lyte interface) that makes Al-Soudi's proposals self-consistent). The solution to the claimed problem is very simple and straightforward, and is, in fact, used by many freshman texts. One needs only to apply the following well-established principles: I . Oxidation ocrurs at the anode, and reduction at the cathode. 2. Anivns move toward the anode, and cations toward the rathode. 3. In the external rrrcuit. electrons flow into the cathode, and out of the anode. 4. Electrons flow from a given location to one which is more positive. Some brief comments about these principles (1 refers to principle 1,etc.): I. "Electruchemical ears are always red", as I learned Some 40 yews ago as a tenrhmg assistant for Frank Martin at I'urdue. 2. "Red cats eat cations." Principle 2 dates from Fareday's work on electrolysis (2)-Faraday's Laws-in the 1830's. Combined with item 1and negatiue electrons, it makes anions always negative and cations always positive, a convenience that chemists are unlikely to abandon. 3. "Red cats eat electrons." Aetuallv. ,.this follows from item 2 since nature requires the result of cell operation to be electrically neutral; it ir also to be expected from item I , hecause reduction requires n ruuply of electrons. 4. "What you $re depends on where you stand." In a discharging cell, the point of connection hetween anode and external circuit is negativecompared to the point of connection between the cathode and external circuit (so electrons flow from the anode through the external circuit to the cathode) but is positive compared to the anode-electrolyte interface (so electrons flow from this interface through the anode to the external circuit). AlSoudi'a view of the "origin of electrons" (item 2 in the second paragraph) appears to arise from "standing in one place" when examininggalvanic cells and "standing in adifferent place" when elremining electrolyticcells. 5. Neoer use a rule that sayslimplies that the cathode is always negative. 6. For the novice who knows that electrons have negative charge, items 1-3 may be readily recovered from the ridiculous statement "Red cats eat electrons and cations", i.e. reduction at the cathode, electron and cation flow toward the cathode. Whether the cathode is positive or negative "depends on where you stand viewed from the anode, the cathode is positive in galvanic cells (electrons flow spontaneously from anode to cathode), but is negative in electrolytic cells (electronsare pumped into the cathode and extracted from the anode by an external power source). If Al-Soudi. or others. wish to eliminate "dominatlionl . .bvthe jargon of'oxidationand reduction" (I) and emphasize the role of electrons. she needs to change only Principle 1to something like this: "~lectronsare capLred (by some chemical entitv) a t the cathode, released at the anode". (In this sense, thecathode is always the origin of the electrons (see but Al-Soudi does not use this item 2, second paramaoh), - sense ionsistenily.) If there are U.S. texts andlor British texts that state that the anode is always positive and the cathode always negative, as Al-Soudi claims, the solution is to correct those texts and their authors, not to abandon a century of well-established theory and thoroughly proven practice, which, contrary to Al-Soudi's assertions, are convenient, logical, selfconsistent and completely in accord with the "laws of physics"; in fact, many electrochemical concepts pre-date the related "laws of physics" and were very useful in establishing those laws (2). ~
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Literature CHed 1. Al-Soudi. H.J.Chem. E d w . 1989.66.630. 2. Oespeq R.E.; Spefer, M. Scientific Monthly 1937.45,535-546
Robert D. Freeman Oklahoma State University Stillwater, OK 74078
To the Editor: The Provocative Opinion, "Confusion over ElectrochemicalConventions" by Helen Al-Soudi [1989,66,630] unfortunatelv onlv perpetuates the errors that the author deolores. The diffic&ts ia not so much confusion over conventions as the artual wrong use of terminolops. The error lies in defining the positive electrode as the-anode and the negative electrode as the cathode. As the author points out this is a common statement in American texts. However, the author states with approbation that British texts refer to the cathode ". . . alwivs as the minus electrode". But this statement is also incorrect. The point is that there is not aunique connection between the polarity of an electrode in an electrochemical cell and whether it is an anode or cathode. The terms "anode" and "cathode"on1y have meaning if a net current is passing from the electrode to the solution. If that net current is oositive, the electrode is an anode; if that net current is negative the electrode is a cathode. (These are the ICPAC recommendationsonelectrochemical terminology.) Stated slightly differ. ently, the anode is the electrode at which oxidation occurs. while the cathode is the electrode a t which reduction occurs. It is not appropriate to use the terms, anode and cathode, in connection with an electrochemical cell that is at equilibrium. In anv electrochemical cell. one electrode is nositive and the other electrode is negative (relative to one another). We can determine which is which bv com~arinetherellwith one of known polarity using a poientioiete; circuit or other device. Passina a current one way or the other to the electrode does notchange this polarity. Thus, in the usual zinccopper cell the zinc electrode is always. the negative electrode, (or the negative plate) and the copper electrode is always the positive electrode (or the positive plate). (It is also worth noting that the polarity of the electrode, being a physical property, has nothing to do with whether the electrode is on the right or left side of the cell!) The polarity is in complete accord with the physics of the situation; the negative electrode has more negative charge or less positive charge than the positive electrode. If the zincxopper cell provides power, zinc is oxidized to zinc ion so the zinc electrode (the negative electrode) is the anode; copper ion is reduced to copper at the copper electrode (the positive electrode) which is the cathode. Reversing the current through the cell by imposing a potential difference from an external power source oxidizes the metallic copper so now the positive copper electrode is the anode. At the zinc electrode some reduction reaction occurs (possibly the reduction of zinc ion to metallic zinc, more likely the reduction of either hydrogen ion or water to produce hydrogen). So the negative zinc electrode is now a cathode. The situation is more easily illustrated by a secondary cell such as the lead storaee batterv. (Unfortunatelv. the term "secondary cell" also was misusedin the ~ r o u o c i t i u eOpinion. A secondarv cell is not an electrolvtic cell but is a recharzeable electroch~micalcell.) In the lead storage battery, the lead dioxide electrode is the positive plate and the lead electrode is the negative plate. On discharge, the lead dioxide is reduced to lead sulfate so the positive plate is the cathode. The lead is oxidized t o lead sulfate so the negative plate is the anode. On charging, the polarity remains the same; lead dioxide is positive, lead is negative. But a t the positive plate (which is now the anode) lead sulfate is oxidized to lead dioxide, while at the negative plate (which is now the cathode) the lead sulfate is reduced to metallic lead. Schematically, Electrode Positive Negative
Discharge Cathode Anode
Volume 67
Charge Anode Cathode
Number 11 November 1990
991
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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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)
