OXIDATION NUMBER IN AUTO-REDOX REACTIONS LAWRENCE P. EBLIN Ohio University, Athens, Ohio
THE value of the concent of
number. somelies in its practical;tility, timescalled valence In particular, a bowledge of oxidation state provides the meam for finding out which elements are oxidized and which elements are reduced in redox reactions. firthemore, it provides a means for balancing redox equations. Most present-day textbook^ carefully note that there is often no apparent relationship between oxidation number and the nature of the bonds by which atoms are united. Consider, for example, the following quotation from Jones' "Inorganic Chemistry" :' ~
~
~~
~~~
~
The structures assumed for the various scidsof phosphorusare:
IT
H -X :.oo :.. HXO?P$O: 7.0.. H
Hypophosphorous acid
Orthophosphorous acid
H H?O~EO: XO ..ox
..
.. ..
.. ..
H
H
: o : : .. o:
Metaphosphorie acid
Pyrophosphoric acid
..
.x
.X
:o: :o: Hrij:+. . 0 0
00.-
:o: .. :o: ..
Hypophosphoric acid
H
:o: if
H
H
sisler, vanderwerf, and ~ ~ ~point i out d the$ ~ by ~ the application of the conventional rules, we arrive at a for carbon in each of the different oxidation following compounds~ -4
CH,
+I
-%
CHZCI
~H,CI,
+I
CHCIS
CCI,
Thus, while the valence of carbon remains constant a t 4, its oxidation state varies all the may from -4 in methane to 4 in carbon tetrachloride.
+
Some of the anomalies of oxidation numbers are thoroughly consistent with the fundamental rule that the numberinofethane any element in the free state is zero.oxidation For example, C2He,the oxidation nnmber of carbon is calculated to be -3. electronic formula for ethane,
Considering the
:o: 7..
H Orthophosphoric acid
It will be noted that in each of these formulas the phosphorus atomis sharing sll five of its outermost electrons, and that in each case, save in that of metaphosphoric acid, the phosphorus atom is surrounded by four other groups. The real valence of phosphorus is, therefore, in each instance five, while its oaardination number is in each case four, save in the case of metaphosphoric aeid in whioh it is three. The various oxidation numbers whioh may he calculated for phosphorus in these compounds are: Hypophosphornus acid (HaPo*) 1 Orthophosphorous soid (&POa) 3 Hypophosphoric acid (HP.Oe) 4 Metaphosphorio acid (HPOa) 5 Pyrophosphoric soid (H4P.07) 5 Orthophosphoric acid (HaPo,) 5 Permonophoaphario acid (HaP06) 7 ~ e r d i ~ h o s ~ h oacid I i c (HIPSOB) 6
+ + + + + + + +
JONES. W. NORTON."Inoreanic Chemistrv." .. The Blskiston ~ o r n ~ a n y~hiladelphia; ,' 1947, pp. 374-5. 1
From the above structures, it is apparent that these numbers are impinary, since they represent no reel valence. Even so, however, they are useful in classifying the compounds of phosphorus ,d reactions, and they are especially useful in balancing oxidation-reduction equations for such reactions in which they participate.
it is apparent that the oxidation number of carbon is numerically equal to the sum of the covalent bonds exclusive of homatomic bonds. The result is that in the alkane series, CnH2,+2,the oxidation number of carbon varies from -4 in methane to -3 in ethane, -Z2/a in propane, -Z1/% in butane, and so on, approaching -2 as a limit. Such "strange" oxidation numbers as -Z2/a are easily explained as averages. Thus, in propane, CsHs, each of the two terminal carbon atoms is bonded to three hydrogen atoms, and the oxidation number of each of these carbon atoms may be said to be -3 in the same sense that the oxidation number of each carbon atom in ethane is -3. Since the middle carbon atom of the propane molecule is bonded to only two hydrogen atoms, the oxidation number of this atom may l i k e wise be regarded as -2. Since the sum of the three or - Z 2 / ~ . oxidation numbers is -8, the average is In this way we "explain" quite satisfactorily the oxidation number -22/3 as an average. Since in all alkanes the oxidation number of carbon is algebraically less than -2, the combustion of any alkane to give carbon dioxide, wherein the oxidation a S ~ H. ~H., ~C. A. ~ VANDERWERP, ~ , AND A. W. DAYIDSON, "General Chemistrv. A Svatematic A-~ ~ r o a c h .The " Macmillau Company, New ~ d r k 1949, , p. 219.
