1124
JOURNAL OF CHEMICAL EDUCATION
JUNE,1932
In view of what is stated in the foregoing, it appears to be desirable that the assertion that Paracelsus observed the evolution of hydrogen during the interaction of sulfuric acid with iron or other metal should be permitted to disappear from current literature. LEONARD DOBBIN BLACKSHIELS, SCOTLAND
BALANCING CHEMICAL EQUATIONS DEAREDITOR: EDUCATION have The last few numbers of the JOURNAL OF CHEMICAL contained several letters concerning various methods of balancing oxidation-rkduction equations. Since the inquiry of A. W. S. Endslow U. CHEM.EDUC.,8, 2453 (Dec., 1931)l there have been nine letters on the snbject. Four of these make no attempt to give Mr. Endslow the information he asked for, namely, references to published work on the algebraic methods of balancing equations. Instead they give detailed methods of balancing equations which in most cases have long been known and used. All of the methods are based on the idea of arbitrarily assigning valences or oxidation numbers to individual atoms in the molecules which react and are produced. The equation is then balanced by considering the changes in valence which occur during the reaction. That such assignment is purely arbitrary from the point of view of balancing the equation may be shown by fixing the valence of, say, th$nitrogen and allowing that of the oxygen to change. The following equation results. 0
I
+l +5 -2
loses (2 X 3) +2
+6 -2
1
t i -5
+I -2
+ 8HNOs = 3Cu(NOa)n + ZNO + 4Hs0 I gains (3 X 2) 1
3Cu
One may assign any numbers whatever so long as the sum of all the valences equal zero for a compound or equal the charge if an ion is considered. The methods run into difficulties when such constituents as H,Oz, SCN-, NHpOH, etc., are present. Valences must be assigned to sulfur, carbon, nitrogen, etc., and this is often difficult, a t least for beginners. Jette and LaMer in their so-called "ion-electron" method [J. CHEM. E ~ u c .4, , 1021,11.58 (1927)l have avoided this difficulty by considering the loss or gain of electrons when an oxidation or reduction reaction takes place. Thus, for the above reaction,
+ ., + +
3Cu = 3Cu++ . 3(2e) 2NOa- f 8Hf = 2 N 0 4H20 - 2(3e) 3Cu ZNOs8Hf = 2 N 0 4H.O 3Cu++
+
+
+
oxidation reduction complete
VOL.9, NO. G
CORRESPONDENCE
1125
The advantage of this method is more apparent when the reaction between H2O2and MnOn- is written.
+
5H~0.
+
+
16HC = 2Mn++ 8HaO - 2(5e) ]OH+ 5(2e) 5 8 0 = ~ 50. 2Mn046H+ = 2Mn++ 8 H ~ 0 502
2MnO'-
+
+
+
+
+
reduction oxidation complete
Nothing is said about the valence of any single atom yet it is obvious that when the first reaction takes place, ten electrons are gained and that the ten are lost when the second reaction occurs. The advisability of teaching any of these more complicated methods to elementary students who do not expect to major in chemistry may be questioned. However, the students who do expect to go on might well be taught a method which will find application in all their later work. Those who object that the newer methods are too difficult are cited to the Editorial, "Of Cabbages and Kings," which appeared in the very same number (December, 1931) that contained the letter starting this discussion. Yours truly, GRANTWERNIMONT PURDUEUNIVERSITY LAFAYETTB. INDIANA
* * * *
DEAREDITOR : Regarding the discussion of balancing chemical equations in recent issues EDUCATION, I hould like to make some comof the JOURNAL OF CHEMICAL ments for publication which will amplify my stafkment on this subject in the May issue. In any oxidation-redudion reaction, the important point is that oxidation and reduction must occur together and to the same extent. As oxidation and redudion is the loss and gain of electrons, respectively, then the numbers of electrons lost (oxidation) must be equal to those gained (reduction). By writing the oxidation and reduction reactions separately and multiplying each so that the electrons lost and gained are equal, the final equation is obtained by adding the two separate equations. This method, the ion-electron method, is superior to the valence-change method not only because it is consistent with the f a d that ions are in solution, but i t also gives information as t o the acidity of the solution. For instance, the reaction between Bi+++ and HSn02-, which is written and balanced by the ion-electron method as follows:
+
2 X Bi+++ 3c e Bi 280 3 X HSnOn- 3 0 H - - 2e e SnOs-2Bi+++ 3HSnO.c SOH-S2Bi 6H90 3SnOsc-
+
+
+
+
+ +
not only shows what ions are involved and how much, but also that OHis necessary for the reaction. But if the valence-change method is used,
