I am glad that Professor Lehmlan voiced his objection to my ignorance with respect to the English language for it gives me opportunity to emphasize once more that there is occasional danger of precipitation of Ba++ with the sulfides of the Cu-Sn group. ROCHESTER. M ~ s s ~ c ~ u s ~ r r s WILL- T. HALL Nan-stoichiometric Equations
To the Editor: Under this title, Otto F. Steinbach' points out that an equation may balance algebraically and yet not represent the truth. This fact is one that probably has bothered every student of chemistry a t one time or another, and every teacher frequently has students ask why a certain equation is marked wrong in spite of the fact that i t balances. Chemical equations are commonly written for two purposes: (1) to show the probable reaction products, and (2) to show the quantities of materials that enter into reaction. If an equation does not fulfill the latter requirement it can be termed "nonstoichiometric." The reactions that are used in quantitative analysis must be stoichiometric or they have no value. With most reactions of organic chemistry the equations are idealistic and nonstoichiometric, but nevertheless have value. Recognizing this fact, it is the common practice of all teachers of organic chemistry to pay little attention, for example, to balancing equations of oxidation and reduction. Thus if it is a matter of oxidizing an alcohol to an aldehyde or an acid with permanganate, the instructor merely writes the graphic formula of the alcohol and shows that the element oxygen, or the radical OH, can replace hydrogen in the formula to give the desired product. This practice is, of course, related to the old dualistic conception of oxidation by permanganate which was based upon the fact that the acid anhydride, Mn20,, was unstable and easily lost oxygen. The formula of permanganate was commonly written, in the good old days, K20.Mnz0, rather than KMnO.. Today it is common practice to consider the oxidation by permanganate as related to the reduction of manganese from a valence of seven to a valence of two (or four in some cases). From the valence-change standpoint, few students of organic chemistry can balance equations for the oxidation of organic compounds, and there isn't much use in doing it unless the reaction is stoichiometric. I n such cases the valence-change method is easy to apply if one is willing to admit-as a few organic chemists are-that the valence bonds of the carbon atom may be either positive or negative, as illustrated by the compounds CCla and CHI. The non-stoichiometric equations cited by Dr. Steinbach are all equations of oxidation and reduction involving hydrogen peroxide, sodium hypobromite, chloric acid, potassium hypochlorite, and potas. -
sium chlorate. These compounds, for the most part, are not very stable and, as he points out, may give rise to more than one reduction product. In the case of hydrogen peroxide, however, the statement that the reaction between it and potassium permanganate is a good example of variable coefficients is absolutely wrong and cannot pass unchallenged. The reaction between KMuO, and HzOz in the presence of H2S04 has been known to be stoichiometric for many, many years and was studied by C. F. Schonbein (1799-1868) who found that the reaction as expressed in our present nomenclature, is In accordance with Schonbein's theory, which is not bad even today, half of the oxygen evolved comes from the KMn04 and half from the HzOZ. All the other equations which Dr. Steinbach suggests are obviously incorrect. However, since he is bothered by the fact that he can write a t least nine different equations to express such a reaction and many college freshmen and high-school students are troubled in the same way, just a word or two may be said in explanation. Hydrogen peroxide in dilute acidic solutions is quite stable. I t slowly decomposes, however, into HzO and 0,. During the course of a titration of a solution of hydrogen peroxide with permanganate there is no appreciable decomposition of this kind and the volume of oxygen evolved as compared with the volume of permanganate reduced is perfectly definite. The hydrogen peroxide content of an aqueous solution can be determined accurately either by measuring the volume of permanganate solution required to oxidize the peroxide or the volume of the oxygen evolved. In terms of the old ideas of oxidation, the correct equation may be written 5Hz0z 50 = 5H10 50s
+
+
and the equivalent weight of HzOz(or the weight equal to an oxygen atom of oxidizer) is the molecular weight divided by two. I can write equations such as these:
but they are wrong and have no value because they do not correspond to the equivalent weight of hydrogen peroxide as has been determined experimentally. In other words, the eight incorrect equations written by Dr. Steinbach do not represent the empirical value of H202as a reducing agent; it is equivalent in reducing power to two atoms of hydrogen. This has been proved experimentally, but the astounding fact is that by adding oxygen to the water molecule we get H202which will, (1) by adding a little sodium hydroxide and heating, decompose into water and oxygen, (2) act as an efficient oxidizing agent, or (3) act as an efficient stoichion.etric reducing agent with permanganate and with oxides such as Rhoz.
