edited by: DAVIO A. PHILLIPS Wabash Coiiege Crawfordsville. IN 47933 PRUDENCE PHILLIPS Crawford~villeHigh School Crawfordsville,IN 47933
Electrochemistry
Goals in Teaching Electrochemistry
Ronald I. Perkins Greenwich High School Greenwich,CT 06830 Electrochemistry is one of the most importarit topics in my chemistry course. I t easily relates to attent.ion-getting activities and visually exciting demonstrations; it explains manv" uhenomena observed in the real world: an d electro. chemistry can serve to unify an introductory course because i t is related to almost every other topic in chemistry.
J. T. Maloy Seton Hall University South Orange, NJ 07079 I have started more than one lecture in electrochemistry by asking the class to arrange the known kinds of chemical reactions in order of increasing complexity. After a few minutes of weighing the intricacies of double displacement against of those of backside attack, I provide them with my own list:
Electrochemlstry Excltes I t is no accident that many early chemists such as Davy, Berzelius, and Faraday chose to investigate electrochemical phenomena. Many students relive some of the old masters' feelings of excitement when they watch bubbles of gas form in a two to one ratio around two inert electrodes immersed in a solution of acid; or by observing orange copper deposit on a graphite electrode immersed in a blue copper sulfate solution and then disappear when the battery connections are reversed. One does not need t o understand much chemistry to appreciate the beauty of vanadium or manganese solutions undergoing oxidation numher changes. Few will ever forget the electrochemical demonstrations-the grandest in the chemist's arsenal-of the exploding hydrogen and oxy~. gen balloon or the thermite reaction. One soon concludes that most of our favorite chemical changes are examples of oxidation-reduction reactions. Electrochemistw. Exolalns . Interest in the study of chemistry can he stimulated by color changes and explosions that are often remembered for along time. The power of chemistry, however, comes with its beautiful theories and models that help us first explain and then predict real world phenomena. I can think of no more useful topic to help us understand the world in which we live than thembject of electrochemistry. The study of reduction potentials answers questions such as why only a few metals, such as gold, silver, and copper, exist free in nature. The study of electrochemical cells explains the purpose of a salt bridge, and how to change the concentration of a solution in an electrochemical cell to increase the measured volta~e.Some are surprised that two dissimilar metals stuck into a grapefruit ran serve as a batterv. Althuuah rnanv f ~ n dthese endeavors intellectuallv satisfying, others are stimulated by mure practical examples. All itudenu seem to be interested in rheinistrs as applied to more practical "real-world" problems. The topic ofelectrochemistry is especially good a t answering practical questions. Why does one avoid connecting two dissimilar metals, whether in a work of a r t that will he exposed to the weather, or in household plumbing? Why do cities have building codes to define how a water pipe should be connected to the city system? Which is better, to use iron nails in a copper
1) electron transfer 2) proton transfer, and 3) other. Once the jeering subsides, I inform the class that electrochemistry should he important to them because i t deals with the simplest kind of chemical change: electron transfer across an interface. The simplicity of the kind of chemical chnnee " that is considered in electrochemistrv allows us to develop a high level of sophistication in our understanding of the nhvsical changes that accomuanv it. This is noteworthy b;ca;se physical changes accknpany every detected chemical reaction. Thus, while heing in itself important, the study of electrochemistrv also provides us with a unique o~portunitvto investigate the-interplay between chemical-and physical ohenomena a t a fundamental level. I contend that this interplay determines every facet of our temporal existence. This conviction ~rovidesthe basis for my primary goal when I lecture on electrochemistry. I look a t electrochemistry in the same way the Music Man viewed the game of billiards: electrochemistry allows one to develop a "keen eye" and "horse sense" about the necessary interplay between chemistry and physics. With this general goal in mind, let us consider some specific examples. Theory of Matter Electrochemistry is essential to the way that both chemists and physicists think about matter. Through bulk coulometry (chemistry) one may establish the relationship between the mass of an electrodeposited element and the charre necessarv to deoosit it. Because the charge of an eleciron is now known (thanks to Millikan's physics experiment), it is possible to count the numher of electrons passed in this deposition and, if the redox reaction is known, to use this numher t o count the numher of atoms contained within (Continued on page 1019, Col. 2 )
I In planning a chemistty course, some of the most important decisions a teacher must make involve me selection of material to be covered and the time to be devoted to each topic. For each column in tiis series, a high school and a college teacher have been invited to discuss why they feel a particular topic is important and how it
contributes to ma students' understanding of chemislry. (Continued onpoge I019Col. I)
1018
Journal of Chemical Education
(Continuedfrornpoge 1018, Col. 1)
(Continuedfrom page 1018, Col. 2)
roof or copper nails in an iron roof? How does one reclaim the silver from photographic fixer? Observing a 500-mL heaker full of fluffy silver recovered from 45 gallons of photographic fixer seems to impress all students of the relevancy of electrochemistry.
the known mass. This permits either the experimental determination of the atomic weight of the element if the Avogadro number is known, or the experimental evaluation of the Avogadro number if an element having a defined atomic weight is electrodeposited.
