Electrochemical Conventlons: Responses to a Provocaflve
Oplnlon To the Edltoc
There are two major deficiencies in Al-Soudi's ( I ) Provocative Opinion: the cited problem and the proposed solution. Her first sentence. "The nresent teachine" of electrochemistry in U S . texts leads to confusion", implies a pervasive problem, but no references are given. Of four "standard" texts that I happened to have on my shelf-Zumdahl, Chang, Brown and LeMay, and Bailar et a1.-not one is guilty of the confusion claimed by Al-Soudi. I t is difficult to comment further since Al-Soudi did not tell us where the confusion exists, other than "in U S . texts". Al-Soudi's proposed solution is completely unacceptable. (1)She proposes that the negative electrode always he labeled as the cathode, i.e., in both galvanic and electrolytic cells. The words cationlanion mean-were created by Faraday and Whewell to mean (2)-"that which moves toward the cathodelanode": hence. her ~ al make the . r o.~ o s would S042- ion an anion in a charging lead storage battery hut a cation in a discharging battery. Furthermore, the definitions
990
Journal of Chemical Education
(2) of anode and cathode require that (positive) electric current flow through the cell from anode to cathode; her proposal violates those basic definitions. (2) Her proposal that ". . . the term cathode.. . should be reserved always for the origin of electrons" is not only incompatible with wellestablished electrochemical theory and practice, it is not even consistent with her proposal that the cathode always be negative. If one accepts her proposal that the negative electrode be labelled cathode, then in a galvanic cell electrons flow from cathode to the external circuit, but in an electrolytic cell electrons flow from the external circuit into the cathode. (The term "origin of electrons" is somewhat ambiguous (see principle 4and comment 4 below), hut I can find no viewpoint (e.g., the external circuit, the electrode-electro-
Editor's Note: Many letters were received in response to Al-
Soudi's Prouocative Opinion. The ones published here represent the earliest submitted and a variety of responses. No further comments on the original article will he considered for publication,
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). ~
~~
~~~
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
