Chemistry 1020Lecture 10Notes
Hour Test 1
The grades on Test 1 were much lower than they should have been. Therefore I am spending the first part of this lecture going over the questions on Test 1, trying to point out where you should have known what the question was asking. I hope this exercise will give you some insight into what kind of preparation you need to make for the next test. (Keys to the test are posted on the bulletin board outside 208 HTL and in the "LightCHM 1020 notebook at the reserve desk in Dirac Science Library.
Apparently I did not work sufficient examples from the text last Tuesday for you to be able to understand the questions on quiz 3. Therefore I am postponing the due date for quiz 3, and will work some examples here.
Using bond energies to calculate energy changes in chemical reactions:
As pointed out in the last lecture, exothermic reactions produce energy. We can write energy as a product of the reaction. Where did it come from? The internal energy of the reactants was higher than the internal energy of the products, so the chemical change has energy left over.
For an endothermic reaction, one must add energy, and thus can write energy as a reactant. In this case the internal energy of the reactants is less than that of the products, so one must provide energy in order to form the products.
One way to think of the reaction as occurring, which is hypothetical only, is to break every bond in the reactants to form individual atoms, then to rearrange the atoms and form the new bonds of the products. By averaging thermochemical data from many reactions, we can come up with average values of the energy required to break a chemical bond. This energy is always positive (involving input of energy) because the energy of the shared electrons in the bond is lower than they would be in the individual atoms. Therefore breaking the bond involves input of energy. When bonds are formed from individual atoms, then, energy is given off. Therefore whether there is a net production or uptake of energy in a reaction depends on the relative energies of the bonds in the reactants and products.
Lets take a look at "Your turn4.6". You are asked to calculate the energy change in combustion of propane (C3H8), and the process is very similar to that worked in the text for the combustion of methane (CH4).
First of all, write the reaction:
C3H8 + 5 O2 à 3 CO2 + 4 H2O
It is useful to write the Lewis structures of reactants and products:
This is per mole C3H8. What about per gram of C3H8?
2016 kJ/mole x 1 mole/44.0 g C3H8 = 45.8 kJ/g