Nicholas J. Mammano State University of New York Plansburgh. NY 12901
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A Chemistry Lesson at Three Mile Island
As a teacher of a large chemistrv - one-semester general course for pre-technical, non-science majors (nursing, home economics, food and nutrition), the problem of developing an intense overview of the basic principles of chemistrywichin 15 weeks while a t the same time maintaining the interest and attention of a "non-captive" audience is almost a daily challenge. The course is not a terminal course in science since i t is meant to he a vrereouisite for oreanic chemistrv, " . hiochemistry, and various preprofessional courses in nutrition and nursing. Conseauentlv it serves a svecial student. neither the sciencemajor, who can be expected to take his chemistry "straight", .. . nor the liberal arts maior, who mieht be fulfilling a graduilte rrquiren~entin s~.ience.For the lnttrr itudent, n quite :tci~drmiwllvcrvdihle appr(,ach m a b t I I to ~ emphasize what chemistry i i "about", i:e-., the "process of chemistry2' rather than its facts and details. It is thus the "in-between'' student t'ur -.whomthe instruction of chemistry raises special prol~lemi;the studeut who at worst actively hates chemistry ur d t c n at best uueativns its necesiit\,. While the need tor a solid foundation'in chemistry for nurses and dietitians may be clear to many of us, it is not necessarily the case for students in these areas. After several years of teaching freshman chemistry to students who have little or no interest in it I would like to suggest an approach to this task which might just capture and maintain student interest without compromising the rigor and honesty of the discipline. That is, that it is necessary to make the course as interesting and exciting as possible in order that students learn the material, almost in spite of their indifference. The approach is not unlike the doctor who sugarcoats a bitter pill for the good of the patient. While the analogy might he distasteful to my colleagues, I do not believe the anvroach is condescendine or vaternal: it is sirnolv an effort to come to terms with the sekeiident fact that thkre are manv colleee students who find chemistrv vainful. horine or irrelevant intheir lives yet need to know o;.'r t h k group it becomes necessarv for the colleee teacher to activelv and energeticslly seek to excite and motivate, to present rhe ma. rerial in a context t h indiffenmt ~ freshmen ran relore t ~The process I am suggesting is time-consuming since it means scouring the popular literature and media ahout science news, keeping abreast of publications such as Science, J. CHEM. EDUC., Science News, the daily newspapers, as well as TV. It means lecture demonstrations as often and as svectacular ar posiit)le. It m r m s concrete and relevant examples of the close connection of(:hemistrv i)rincii)les with the dail!. lives of students. It means not avoiding t i e inclination to "ham i t up" by injecting some humor, spontaneity and drama into a lecture or laboratory. Clearly many college teachers are neither personally nor professionally inclined toward such an approach. I would suggest that many are; that in fact most teachen of college chemistry who take the trouble to read this Journal may indeed he so inclined. The real challenge is to develop these examples so that the integrity of the chemical principle is not sacrificed. While there are any number of instances that arise in a given semester. the recent nuclear accident a t Three Mile Island ('I'MI) providrd a spectacular case in point. As the news a(.cwlnu unfolded during the earlv davs o i the accident. the incident became a focus of some liveiy and instructive'discussion in my freshman chemistry class. In fact, I scrapped my original lecture plan and developed a completely new
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286 1 Journal of Chemical Education
presmtation of nuclear chemistry, gas laws, 1.e ChAtelier's pr:nciple and equilibrium, and swichionletry. The whule topic of nuclear chemi-try which mdinarily rakes atwur 3 lectures and includes the &indiird trearment of radioactivity. penetrating power, rem dosage, fission, fusion, half-lives, etc. was treated by continual reference to TV news accounts and allusion to the suecifics of the Harrishure incident. For exof U-235 is 1-131. ample, one of th; possible fission The serious . nhvsioloeical effects of exposure to 1-131was used . as a voint of devarture for the development of fission, critical mass, isotc,pes, hali-liws, etc. Another example was the formation of the hydrugen bubble which was an e w n t totally unforeseen 11y the plant official.; and therrfore particularly dramaticand intert~;ting. I auuld like todevelop in some detail the ~ r u r e d u r eused i in utilizinr the bvdruren lnhhle incident to elucidate basic chemical priuci&es. ' Early reports on the source of the bubble and some informal discussions with chemistry colleagues suggested two possible mechanisms of bubble formation, each of which is consistent with the "high" temperatures which developed from the nuclear accident. 1 j Thermol dwrrmporilion olwoter, coupled with a reaction of the oxvnen pn)duced with hocmetal in the core, since was mostly hydrogen. As the presentation the gas developed in class, I suggested that the metal reacting with O? he Fe in the core construction or Zr in the rod casing. . mieht " ~ Discussions with colleagues familiar with the construction i f nuclear vlants as well as readina about their construction itrmgly suggested that %rwas the likely reactant. Thus thc source of tbr bubble would he tu.0 reuctiuns at high temperature (1) 2HzOk) 2Hzk) + O d d (2) On(g) + Zds) ZrOn(s) 2) Direct reaction of water with a n active metal a t high temperature. Zirconium in the cladding again appeared to he the probable reactant: (3) 2H20(g)+ Zds) ZrOds) + 2Hdg) (A later report in Science' stated that reaction (3) ( t > 2700°F) had been established as the source of the bubble.) ~~
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Thus using the dramatic incident of hobble formation a t TMI and reproducing a diagram from the local newspaper, showing how the bubbie imp&ed the flow of cooling water and comoounded the overheat in^ problem, provided a gripping focus for the development of;ome chemical ideas in lecture. A party balloon containing hydrogen was ignited in lecture and the Hindenburg disaster (and movie) was referred to, underlying the seriousness of the danger a t TMI. In this rather "hyped" atmosphere it was not too difficult to get student attention to present or emphasize the following basic principles. Le Chbtelier's principle
For the thermal decomoosition of water the equilibrium is far to the left, i.e., water is stable with respect tddecomposition to its elements a t room temperature. Since AH for reaction (1) is +58 kcallmole HzO a t 25"C, according to Le CbA1 Science,
594 (11 May, 1979).