~
2
JOURNAL OF CHEMICAL EDUCATION
222
number of carbon is +4, involves an algebraic increase in the oxidation number of carbon. Since oxidation is defined as an algebraic increase in oxidation number, it is gratifying to be able to say that the combustion of any alkane involves the oxidation of carbon. Even if, as Jones contends, some oxidation numbers are "imaginary," the foregoing discussion shows that it is possible to explain many such oxidation numbers as averages. Even the oxidation number zero for the single carbon atom in CHGl? can be interpreted as an average of +2 and -2. Once it is agreed that the value of the oxidation number concept lies in its utility, then the measure of its value lies in its utilitarian reliability. In particular, the oxidation number concept is well justified if it facilitates the comprehension of redox reactions, or even if it merely facilitates the balancing of redox equations. In the reaction: 2Na&Oa
+ 1,
-
+
N&%Oo 2NaI
viously referred to as having the oxidation number +6. In other words, the sulfite sulfur has been oxidized from +4 to +6: 1 atmm oxidized 2 units
u 1 atom reduced 2 units On the other hand, if one ignores the question of structure and simply gives sulfur the apparent oxidation number +2 in thiosulfates, the sulfite sulfur in this reaction will be said to undergo reduction from +4 to +2:
r
1 atom reduced 2 units 1
I
the iodine is clearly reduced, and from this fact alone it would be inferred that the sulfur is oxidized. By the rules for determining oxidation number, the sulfur is 1 atom L A oxidized, since the oxidation number of sulfur is inoxidized 2 units creasing from +2 in the thiosulfate to +2'/* in the tetrathionate. Furthermore, the assumption that the The elementary sulfur will be ozidized if its valence oxidation number of each sulfur atom increases number changes from 0 to +2, but it will be reduced if unit makes it possible to balance the equation correctly its valence number changes from 0 to -2. by the oxidation number change method. Surely, the A similar problem is posed in connection with the deimportance of the oxidation number concept lies in the composition of the hypothetical tbiosulfuric acid: fact that it "works" so well, even in situations where the oxidation numbers themselves do not seem to "make sense." What "sense" is there in the statement that the oxi- Is the liberated sulfur the product of oxidation or of redation number of sulfur in N&Oa is +2? The only duction? The answer depends on the oxidation state possible meaning in it is that +2 is an average of the from which it is assumed to have come. If it has come "real" oxidation numbers of the two sulfur atoms. In from the oxidation state +2, it has been reduced; if it has come from the oxidation state -2, it has been the words of Sisler and co-authors? oxidized. From the electronic formula and by analogy with the sulfate Earlier in this discussion, it has been made quite ion, this [the valence +2 for sulfur] may be interpreted as an apparent that it is the average oxidation number that is merage between +6 for the central sulfur atom and -2 for the customarily referred to by the term oxidation number. coordinated sulfur atom. There are certain instances, however, in which this is But if this is agreed to, then in the iodine-thiosulfate never done. For example, the formula for ammonium reaction, not only the iodine but part of the sulfur nitrate is usually written NHdN03, and this clearly undergoes reduction: implies the oxidation state -3 for one of the nitrogen atoms and +5 for the other. If it is good chemical 2 atoms reduced 3'/2 units each etiquette to represent sodium thiosulfate as Na&03, perhaps it could be permitted to write the formula for 2 atoms reduced ammonium nitrate NzH403. If we do so, the oxidation number of nitrogen appears to be +l. It cannot be said that this is really "wrong," for + l is simply the average of -3 and +5. If one is permitted to write the equation for the decomposition of ammonium nitrate in the following 2 atoms oxidized 4'/* units each manner: Similarly, in the combination of sulfur with sulfites, it is really the sulfite sulfur that is oxidized. For this atom becomes. in the thiosulfate. the central atom ore- the interesting implication results that the reaction is not a redox reaction! Similarly, the decomposition of