The reducing action of hydrogen peroxide can be explained in several ways. We can, for example, assume that one atom of oxygen is united with another atom of oxygen so that the structure may be represented thus: H
4
H
a
I
If we attempt to recognize positive and negative ends of each*valence bond, we find that one atom of 0 is negative toward the H atom but positive toward the other 0 atom. In other words, the extra atom of 0 of H20zis in a sense a neutral atom, and the two atoms of 0 hold two atoms of H just as one atom of 0 does in water. Now if the Mn of permanganate, in ping from the valence of +7 to +2 momentarily oxidizes this neutral 0 to Off, the latter can combine with the 0 of KMn04 which is normal and has a valence of -2. Really, all this means is that the 02loses two electrons and becomes two atoms of 0, each with a valence of -2. In the case of oxidation of H z O ~ ~ C r it~ isO harder to get conditions whereby the normal reaction
is stoichiometric and has been used successfully for determining very accurately iodate, iodide, or acidity. Practice in writing equations of oxidation-reduction are very useful in teaching beginners, hut the practice of asking students to try to balance equations without knowing what the products of the reaction are likely to be should be discountenanced. The useful reactions of inorganic chemistry are almost always stoichiometric, and it is of fundamental importance that the student should know the most likely equivalents of the reactants. WELULMT. HALL ROC KBSTBR, MASwCmSBmS
To the Editor:
In the February issue of the 5, CIIBM,EDUc.there is an article entitled "Nou-stoichiometric equations" by Otto F. Steinbach. Though he seems not to realize i t the author brings up the old, old error involved in adding equations for concurrent reactions. See, for example, Smith's "Inorganic Chemistry," 3rd ed., page 485 (The Century Company, New York, ~ 1917). The rulethat should be followed is: equations for consecutive reactions may be added; equations for concurrent reactions may not be added. K,CrsO, + 3Hx& + 5HIS0, = 2KHS0, + 7H20 + 3 0 ~ Actually an infinite number of "equations" that are balanced but do not express actual stoichiometric will be stoichiometric. The reaction relations can be "derived" by lumping together the &C& 5 & 0 ~ 5&S01 = 2KHSO4 CrdS03r 9&0 for concUTTentreactions. one need only 40% multiply the coefficients in the equation for one of the is given as the normal one by Dr. Steinbad, but this reactions by n before adding i t to the other. Or the is obviously incorrect as this makes 6 equivalents of the two or more equationscan be in of K2Crp0,equal to 5Ha02. multiplied by diierent numbers. Taking, as the The reaction between KCIOl and HC1 is not likely author has without realizing it, concurrent chemical to be stoichiometric but will vary with different con- changes which have a reactant in (H&), centrations and at different temperatures. Perhaps and adding their equations the simplest reaction will be
+
+
ZCIOa-
+
+
+
+ 2C1- + 4H+ = 2C10? + C4 + 2H20
in which the C1+5 goes to Cl+' and the C1- goes to ClO. A reaction such as 4C10s-
2KMnO.
ZKHSO,.
and n timer 2H102
+ 12U- + 16H+ = 2CIOx + 7CL + 8Hn0
which is the one Dr. Steinbach prefers, is complicated by the fact that four different valences of C1 are involved, and half the C1 of the &lorate is reduced to CIOz while the remainder goes to Clz. This is pretty bad, but Dr. Steinbach's explanation that "chloride is oxidized to chlorine while the chlorine of the chlorate is reduced to chloride and chlorine dioxide" is even more in such a case as this, I know of no way of tagging the chlorine atoms and finding out just where d the Cl, formed originated. I t is known that in quantitative analysis the reaction CIO1-
+ 5C1- + 6H+ = 3CL + 3 8 0
can he made to take place although this possibility did not occur to Dr. Steinhach. In iodimetry the equation 10.-
+ 51- + 6Hf
=
3L
+ 3H.O
+ 5H20, + 4Hd01-r
-
+ 2MnSOa + 8H10 + 50r
n times 2H.O
+ n times 0*
(1)
(2)
we can obtain as many equations as we wish. We can get still more by adding rn times to times (2) where and rn are numbers, Of course these equations are false, as they do not state the actual stoichiometric relations between the substances iuvolved. In some cases, as in Dr. Steinbach's fifth set of equations, there is no constant relation between the quantities of substances expressed by his equations. The ratio of the quantities will depend tions. Dr. Steinbach's second set of equations, in which by the way he chooses an incorrect equation as the correct one, is obtained by adding times KaCr*Or
+ 3Ha0a
f
5HnS04+
+
~ K H S O , Cr2(S03.
to n times 2H,02
-
ZH20
+ O1
+ 7H.O + 3OZ
