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Offline hmx9123

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Teaching Oxidation Numbers
« on: March 15, 2006, 06:17:23 AM »
This came from the 'Oxidizing Agents' thread in the HS Chemistry forum.  The main point was that the rules taught to HS students weren't simplified, but rather simply wrong at times.  I began thinking of another way to teach oxidation numbers to high school students.  We would need something easy to understand, simple but never incorrect, and something that can be build upon for more advanced learning.

Rather than the algorithm process that is usually studied (alkalis are +1, fluorine is -1, oxygen is usually -2 but can be -1 or higher...), why not teach by structure?  By this point, students have been drawing lewis strucutures, and it is all too seldom in high school or even general chemistry that students think of the structures of their compounds.  This would alleviate many problems, as it is obvious once you look at the structure of certain compounds that certain atoms of the same type are chemically inequivalent.  For example, the previous discussion used potassium peroxydisulfate and sodium thiosulfate for discussion.  They are particularly suited because of different oxidation state oxygen atoms in the potassium compound and the different oxidation state sulfur atoms in the sodium compound, even though the simple method gives you an average oxidation state of the sulfur atom, which is generally incorrect.

The disadvantages to teaching an oxidation number by structure method would be that:
a. you would have to be able to draw correct lewis structures (not always trivial)
b. you would have to have a periodic table nearby with data on it, or memorize most of the electronegativies thereon
c. it is slightly more complicated than memorizing rules and following algorithms (although perhaps more correct)

The advantages are:
a. students see structures of compounds that would otherwise be meaningless molecular formulae
b. inequivalent atoms are much more apparent
c. you get practice drawing lewis strucutres. :)

Are there any other methods you can think up?  Perhaps we can come up with some and maybe even list out a new set of rules for determining oxidation numbers for atoms in different polyatomic ions.  Hopefully this doesn't seem too pointless to anyone, but it seems like a worthwhile discussion at least.  I'd be very interested to see how different people approach teaching oxidation numbers.

Offline Mitch

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Re:Teaching Oxidation Numbers
« Reply #1 on: March 15, 2006, 11:27:24 AM »
I think the current system is good, but highschool teachers need to point out the examples where this is not the case and how one can approach the problem if faced with a molecule that deviates significantly from experience.
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Offline constant thinker

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Re:Teaching Oxidation Numbers
« Reply #2 on: March 15, 2006, 03:55:05 PM »
I would just like to note that some teachers are crackpots and should not be teaching. Some do not know what they are talking about. I have retaught friends in the past all about the periodic table because they had a teacher who did not know a thing. I also have done the same with geometry and algebra. English is the only subject I haven't taught anyone about because I am terrible at English. I never understand completely what Shakespeare, Swift, or other guys like them are ever talking about. History classes are generally straight forward.
« Last Edit: March 15, 2006, 03:57:34 PM by constant thinker »
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Offline hmx9123

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Re:Teaching Oxidation Numbers
« Reply #3 on: March 15, 2006, 08:55:47 PM »
Well, there definitely are some crackpot teachers out there.  My general chemistry high school teacher was terrible.  Fortunately, I had a much better teacher the next year for AP.  Anyway, back to the subject at hand.

If we continue along the pathway of teaching oxidation numbers by the counting method, we still have the problem of having quite a few exceptions to the rule.  I was looking through my gen chem books at home, and they have some of these compounds (one even had sodium thiosulfate!) listed in their problem sets, and their answers were given incorrectly--they used the methods that I had illustrated earlier in the HS forum.  If we point out how to approach the problem, then we are essentially teaching two methods of doing the same thing, are we not?  I dunno, I hate redundancy for things like this, because they can get confused.

How about this, at least as a start.  Add, change, edit, whatever, how you like.  I'm trying to come up with a general algorithm that will hold for as many compounds as possible.

Steps for obtaining correct oxidation numbers:
1. Draw (or look up) the correct lewis structure of the compound in question, including labelling formal charge, etc.
2. Count the number of bonds around each atom, counting in the following manner:
- If the bond attaches to a more electronegative atom, count it as -1
- If the bond attaches to a less electronegative atom, count it as +1
- If the bond attaches to an atom of the same type, count it as 0
- Add any formal charge that may be on the atom
3. Add up the numbers to give you an oxidation number for the atom in question.

This doesn't hold for everything.  Looking at the peroxydisulfate anion it seems to, but the thiosulfate doesn't.  In the peroxydisulfate, each sulfur has a +6, and all oxygens but the peroxides have -2, and the peroxide oxygens have -1.  In the thiosulfate, using this algorithm, the sulfur doubly bound to the central sulfur has an oxidation state of 0.  Hmm.  Maybe instead of 'attaches to an atom of the same type' we could say 'attaches to an atom of the same type in a chemically equivalent environment.'  Does that solve our problem?  I don't really know if a simple algorithm is doable, but I think it would be useful to see if we can make one.  What do y'all think?
« Last Edit: March 15, 2006, 09:00:06 PM by hmx9123 »

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