If oxidation numbers (ON's) merely serve as bookkeeping numbers for balancing equations, then Ludwig's method (J. Chem. Educ. 1996, 73, 507) would be acceptable. However ON's have a chemical reality in that they can show, albeit in an exaggerated fashion, the direction of charge distribution and its change in a reaction. The claim, for example, that reversing the electron distribution in the cyanide ion from
is chemically reasonable is not supported by any evidence (see J. Chem. Educ. 1988, 65, 45 for conventions used). In fact semiempirical or ab initio calculations show that nitrogen is the negative partner, as does the experimentally
derived electron distribution obtained from the X-ray structure of LiCN. (N varies from 0.046e more negative than C on PM3 to 0.295e on 6.13G**).
Students are likely to be more confused than
enlightened when cations can have variable negative and
sometimes nonintegral ON's. They need a systematic
procedure to generate consistent integral numbers linked to
concepts such as electronegativities and Lewis structures taught
in their courses.
Also, Ludwig's method is hardly a great time saver
over conventional balancing. For example, in dealing with
the third equation he has to assign conventional ON's to all
but one of the molecules. The conventional numbers
required in the remaining molecule are shown.
The increase in overall oxidation number has to be
balanced by the 5-electron reduction Mn(VII)
-> Mn(II). Thus 558/5 permanganates are required to oxidize each
molecule of the chromium complex.
|
Mean ON's in reactant |
ON's in products |
No. of atoms |
Increase in ON |
| Cr |
18/7 |
6 |
7 |
24 |
| C |
132/42 |
4 |
42 |
36 |
| N |
-3 |
5 |
66 |
528 |
Interestingly in the second equation Ludwig
considers the CN group on the thiocyanate and cyanide ions to be
unchanged. In terms of conventional ON's this is only true
if the former is formulated as
since the electronegativities of C and S are so close;
otherwise the ON's of C and S both vary and only N remains
constant.
|
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