I
i
i
I
APRIL, 1951
ammonium nitrite may be made to appear not a redox reaction:
in cases involving auto-redoxion. Such cases always involve, either as a reactant or as a product, a compound wherein one atom of an element has a positive oxidation number and the other atom a negative oxidation number. The writer is convinced, on the basis of A slightly more complicated example is the reaction the foregoing considerations, that it is advisable to disbetween mercuric chloride and sodium thiosulfate when pense with the concept of average oxidation number in the two compounds are present in solution in the ratio such instances. For it borders on absurdity t o cou1:l. tend, for example, that the liberation of elementary nitrogen by the decomposition of ammonium nitrite is not a case of redoxion. Having made our decision, then As formulated above, this appears to be a redox reac- for the sake of consistency we must avoid such praction, involving the oxidation of one sulfur atom, and the tices as designating the oxidation number of sulfur in reduction of the other one by an equal amount. If, thiosulfates to be +2. Applying this recommendation t o peroxy acids, the however, the "real" oxidation numbers of the individual sulfur atoms are considered, the reaction appears to in- writer h d l y must register his objection t o the oxidavolve no change in oxidation number, whatsoever. Is tion number which Jones assigns t o phosphorus in the this a redox reaction or is it not a redox reaction? Ap- acids having the formulas H3PO6and H4P208. Since parently, if this one is considered to be a redox reac- these compounds contain the peroxide linkage, it is tion, then the decomposition of ammonium nitrate meaningless to deduce an oxidation number for phosmust be judged not to be a redox reaction. It would phorus on the assumption that the peroxide linkage is hardly be consistent to think in terms of average oxida- absent. Since hydrogen peroxide has the formula tion number in the one example and not do so in the HzOz, the oxidation number of oxygen therein is - 1, i. e., the peroxy radical has the oxidation number -2. other. Other examples could be cited that present the same Writing the formula for H3P06 in a pseudo-structural dilemma as is posed in the foregoing cases. All of them manner, HsPO3OZ,we can readily infer an oxidation are reactions of the type which may conveniently be number of +5 for phosphorus: called auto-redoxion. There is some confusion apparent in textbooks over the use of the two terms, auto-redoxion and internal redoxion. For the sake of Similarly, the oxidation number of phosphorus in clarity, the writer suggests the adoption of the follow- H4P208is +5: ing usage: Auto-redoxion should be used to designate +4 + I 0 -IP -1 any redox reaction wherein one atom of an element oxiH, P* 0 s 0 2 dizes,another atom of the same element. Internal redoxion should be used to describe a redox reaction By the same reasoning, the writer concludes that the wherein the atom that is oxidized and the atom that is oxidation number of sulfur in HzSOs and H&Os is +6, reduced are within the same compound; if the two rather than +8 and f7, respectively. This puts him atoms in question are atoms of the same element, we in agreement with Sisler, Vanderwerf, and Davidson, have a case of internal auto-redoxion. who write:4 Examples of auto-redoxion: It should be noted that the oxidation state of sulfur in the
Examples of internal redoxion:
Examples of internal auto-redoxion:
As stated above, the apparent dilemma over which atom is oxidized and which atom is reduced arises only
peroxysulfates, just as in sulfates and pyrosulfates, is +6. The oxidizing action of peroxysulfates invalves no change in the oxidation state of sulfur, but rather a change of the "peroxy" oxygen atoms from -1 to -2.
. It will be noted that Sisler and co-authors use the prefix peroxy- in preference to per- in referring t o acids and salts containing the peroxy group. This should be made standard practice to avoid confusion with perchlorates, permanganates, etc., which are compounds that do not contain the peroxy group.
' Loc. n't., p. 481,
