Oxidation-Reduction
Harry H. Sisler and Calvin A. VanderWerf Unlverslty of Florlda Gameswlle, FL 32601
An example of chemical sophistry
It is an honor to he asked to contribute to a group of discussions which are assembled in trihute to Dr. W. T. Liooin.. colt ulio for thlrteen years has scrved su effectively as rhe editor of thedOLIRNAL OF CHKMTCAL E1)IICA'I'ION. It seems appropriate to present a brief discussion of one of the more apparent sophistries in chemical education in this context, for Tom Lippincott has been most effective in speaking out against specious reasoning in the field of chemical education and in seeking to rid the field of outworn concepts. We have recently reexamined oxidation-reduction in the current ~ o ~ u l textbooks ar of eeneral colleee chemistrv. There has bekn virtually no progr& during thewpastthree decades in the manner in which theconceot is commontv defined. Nor has there been diminution in the shallow sophi& with which we chemists try to cling to these outworn definitions. For example we find in one of the best selling of the currently popular texts the following: "What do we mean by the term oxidation-reduction reaction? In the table we assign positive and negative valence numbers to some of the elements. For example, hydrogen has a valence number of plus one and oxygen is minus two. Now we know that the negative charge designates an excess of electrons and the positive charge results when there is a deficiency of electrons." In still another very popular text, we find this definition: "Oxidation is defined in a broad sense as a reaction in which atoms or ions lose electrons." It is generally true that very few of the currently popular college chemistry texts define oxidation-reduction in any other way than as the loss and gain of electrons. As we reviewed the treatment of oxidation-reduction by the currently popular general chemistry texts, we were struck by a strong feeling of deja uu and recalled a paper which we published in September 1945, in collaboration with A. W. Davidson in which we analyzed in detail the faults in the treatment of oxidation-reduction reactions and the inron~ ~ ~ sistencies in its'definition. To my knowledge, that paper has never been attacked by any other published document. What we suppose is much worse is that it has largely been ignored, for the current treatments of oxidation-reduction disnlav precisely the same faults as those which stimulated the writing of the original paper over 30 years ago. With a few exceptions, textbooks today still base their definitions of oxidation-reduction on loss and gain of electrons, or perhaps in an effort to avoid some of the more ohvious deficiencies of this definition, they define the concept in terms of loss or gain in electron density. Of even greater futility, hut in recognition of the lack of validity in the loss or gain definitibn, we find one or two definitions which add the phrase "apparent loss or gain of electrons." Still others, who ~
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42 / Journal of Chemical Education
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recognized the weakness of the effort to define oxidationreduction reactions in terms of electron loss and gain, defined this concept in terms of change in oxidation numhers hut then proceeded to wipe out the progress they had made by implying a consistent relationship between calculated oxidation numhers and the electronic states of the atoms concerned. Only a very few recognize what is, in fact, true--that there is no consistent basis for distinmishine so-called oxidation-reduction reactions from 0th; chemical reactions, other than by changes in oxidation numhers; and that these numbers have no consistent physical basis hut are calculated on what must now he recognized as arbitrary rules. In fact, there are those who have suggested that the concept of oxidation-reduction is little more than an artifact in the history of the chemical sciences. Let us look a t some chemical reactions in which we seek to aoolv the electron transfer definition of oxidation and reduicon. It is obvious that the ideal definition would first of all he one which can he applied on the basis of the initial and final states of a reaction without regard to mechanism. It is also clear that the identification of oxidation with electron gain is entirely valid in reactions involving only elements and simple ions. This relationship . may . he illustrated hv means of such reactions as
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Zni.)+ Cu(.J+ 2Na(,1 + C121,) 2Brie& + Cln(,i
--
ZniJ+ + CU(,I 2(Na+, CIWi,) B w ~ )+ 2C4aq~-
In each case, the first reactant has lost electrons and has been oxidized whereas the second reactant has mined electrons and has been reduced. All clear cut cases of complete electron transfer in chemical reactions can he classified readily as oxidation-reduction under the electron transfer definition. However. to extraoolate from this relations hi^ that all oxi~ dation-reduction reactions can he related simila& to electron transfer is a fallacy. Consider, for example, the complete combustion of carhon in the atmosphere:
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C(S1 + 0 2 1 d
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Cozla)
Bv universal agreement. the carhon is oxidized hv four units. ~ h w e v e rthe , &sumption that carbon has lost fo;r electrons in its oxidation to carhon dioxide reauires the oostulatinn of the existence of carhon in carhon dioxide as the C4+ ion, a postulate which is, of course, completely inadmissable. It might he argued that four carhon electrons are shifted partially from the carhon atom toward the oxygen atoms, hut this claim can hardly he made in the case of the oxidation of carhon to carbon monoxide; the very small dipole moment of carbon
monoxide indicates that little, if any, electron displacement has taken place in its formation. The case for complete electron loss in oxidation is even more difficult touphold in such reactions as the oxidation of acetaldehyde to acetate ion in alkaline solutions:
XH,C
/P: + ZMnq,., + 2H,0,1,
\..
o:-
