.