crppliccrtion/ and analogie~ Electrochemical Errors Ron DeLorenzo Middle Georgia College Cochran, GA 31014
When students question instructors about the value of chemistry, there are several approaches (I). However, it is not always necessary for instructors to wait for the question, and many instructors allude to applications throughout their lectures. Another approach is to point out to students some of the unexpected consequences that have resulted from the ignorance of basic chemical principles. This paper provides some examples that can be used in teaching electrochemistry. Much of chemistry is involved with the sharing or transferrine of electrons between atoms. and the electrochethical seriesys a useful device that correlaks this chemical behavior. Metals listed a t the too of the electrochemical series are verv good reducing agents,'and metals listed a t the bottom of thg series are very good oxidizing agents. When a free metal listed a t the top of the series makes contact with an ion of a metal listed a t the bottom of the series, a redox reaction can occur spontaneously, i.e., the free metalmay lose one or more electrons to the metal ion. For example, Zn
-
+ CuZ+
Cu + Zn2+
Caution should be exercised a t this point. Students should be reminded that not all theoretically spontaneous reactions occur quickly, and under some conditions, theoretically spontaneous reactions may not even take place. For example, hydrogen and oxvgen mav s~ontaneouslvreact to form water, b"t i n a mixturelhe tw" (without a catalyst or spark) may coexist a long time without rhe noticeable production of water. Also, when two disparate metals are connected in moist air, the more active metal has a tendency to corrode. However, circumstances sometimes exist which thwart this corrosion tendency. For example, oxidation and reduction does not usually occur at a CrIAl junction because both metals have protective oxide films. The following material illustrates the importance of basic electrochemical knowledge in everyday affairs.
424
Journal of Chemical Education
edited by: RON DELORENZO Middle Georgia College Cochran, Georgia 31014
The Statue of Liberty
The Statue of Liberty will be one hundred years old in 1986. Unfortunately, the statue was constructed by supporting its 200,000-lh copper skin with about 2000 iron armature bars. Since copper is less active than iron, over the years the electrolytic action a t the ironlcopper interface has resulted in the deterioration of the suooortine frame and has reduced the frame to less than one-half of its original thickness (2,3). As discussed above. oxidation and reduction do not alwavs occur when two disparate metals are connected in moist air. However, one should always he aware of the possibility of electrochemical corrosion *hen different metals are physically connected. In the case of the statue of liberty, reactions such as the following occurred:
Three Mile Island
The 1979nuclear accident at Three Mile Island (TMI) included the formation of a hydrogen bubble, an event unforseen by most plant officials. Zirconium (used in cladding and in rod casings) is more active than hydrogen and can react with water to produce hydrogen gas. Zr does not displace hydrogen from water a t room temperature for the same reason that A1 does not, i.e., both Zr and Al have protective oxide coatings. However, these coatings break down a t higher temperatures such as those reached inside the TMI core when a water pump failed. Zr
+ 2Hz0
-
ZrOz
+ 2Hz
The unexpected electrochemically produced hydrogen bubble posed two threats: (1) its growth would further impede the flow of cooling water, and (2) the presence of a nearby m i s feature presents a collection of descriptive applications and analogies designedto help students understand some of the difficult concepts frequently encountered in chemistry. Contributionsthat will produce a greater appreciation and knowledgeof political, religious, economic, historical, and scientific aspects of life are encouraged.
spark could cause the bubble to explode. Either of these could have led to a meltdown ( 4 4 ) . Artificial Reefs In the early 1970's, old cars littering the Hawaiian landscape were tossed into the sea surroundine Honolulu Countv, where they were expected to serve as artififial reefs to providi shelter for fish. Platinum, which is less active, than iron. is naturally found in ocean water. The cars are now covered with crusts of valuable P t (7).