telier the reaction a t equilibrium should shift to the right as the temperature is raised, which would result in a large in-~ crease in HP. Moreover removal of 07-hv,reaction with Zr wc~uldresult in even more H1 consistent u,ith the concmtrae . eauilihrium was tion elfcct of 1.e Chfitelier'i ~ r i n c i ~ lThe also discussed from the viewpoint of pressureberturhation. ~
Energetlcs I t was shown how reaction (3) is the sum of reaction (1) , , and (2), and that it was possible &add and subtract reactions in this manner to "obtain new reactions." While a discussion of the First law implications was beyond the scope of a freshman chemistry student, a calculation of AH of reaction (3) . . from the Al13;of reaction ( 1 1 and (2) a:a pwtbrmed showing how W'>cot~ldbeadded in this wav: a~ecificalls.Wt31= 21t111) AH(2) or AH(3) = 2(58) (-217) = -i01 Kcallmole. For those students who proceed to a course in hiochemistrv the treatment provides an introduction to concepts involved in electron transfer half reactions, for example, which are the energy providing steps in many biological pathways. Reaction (3) was discussed from the point of view of the Activity Series; since Zr lies above HZit can reduce water to Hz (hence the hydrogen bubble). The fact that this does not occur a t room temperature was explained in terms of the passivity of certain metals, e.g., Al, Zr. At higher temperatures the protective oxide coating presumably breaks down exposing free metal surface. Thus the metal becomes reactive a t high temperature for kinetic or mechanistic reasons (activation energy was briefly mentioned here) even though the negative AH for reaction (3) sliould make the reaction less favored at high temperature according to Le Chitelier. I t is a fine example of how thermodynamic or energy considerations (Le ChAtelier's principle) are not the whole story. ~~~~~~
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Gas Laws The bubble was described as having a volume of 1000 ft3 (filling a 10 X 10 X 10 ft room, for example). The Perfect Gas Law was used to calculate the moles of Hz using published estimates of the pressure and temperature of the huhhl6. Using values o f t 250°C,p 1000 psi, the volume, temper.28,000 liters, T = ature and pressure were converted to V ; 500°K, P 70 atm and n(Hz) = PVIRT F. 50,000 moles Hz.
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Stoichiometry I t was shown how a simple mole-weight problem could he set up to estimate the enormous damage at TMI. Using re-
action (3) the weight of Zr reacted is determined from a simple factor analysis problem: 5 X 10' rnnlps H>X I mole %r/2 molc.; Hz X 9 I g Zr/l mole Zr 2 X 11)" Zr: 'L X 10%.X I Ib 454 c X 1 ton L X 10' It, 2 tons Zi!2
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Boyle's Law There was some concern over the excessive pressure in the core. The decision not to relieve the situation by lowering the pressure was a direct consequence of Boyle's ~ a ws,i n c e h ering the pressure would increase the volume of the bubble. exposing more core and exacerbating the problem of overheating and possible meltdown. If, for example, the pressure on the sample of hydrogen gas a t 1000 ft3 were reduced from 1000 psi to 500 psi the bubble volume would double to 2000 ft3, overheating more of the reactor rods, increasing the temperature and producing more Hz, (also increasing volume by Charles' Law) with possibly catastrophic consequences. Thus, by exploiting the excitement and drama of the TMI accident, developing the appropriate chemical principles within that context, it was possihle to present some rigorous chemistry in a spirited and interesting manner. Note that this is not talking "about" chemistry in a vague way using standard examples. The details of the principles involved were worked into the lecture presentation almost ad hoc but with the specific numerical data available from news accounts and a t a level of difficulty at least equal to the textbook. I t afforded a freshness and vitality to my own preparation that was clearly communicated to the students judgina - . by . their attendance and classroom response. We live litefally in the Age of Science. Modern life is filled with examples, good and bad, of events and topics that show how the science of chemistry is relevant, exciting and concrete. It behooves us as teachers of chemistry to use these examples to the fullest extent possible, especially when dealing with students who are not inclined toward a career in scienie. ~
This number probably represents an upper limit since during normal operation the cooling water is charged with extra hydrogen to scavenge oxygen and hinder corrosion. Some of the hydrogen in the bubble may have resulted from the release of this dissolved hydrogen when the pressure fell accidentally during the sequence of events at TMI.
Volume 57, Number 4, April 1980 / 287
