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Oliver G. Ludwig in his recent article
(1) balances Stout's "redox challenges"
(2) by unconventional oxidation numbers. I agree that it is pedagogically useful to show
to the students that the oxidation numbers are
conventional tools for interpretation and balancing redox reactions.
However, in my opinion, balancing chemical equations
by nonconventional oxidation numbers is permitted in case
of need only. It may be a powerful method for balancing
complicated chemical equations such as double
disproportionations or double redox reactions.
For example, the equation
P2I4 +
P4 + H2O ->
PH4I + H3PO4
published in this Journal
(3), is a really serious "redox
challenge" for balancing by conventional oxidation numbers
(4–6). Introducing unusual oxidation numbers we can
convert this double disproportionation into a simple redox
reaction as follows: Taking the oxidation number of P in
P2I4, and PH4I equal to its conventional value in
H3PO4 (P: +5), we get only P is oxidized in the reaction. If we select iodine
as the other redox element among those kinds of atoms
that occur in only one substance on each side of the equation
(I, O) and assign H and O their usual oxidation numbers
of +1, and -2, respectively, we get unconventional
oxidation number of -2, 5 for I in
P2I4, and -9 in
PH4I. Thus, the P is oxidized from 0 to +5, and I is reduced from -2, 5 to -9,
giving stoichiometric coefficients of 6, 5/4, 5/4 and 5 for
P4, P2I4, and
PH4I, respectively. After balancing in P, then O
atoms, and multiplying through by 8, we obtain the balanced
equation:
10P2I4 +
13P4 +128H2O =
40PH4I + 32H3PO4
Literature Cited
- Ludwig, O. G. J. Chem. Educ. 1996, 73, 507.
- Stout, R. J. Chem. Educ. 1995, 72, 1125.
- Carrano, S. A. J. Chem. Educ. 1978, 55, 382.
- Kolb, D. J. Chem. Educ. 1979, 56, 181.
- Cardinali, M. E., Giomini, C., Marrsu, G. J. Chem. Educ. 1995, 72, 716.
- Cardinali, M. E.; Giomini, C.; Marrosu, G. Educ. Chem. 1996, 51.
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