Correspondence OXIDATION-REDUCTION EQUATIONS MYDEAR MR. REWMUTH: Your recent articles in the JOURNAL dealing with oxidation-reduction reactions are not only thought-provoking but, as I hope you will find, correspondence-provoking also. It seems to me that you have outlined the subject on the basis of the theory of atomic structure in a very satisfactory way. My personal reaction is favorable-on the whole. But certain points in connection with your reply to the inquiry by C. A. G. in the May issue lead me to write to you in a somewhat controversial spirit. Frankly, I do not believe that you are in accord with the majority of teachers of chemistry who have to teach oxidation-reduction reactions and who a t the same time have tried to keep in touch with the modern trends of theory as applied to this most difficult type of reactions with which elementary or second-year students have to deal. My contentions, made in the interests of chemical education, are these. First. On page 1182 of the May number of the JOURNAL YOU state: "The fact is that KMn04 does not yield 'nascent' (or atomic) oxygen." Mr. Editor, your evidence? As a matter of opinion I agree with you but searching of the literature has failed t o produce the desired d e n c e . In fact, the evidence, as far as i t goes, is against us; KMnOl in the presence of fairly concentrated sulfuric acid does yield oxygen. Other teachers of chemistry besides myself would he gratified, I am sure, to have some real, convincing evidence that the same thing may not happen at all in more dilute solution. EDUCATION has weighty influence Second. The JOURNAL OF CHEMICAL among teachers of chemistry; it is regarded as authoritative. Lest your personal preference in avoiding assigning valences t o the individual atoms in an ion be regarded by readers of the JOURNAL as an officialpronouncement of the Division behind the JOURNAL, let me be one to register opposition t o your views on the mattcr. We assign valence to carbon in writing the structural formula of acetic acid used in teaching organic chemistry; why not retain that valence when dealing with the acetate ion? We assign valence t o sulfur when we explain the formation of sulfuric acid from sulfury1 chloride and water; why not retain it when dealing with the sulfate ion? Why should we go consciously out of our way to avoid assigning valences t o the atoms in an ion when those same avoided valences must be assigned t o deal intelligently with the molecule in which the valence forces must be substantially the same? Why not assign valence t o the Mn in Mn04-? 1688
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CORRESPONDENCE
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Third. The reaction between ferrous sulfate and potassium permanganate in the presence of sulfuric acid would be represented, if I am not mistaken, according to your method, by:
+
5Fe++ ---t SFe+++ 5r 8H + 5c --f Mnf + (2) WnO1(1)
+
+
+ 4H.O
Reaction (1) probably represents the real mechanism; the fact that the oxidation of the ferrous ion may be accomplished with salt bridge and wire connection is evidence of this. Reaction (2) may be correct as a "summed up" reaction but as a representation of the mechanism i t is unnecessarily complex and it is not the most reasonable-it implies that the reaction is of the ninth order-that reduction of the M n 0 c may take place only when "bombarded" simultaneously by eight H+ ions in the presence of the available electrons. Is i t not more reasonable and more logical-and it is certainly simplerto assume the existence of heptavalent manganese ions? With this assumption the reaction comes down t o this: (1) 5Fet+ ---t 5Fe+++ (2) M n O l - e Mn+' f 40--
+
8H T 1
+
4Hp0 Mn+' +5e+Mnf+
Modem chemists generally believe in the near-universality of equilibria in solutions; they believe that even nitric acid and sulfuric acid may yield some OH- in aqueous solution; they believe that in the ionization of water itself, relatively slight as it is, there is some secondary ionization yielding 0--. We know that MnzOl exists. Why not then assume the existence of heptavalent Mn ion, the equilibrium tending strongly to the M n O d side? Also is i t not generally believed that when water forms in solution from H + it does so by union with OH- or 0--? So it seems to me that in the interests of chemical pedagogy you should give publicity in some manner or other to this other view which I am firmly convinced will be in closer accord with the views of those actually teaching the subject. This mechanism is more readily visualized by the student, is infinitely simpler to teach and, more important than either of these, it is probably a more nearly correct picture of the actual mechanism. W. B. MELDRUM HAVERFORD COLLEGE HAVERFORD. PENNSYLVANIA
* * * * *
DEARDR. MELDRUM: Your letter is representative of a number of comments called forth by the correspondence on oxidation-reduction in the May JOURNAL ( p p 11804).
