All of this is quite consistent with our physical picture of the direction of the current flow both in the cell and in the external circuit.
One concludes that there are no shortcuts in teachine the electrochemistry of galvanic cells; the processes in eacg cell must be treated holistically.
Gilbert W. Castellan DBpt. of Chemistry & Biochemistry Universily of Maryland at College Park College Park. MD 20742
A. A. Woolf Bristoi Polytechnic Bristoi BS 16 1QY United Kingdom
To the Editor:
A recent article [1989,66,630] intended to remove confusion succeeds in compounding it by various misstatements. We are asked to consider only electron movements, the "physics", rather than the observed redox reactions, the "chemistry", and then by using the IUPAC conventions all confusion is supposed t o vanish. However the choice of convention is secondarv in mv. ooin. ion since any convention correctly applied must be consistent with observation. The main problem resides in giving a sign to an electrode or an electrode potential. This will depend on (1)the counter electrode in the cell, (2) whether the cell is discharging (self-driven) or charging (driven) and (3) the conventions used. O~erationallythe historical convention of current flowing away from positive terminal, the opposite to the real electron flow, gives the sign to an electrode but it is the "chemistry" which decides which electrode will receive this flow. Oxidation or de-electronation always occurs at an anode as opposed to reduction or electronation at a cathode; the polarity depends on how the cell is -- used. This can be clarified by considering the Daniel cell with the IUPAC conventions. In a self-driven cell the less noble metal (i.e., the one with the more negative relative reduction potential) will dissolve. ~
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To the Editoc
I have appreciated the "Provocative Opinion" of H. AlSoudi [1989,66,630] very much. Inmy high school chemistry textbook (La Chimica ragionata (Chemistry Explained) Istituto Geografico De Agostini Editions) the same convention is adopted: the cathode is the negative electrode in both primary and secondary cells. I, too, propose to refer to the (simplifying) convention of physics. It is sufficient to show what part of the circuit is the electric generator. During electrolysis, the cathode is connected to the (negative) source dfe~ectronsfrom thegenerator. A galvanic cell,on the other side, is itselfthegeneraror,and thus theelectrode from which electrons flow out must be regarded as the cathode (negative electrode) Glovannl Fochl Scuoia Normaie Supariore di Pisa P a dei Cavaiieri P i s , itaiy
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, + reduction at Cu cathode
e Zn I
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Cu2+
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Znz+
-
oxidation at Zn anode This spontaneous reaction and the associated electron flow can be reversed of driven bv another batterv or chmeer - in opposition. reduction at Zn cathode a,?
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Journal of Chemical Education
I read with interest Helen Al-Soudi's orooosal to remove confusion from electrochemical conventi%~ can't disagree that "electrons will flow from where they are to where they are not", but the chemical potential of the electrons, not their "richness" determines direction of flow. One question remains-will they flow through the solution or the wire? The answer to this question also affects the convention of which is the cathode.
Zne
oxidation at Cu anode The anode of the spontaneous cell becomes the cathode of the driven cell and vice versa. The IUPAC convention is advantageous in that the sign of the reduction potential coincides with the observed electrode polarity. Furthermore, because the latter does not depend on the direction of current flow the electrode.potential is also sign invariant. The effect of divorcing the "physics" from the "chemistrv" shows un in the statement that zinc is the electron source in the primary (self-driven) cell. If the cell is viewed as a whole. then zinc is actuallv a sink or rece~taclereceivine electrons & the metal dissolv&. These elections are utilizei for electronation a t the comer interface. which should rooerly be regarded as the s&ce. similarly, in the driven cell zinc becomes the source for reduction, albeit, with the electrons originating in the external ~ o w e supply. r The statement that "the term cathode should be reserved always for the origin of electrons" is erroneous; rather, i t is the-electrode a t which electronation occurs wherever electrons originate.
Q92
To the Editor:
Llnda M. Sweetlng Towson State University Towson. MD 21204
To the Editoc
In relation to the article bv Al-Soudi. "Confusion over Electrochemical conventions" ,1989, 66, 81, the solution may be to use the etymological meaning of anode = Way up cathode = Way down in any cell "anode" is the way in which the electrons flows out the cell, and "cathode" is the way in which the electrons came into the cell. M. and MaT. Martln-Shnchez E. U. Pabio Montes no SBntis.maTon dad 37 26010 MADR D (Spa n)