Pencil Sharpeners Disposable aluminum-bodied pencil sharpeners were sold in the 1960's. (A disposable pencil sharpener usually contains a single razor hlade, and the sharpener is discarded when the hlade dulls. Substituting aluminum bodies for the typical plastic hodies was probably considered a good idea because aluminum resisted cracking better than the plastics used a t the time. Also, aluminum is resistant to oxidation and light in weight. Unfortunately, since aluminum is more active than iron, the usually noncorrosive aluminum bodies did oxidize as electrons from Al tunneled through the aluminum protective oxide coating into the iron blade. Al
+ Fe3+APf + Fe
I t is interesting to see one of these relics today with its corroded body and rust-free hlade. In the 1960's an American automobile manufacturer produced cars with aluminum water pumps attached to cast-iron engine blocks. The water pumps cont&dly sprung leaks for the same reason that the A1 pencil sharpener bodies failed in the above example. This should be sobering to student8 who proclaim that they do not need to h o w any chemistry because they will spend the rest of their lives constructing pencil sharpeners or designing water pumps. Alchemy During the Middle Ages (r.400-1400~.u.~,nlchemis~ were falsely cmmuraged to continue in their futile search for the philusopher';i stone, 3 stnnr that would turn base metals into gold. The false encouragement rrsulted when alchemists ubserved the .'transmutarion" of iron into n m ~ r throurh r the use of a "magical" blue solution h o w n today as copper sulfate (8).
-
+
Fe + 3Cu2+ 2Fe3+ Cu Dentistry Some dental patients experience a persistent metallic taste when their dentists position slightly oxidized gold inlays that make contact with existing fillings. Fillings contain dental
nmalgam (an alloy of silver, tin, copper, and mercury), which diisolvrs under these conditions, producing the metallic taste (91.
Tin Cans Tin cans are actually iron cans coated with a thin film of tin. T h e DurDose of the tin is to wrotect ohvsicallv the iron from rust&g by sealing out oxygeLand water. ~nfo&mately,when such "tin" cans are scratched or dented. causing the iron to be exposed to oxygen and water, the iron protects the tin from oxidiinr hv H Drucess called cathodic ~rotection.'l'in is less active thaniron; the iron will rust as i t protects the tin.
-
Fe + 3Sn4+ 4FesC+ Sn Galvanized Pipe Galvanized pipe is steel coated with zinc. Sometimes galvanized pipes are connected to copper pipes. Because zinc is more active than copper, such a connection results in the oxidation of the zinc coating which in turn exposes the steel beneath it. Zn + Cn2+- Z I P
+ CU
Now, since iron is more active than copper, the iron rusts as i t protects the copper from m t i n g through electrochemical action. Fe
-
+ 3Cu2+
2Fe3++ Cu
Chemkal Industry The chemical industry may be wasting large quantities of electricity. For many spontaneous chemical reactions, it is possible to direct the movement of electrons from one chemical to another through a wire (voltaic cells) t o obtain ausahle electric current. Converting industrial chemical reaction energy into electricity may become an attractive economic option in the future (10). The importance of basic electrochemical knowledge should he obvious to students exposed t o examples such as those presented here. From pencil sharpeners to nuclear power plants, interesting (and sometimes potentially fatal) consequences can result from the ignorance of basic chemical principles. Literature Cited (1) DeLorenzo,Ronald, J. CHEM Eouc.,58,426 (1981). (2) Infosyderns. I8 (June. 1983). (3) PIeming.Thomes.Rooder'8 Digest, 49 (July, 1983). (4) Mammano, Nicholas J., J.
CHEM. Eouc., 51,286 (1980).
151 Time, 8 (April 9,1979). (6) Marshall, Eliot, Science, 591 (May 11,1979). (7) Science World. 18 (December 13.1973). (8) DeLorenzo. Ronald A,. "Problem Solving In General Chemistry,(( D. C. Heath and Compeny. 1981,~~ 39&396,416.
(9) Treptow, Richards.,J.CHEM. EDUC.,SS, 189 (March, (10) Cham. Ew. Noas,14 (July 18. 1983).
Volume 62 Number 5
1978).
May 1985
425