Electrochemistry Connects Electrochemistry is so pervasive that I use i t t o tie together the various topics in my course. At the beginning of the year, we study stoichiometry using the electrolysis of water and the whiskers of silver growing on copper wire immersed in a silver nitrate solution. We verify the gas laws by collecting hydrogen in a eudiometer tube from the oxidation-reduction reaction of magnesium and acid. When studying atomic structure, we apply Faraday's laws to an electrolysis experiment so that students can determine the charge on a single electron. This helps answer the question of how one can measure extremely small quantities. Bonding is used to explain N- and P-type semiconductors. A t this time, students build and study the electrochemistry of simple diode and transistor circuits. Dead batteries serve as examples of systems a t equilibrium. Equilibrium constants can he determined from a table of reduction potentials using the Nernst equation. In thermodynamics, the Nernst equation gives us a way of determining free energy for a system. By the time the electrochemistry chapter is reached, my students are already familiar with terms suchas anode, cathode, volts, amps, coulombs, oxidation, and reduction. They have connected wires for simple circuits and have observed a number of electrolysis reactions. In addition they have ohserved the year-long project that we carry out in the hack of the room, electroplating silver from used fixer. Instead of being taught as an isolated topic near the end of the course, electrochemistry is used to unify the year's work. Admittedly, in some areas the topic of electrochemistry is fuzzy. Most electrochemists readily admit that they do not understand why certain ingredients make better electroplating solutions than others. I t is often difficult to design experiments that agree with theory. If I were designing the universe, I would probably avoid interactions that cause electrodes to become polarized, and1 would reduce the number of possible half-reactions. However, i t is this complexity that makes electrochemistry such an interesting suhject to study. There is still a hit of art involved.
Slgn Conventlons Two electrochemical sign conventions exist: one is thermodvnamic (chemistrv) - . the other is electrostatic (~hvsics). ~ h e ; m o d ~ n a k i cconvention states that spontane& electrochemical reactions exhibit a positive AE. Electrostatic convention states that the charge carriers in a metallic conductor flow toward the vositive electrode in a spontaneous electrochemical process: Unfortunately, we have developed a system that uses the same nomenclature to describe different properties of matter. While the correct use of either reduction or oxidation potentials permits the determination of thermodynamic spontaneity, the use of reduction potentials fortuitously permits the correct prediction of electrostatic nolaritv from the eiven thermodvnamic s i m convention. A thorough understanding of these prinriples allows I,oth chemists and . ~ h"w i c i s rtodistinguish s between thermodynamic and electrostatic sign conventions in chemistry.
Conclusion The suhject of electrochemistry can he both visually stimulating and intellectually satisfying. My students and I never seem to tire of watching the electrolysis of KI solution, and they enjoy figuring out the electrolysis products of a randomly chosen solution. Many leave my chemistry course with an electroplated leaf or ornament as a permanent souvenir. Because the topic of electrochemistry can be related to so many other areas, i t seems to lend itself to the effective spiral approach to learning, and i t serves well to unify the year's work. Ronald Perkins is Senior Teacher of an eight-member chemistry staff at Greenwich High School, Greenwich, CT. He teaches first-year honors chemistly and second-year advanced-placement chemistry. He oraduated with a~BA and ~ ~ MST dearee ham the UniversiN of New Hampshire de . s a ShellMernt Fellow,a 1982DreyfusMasterTeacher. 1983 Nalmnao Plesiaentia Awardee. 1984 honheasl reglana rec pie m 01 me MCA Catalyst Auara, and a 1985 recop en1 of the hortheast ACS regional award. The past year while on sabbatical, he sewed as the Assistant Director of me institute far Chemical Education at the University of Wisconsin-Madison. He is the 1986 Recipient of the James Bryam Conant Award. ~
