One of the last topics we looked at this semester was oxidation-reduction reactions, and from what I can piece together from student concerns I did a fairly bad job explaining this. I think there are several things I need to remember for when I teach this again, so I’m going to record them here, because why not? In no particular order:

I realized that a lot of students struggled to interpret structures. So in sulfuric acid, H2SO4, students might see “H2″ and not realize that these two hydrogens are not bonded to each other (oxidation state of zero) but to something else (oxidation state of +1). I should have directed students a little more at the beginning as to how to read formulas. And by beginning, I mean much sooner than looking at oxidation states.

One thing that is really on me is not always showing or writing down an oxidation state for each atom in a molecule. Since the oxidation states have to sum to the charge on the molecule, seeing the oxidation state of each atom would probably helped some students. For one student today I showed him how we can sum up the oxidation states to equal the charge of the molecule, but backwards. So if what we know is the total charge and one of the oxidation states, we can solve for x. I wish I had spent more time codifying this equation at the beginning. If we are applying our shortcuts to, let’s say CO2, we see there are two oxygens. Each has an oxidation state of -2, so altogether they are -4. Since CO2 is a neutral molecule, the oxidation state on the carbon must be such that all the oxidation states sum to zero. Well, x + -4 = 0, x = +4.

I also should have made the students do a lot more problems. On the review day, I reminded them that we already looked at oxidation states in the context of electron sharing in bonds. We drew a structure and assigned electrons to atoms based on electronegativity. The short way to get them is to say oxygen is -2, hydrogen is +1, and halides are -1 (usually) and solve other atoms from there.

The students are taking an exam tomorrow , so I guess we’ll see how it goes. For now I need to plan the final exam.