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,
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JOURNAL OF CHEMICAL EDUCATION
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-
JUNE, 1932
the only information gained is that Bi+++ Bi and Sn++ Sn++++. If one is to follow the latter method, it should be expected that adding Sn++ as SnClz to a bismuth solution (Bi+++) would give the same result, viz., the reduction of Bi+++ to Bi. However, if the experiment is carried out, there is no reaction. This result can be explained on the basis of too high H + or too low OH-. If OH- is added in the form of NaOH, the reaction proceeds as expected. Thus the question of acidity of the solution, which often is very important in oxidation-reduction reactions, is answered when the equation is balanced by the ion-electron method, but not by the valencechange. Very truly yours,
LEOLEHRMAN THE COLLEGE OF
THE
OF N E W Y O R K
CITY