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Chemlcal Thermodynamics Electrochemistry is crucial to our understanding of energy conversion devices. For example, the heat released due to e n t h a l ~ vchanee (AH) during an exothermic chemical reaction may he piyiicaliy converted to mechanical energy by means of a heat engine, a device that is subject to Carnot efficiencylimitations. A portion (TAS) of this energy is associated onlv with entrovv changes which take place during the chemiial reaction. when these changes a~sdresultin the liberation of enerev (when A S is necative), the entropic portion of the exothermic AH may be released only as heat; thus, that portion of Alfthat is released by entropy change is inevitably suhject to thermal efficiency limitations when it is converted to mechanical energy. In principle, i t is possible to convert the remainder to mechanical energy through the use of an electric motor, a device not suhject to Carnot efficiency limitations. This energy (which might even he more than the energy available as heat if AS is positive) may he determined directlv bv measuring the voltage of an electrochemical cell in which the reaction is takinhplace. If a high impedance voltmeter is used for this measurement (so that the cell current approaches zero), the measured energy represents the maximum that may he converted to mechanical energy without being subject to thermal efficiency limitations. Thus, this energy is known as the free energy (AG), and the cell voltage " that is used to determine it is known as the electrochemical potential. Since the equilibrium constant mav be determined if the free enerw ... of a chemical reaction is known, the messuremrnt of ~I~rtruchemical potential also provides a basis For the itudy ot'chemiral equilibrium. (Continued on page 1020) Jmph 1. Maioy is an Associate Professw of Chemishy at Seton Hall University, a seif-employed consultant, a Divisional Editor of the Journal of the Eiectrochemicai Society and the Prwident of the Socielv for Electroanaidicai Chemistw. .ISEACI. ~. Afler araduatincl" as a malnemalics major from St. Vlncenl College n 1961. he became rntemsted in a career n cnemcal ea~calianwn le teacn ng a ChEM S t 4 co.rse at Ramsay (PA) Hlgh School in 1963. da entered grad^. ate school as a chemistry major at the University of Texas in 1965 as a participant in the NSF Academic Year Institute Program. Upon receiving his doctorate in 1970, he joined the facuity at West Virginia University where he was selected as a Danfonh Associate in 1978.In 1979 he accepted his present appointment at Seton Hail.
Volume 62 Number 11 November 1965
1019
Transport Phenomena
The term "mass transport" is used to describe those physical changes which take place when matter is transported from one location to another. Mass transport may occur by any of three modes: diffusion (spontaneous movement of particulate matter from where it is to where it is not. as in Brownian motion), migration (movement of charged particles in an electromaanetic field). . . and convection (movement caused by stirring). In a localized chemical reaction (ex.. the comhustion of methane in a Bunsen hurner), the rateof the chemical reaction may he determined hy the rate a t which the reactants are transported to the reaction zone. Since electrochemical reactions are all localized in the interfacial region where electrode meets electrolytic solution, mass transport may he expected to influence the rate of the electrochemical reaction: this chemical rate is directlv measured hv the instantaneous current flowing a t the erectrode. By ;nderstanding how transport phenomena can influence the current. one may gain an insight to current-potential relationships that is invaluable in electroanalvsis. Moreover. since all chemical reactions are localized, we are reminded by our studies of transport phenomena that long-range physical changes (other times and places) may exert an influence on any observed chemical change. ~
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lnterfaclal Phenomena
In electrochemistry, charge is transferred and chemical change takes place a t the electrodelelectrolyte interface.
1020
Journal of Chemical Education
This is an ideal system for experimentation, since events taking place a t the interface may be controlled by external variations in the electrode potential. As in any chemical reaction, the laws of chemical kinetics govern the rate a t which these events take place, hut, because of the interfacial nature of electrochemistry, the pertinent laws are those of heterogeneous rather than of homogeneous kinetics (to which we are more accustomed). Through variations in electrode potential (physics), one may alter the rate (chemistry) of a heterogeneous process while simultaneously monitoring this rate by measuring the current. Thus, through this experiment, one may come to a fundamental understanding of interfacial phenomena, an understanding essential to a full appreciation of the nature of chemical change. Whether we consider the assimilation of glucose in intercellular membranes or the comhustion of coal in a power station, i t is clear that much (if not most) of the chemistry that occurs in tL universe occurs interfacially. In summary, the study of electrochemistry provides us with fundamental information about the nature of matter and the energy relationships that hold when matter is transformed from one form to another. Through the correct application of its conventions, electrochemistry provides us with a rational means for predicting the energetic outcome of these transformations. Most importantly, however, the study of electrochemistry can provide us with an appreciation of the fact that transport phenomena and interfacial kinetics determine the rate a t which matter reaches its thermodynamic conclusion; the study of electrochemistry thereby provides solace for the human condition.
