Having just gone through "the rules" for balancing redox reactions in my Gen Chem class, I once again am faced with a textbook that does things in a different order than I prefer. When I balance redox reactions, I add electrons at the beginning of the process to balance the change in oxidation state/number of the species being oxidized or reduced. Then, at the end, I can use charge balance as a self-check; if the charge balances itself out, then I probably did everything correctly. My current textbook (and many others), have students go through all the steps (add water, add H+, etc), and then add electrons at the end to balance the charge.
When do you add electrons? Chemically and pedagogically, I think that adding electrons at the end of the process to balance charge can be misleading because it ties oxidation number too closely to charge. That's fine if your redox couple is something like Zn2+|Zn, but it seems to throw a wrench into the works if you're using something like permanganate or lead oxide half cells. In the big picture, if the students follow "the rules" correctly and completely, either system will work, but if they only remember a little bit about the process I would prefer that they associate electron loss/gain with changes in oxidation number rather than charge.
Is there some subtle point I'm missing here? Is this an inorganic vs. the rest of chemistry thing, or is it more like the Pepsi vs. Coke debate? Should I start an activist group called "Chemists for the Early Addition of Electrons in Balancing Redox Reactions" and try to get all textbooks to change their ways? Hmm, CEAEBRR isn't a very sexy acronym, perhaps the creative minds behind IONiC VIPEr can make that more appealing. "oxidation state change" would be "OSc" or "OsC" and if "Ar" could be incorporated it would be "OsCAr".....
What a great post!
For a while, I taught both methods in gen chem (we called them the "half-reaction method" and the "oxidation state method"). I think the half-reaction method can be done mindlessly, whereas the oxidation state method requires a little thinking about chemistry.
To verify your concern....I learned the half-reaction method when I was a student and must have slept through the part on oxidation numbers, because I could balance redox reactions but I had no idea what oxidation numbers were. In fact, if asked whether something was being oxidized or reduced, I had to balance the half-reaction and see which side the electrons ended up on (I knew LEO goes GER.)
Bottom line...I'd much rather my students remember something about oxidation numbers and the concept of gaining and losing electrons, instead of some brainless way of balancing reactions. So you can sign me up for your movement. I'll get to work on the acronym.
In reply to I never did understand oxidation states.... by Joanne Stewart / Hope College
I actually use what I call a hybrid method.
Use oxidation numbers, then write out 'half-reactions' for the elements that undergo oxidation or reduction. I agree that learning the chemistry is important and that is why I am fond of oxidation numbers. I also like pulling out the half-reactions because it forces the kids to see that you can't have red without ox and verse vica.
As with class, perhaps an example of this method would be best.
Cr2O72- (aq) + Cl- (aq) --> Cr3+ (aq) + Cl2 (g) in H+
Red: Cr+6 + 3e- --> Cr+3
Ox: Cl- --> Cl0 + e-
Next comes the step that I am not thrilled with (and will gladly take suggestion on) but it is what I do. I call it the formula balance step. You have to account for the formulas of the compounds you are working with.
Red: Cr2+6 + 6e- --> 2 Cr+3
Ox: 2 Cl- --> Cl20 + 2e-
After this you equalize the electrons and then clean up with H+ and H2O.
I don't get complaints when my kids go on to Gen Chem II or analytical and are forced to do the half-reaction method and I like that they think about oxidation states.
I agree that balancing charge is a really bad idea. But I can see problems with trying to deal with oxidation states prior to balancing out the main elements, especially when there is a polyatomic ion, such as (S2O3)2–. I have students follow the "rules", but use the oxidation state changes, NOT the charges to determine the number of electrons. I've posted a movie I made for the class on balancing the reaction:
S2O32– + IO3– ⟶ SO42– + I– (in basic solution)
The link is:
http://www.oswego.edu/~craymond/Teaching/S2O3_IO3.mov