Honors Chemistry Name______________________________
Bond Energy PMP
Mr. McAfoos Period________ Date__________________
Consider the reaction CH4 + 2 O2 à CO2 + 2 H2O If this reaction were constructed using the molecular model kits you used earlier this year it would look like this…
Notice that the pieces used to make the molecules on the right (the products) are already there on the left side (the reactants). So, if you wanted to simulate what nature is doing, you could take the models on the left, take them apart and then build the models on the right. This is what actually occurs. The molecules on the left are ripped apart and the molecules on the right form from the pieces.
A bond is an attraction between two different nuclei and the electrons that they have. In order to break that bond, energy must be put in. How much energy depends on the kind of bond (single or double, between which atoms, etc.). When a bond is formed, two things that are attracted come together. As a result, energy is released.
As a result of understanding this, we can determine the amount of heat involved in the reaction. Energy must be put in to break all of the bonds on the left side and energy will be released as the bonds on the right side are formed. To do this we add up the energy needed to break all of the bonds on the left, and then (separately) add up all of the energy released by the formation of bonds on the right. (These energies can be found on the attached table of bond energies)
Bonds broken = [4 (C-H) + 2 (O=O)] = [4(99kcal) + 2(119kcal)] = 634 kcal put in to break bonds
Bonds made = [2 (C=O) + 4 (O-H)] = [2(192kcal) + 4(111kcal)] = 828 kcal taken out to make bonds
Remember that energy released is negative (exothermic) and that energy added is positive (endothermic). Therefore the energy to make bonds is negative (-828kcal)
So…the energy for the whole reaction is 634 kcal + (-828 kcal) = -194 kcal (an exothermic reaction)
This process can be simplified as follows…
Energy of reaction = bonds broken – bonds made (the minus changes the sign for you). You will, of course, need to draw Lewis Dot Structures for everything. (So…even if you understood none of this…draw ALL of the LDS’s BEFORE class Monday.)
Use this idea to find the energy of the following reactions: