The Gram Electron L. F. AUDRIETH and M. J. COPLEY University of Illinois, Urbano, Illinois
T
HE terms gram atom and gram molecule are introduced early in the course in general chemistry to emphasize above all the quantitative meaning of chemical symbols, formulas, and equations. The term gram ion may also he introduced later, possibly in connection with a discussion of partial equations, such as (1) to (4), which are used to emphasize the meaning of electrovalence and to demonstrate the ionic nature of compounds formed when the indicated electron transfers (gains or losses) occur.
Such half equations may also be given a quantitative meaning, even though the respective processes are not capable of realization by themselves under ordinary circumstances. Thus, one gram atom of potassium is converted into a gram ion of potassium in a process involving the loss of approximately 6.06 X loZSelectrons. I t is proposed that this number and quantity (mass in grams) of electrons be designated the gram electron. The question may properly be raised: Is the introduction of this new term justifiable? Can it be used to simplify the presentation of concepts whose relationships are not ordinarily evident to the beginning student? Actual and successful teaching experience, particularly with students who have had a high-school course in chemistry, has prompted us to submit this idea through the JOURNAL OF CHEMICAL EDUCATION to other teachers of chemistry.
of electrical units. Thus, the ampere is defined as that current which in one second deposits 0.001118 gram of silver from a silver nitrate solution. The coulomb is that quantity of electricity transferred by one ampere in one second. The coulomb may therefore also be defined as that amount of electricity required to deposit 0.001118 gram of silver from a silver nitrate solution. The number of coulombs which are needed to deposit 107.88 grams of silver (one gram atom) is equal to on I", .00 7n.i
or 96,494 coulombs. This number of cou0.001118 lombs has been found by experience to be the smallest amount of electricity which will produce an electrochemical change in a gram atom or gram molecule of any chemical species. I t has become convenient to refer to this quantity of electricity as the "faraday," hut it can just as properly be called a "gram atom of electricity," since the atom of electricity is the electron, or more brietly the "gram electron" as we propose. The gram electron and the faraday are therefore one and the same unit and quantity. Many students experience considerable difficulty in visualizing what is meant by "quantity" of electricity. In emphasizing the fact that it is matter which is transferred this uncertainty disappears. If equations such as (5, 6) are interpreted as we propose, the meaning of quantity of electricity becomes perfectly obvious. Ag+ one gram ion
+ one gram electron
'/,CU++ one-half gram ion
e-
+
e-
+Ag one gram atom (107.88 g.)
(5)
+
one gram electron
I/. Cu one-half gram atom (31.79 g.)
(6)
The gram electron concept is furthermore useful in interpreting reactions which occur at the poles of elecWHAT IS THE GRAM ELECTRON? trolytic and voltaic cells. Electronic equations such The use of the term gram electron emphasizes the as those given above are not only qualitative, but also transfer of ponderable matter in all oxidation-reduction quantitative expressions in that they relate electroreactions, whether chemical or electrochemical. The chemical change to the quantity of electricity and indigram electron represents approximately 0.00055 gram cate the number of gram electrons involved per gram of matter-a not insignificant quantity. This figure is atom, gram ion, or gram molecule. calculable from the mass of the electron (in the rest THE GRAM EQUIVALENT state), 9 X 10WZ8g., multiplied by Avogadro's number, 6.06 X loza,or from the relative (atomic) weight of the The equivalent weight, or the gram equivalent, is electron compared with that of hydrogen. Just as the that quantity of substance whose formation or subsegram atom and the gram molecule represent definite quent reaction involves the loss, gain, transfer, or quantities (masses) and numbers (6.06 X loz3)of atoms sharing of one gram electron.' This is certainly a and molecules, just so does the gram electron represent much more fundamental definition than "that quana related quantity (mass) and the same number of elec- tity of matter which reacts with, contains, or displaces trons. one gram of hydrogen, or e i~ h tgrams of oxygen, or . . ~
THE GRAM ELECTRON AND THE PARADAY OR ELECTRICITY
The relationship to and the identity of the gram electron with the faraday can best be demonstrated by a consideration of the accepted definitions of a number
It has been pointed out to the authors that this definition is a reversal of thelogical derivdtiun of ihc gram electron concept. The gram rlcctron should really he defined in terms of the gram couivalent w e i ~ h t .since the faradav was first rharnctcrlrrd as tdat quantity 'br k~ectricitvwhich will discharae 107.88 grams (one &am eduivalent) of silver from solution
35.46 grams of chlorinen--or any other convenient equivalent quantity of any of the other 92 elements. B$ using t i e gra& electron definition i t makes little difference whether f i: equivalent weight of an oxidizing or reducing agent, of a molecule or of an atom is desired. The equivalent weight of potassium sulfate, a n ionic compound, is a weight in grams equal to the molecular weight divided by two, since the transfer of two gram electrons from two gram atoms of potassium was involved in the formation of one formula weight of the compound. The equivalent weight of hydrogen chloride is equal t o the molecular weight taken in grams, since each atomic species has contributed one gram electron in the formation of one mol of the covalent compound. The equivalent weight of zinc as a reducing agent is the atomic weight divided by two, since one gram atom of zinc may lose two gram electrons (equation 7). (7,
The equivalent weight of permanganate as an oxidizing agent in acid solution is equal to the molecular weight divided by five since one mol of permanganate
takes up five gram electrons as indicated by the followina half reaction.
The use of this new term has justified itself in our own teaching experience. We have found i t especially helpful where students already have had enough of a background in chemistry, so that the semester's work can be started with a thorough discussion of atomic structure. The gram electron is then introduced along with a discussion of the chemical significance of our modern concepts of atomic structure. Formation of electrovalent compounds by transfer of electrons is explained by use of partial (electronic) equations. This gives an opportunity t o bring in oxidation and reduction, both chemical and electrochemical, from a quantitative point of view by using the gram electron concept. The redefinition of equivalent weight follows in lo~ical . sequence. The authors take oleasure in acknowledginc their indebtedness to Professor T. E. Phipps of the University of Illinois and Professor P. W. Selwood of Northwestern University for their valuable suggestions.
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