edited by: DAVID A. PHILLIPS Wabash College Crawfordsville. IN 47933 PRUDENCE PHILLIPS Crawford~villeHigh School Crawlordsville. IN 47933
Why Do We Teach Gas Laws?
Why Teach the Gas Laws?
I believe that most high school chemistry teachers would respond to the question, "Why do we teach gas laws?", by stating that the gas laws serve as a foundation for a more cnmnrehensive model of matter. the kinetic molecular theorv. since the gas laws can be deduced from the fundamental k:lnetic eouation for a oerfect .. eas.. PV = % nmu2. and the equation rdating kinetic enerm tu temperature, 42 n m u 2 = cTor!,Mv'=:, )- RT.. whv . do we soend so much time teachinethe gas-laws? Historically,the study of gases was one line of approach that led to the kinetic theory; the other line was the study of heat. Unfortunatelv, a ~uantitativeunderstandina of the kinetic theory requires thermodynamics and c a l c u l k However, a qualitative understanding of the kinetic theory does not. I t is a qualitative understanding of the kinetic theory that is needed hv an educated person. The kinetic throrv s t i n d as one of thr grwitest and most useful of man's intdlectual structures. A*J w l H. Hildehrand, one of America's finest chemists, wrote in 1963l
Apart from Gay Lussac's Law of Combining Volumes and, to a degree, Graham's Laws of Effusion and Diffusion the classical gas laws are more physics than chemistry. Why then should we devote so much time to them in eeneral chemistrv courses? The reasons seem to me to he by turn historical, oedaeoeic. . " " ..nractical. and aesthetic. Historically, modkrn chemistry was almost literally coniured out of thin air. Lavoisier's elucidation of the nature of the atmosphere, Dalton's musings on gases and atoms, GayLussac's remarkable reaction-volume observations, and Clausius and Maxwell's kinetic molecular theory marked chemistry's progress from hocus-pocus to physical chemistry-a distance greater than is often apparent to the haffled undergraduate. Torricelli's experiment, to which I shall return, is the oldest experiment we commonly describe. Boyle's Law (discovered by Power and Townley and sometimes credited to Mariotte) is the oldest quantitative generalization we describe and test. Charles' Law (first published by Gay-Lussac and Dalton) allows us to aooroach the conceot of absolute temperaturk. Amontons' L& (actually discov&ed and puhlished by Amontons) is left unnamed and obtained by deduction. We read Avogadro's hypothesis like an incantation and 22.4 L is clearly God's measure. We do not merely tell, we show. Is any lecture demonstration more graphic than the crumolin~of a sturdv tin-can hv the insubstantial air? All of these and much mire are expiained by kinetic molecular theory which was one of the earliest and remains one of the mostnearly perfect of all physical theories. Moreover it is surely the most easily visualized of all microscopic models. The veils of mathvmaticnl mwtery fall away and to the music of Maxwell's demon the atoms and molecules dance palpahlv, nakedly, and endlessly before our enraptured eyes. Try saying that about molecular orhital theory or the second law of thermodynamics! One of the main pedagogic hurdles we have to get our heeinnine chemistrv students over (the sentence is as awkward &the process) is that algebra canactually be useful. Teachers of mathematics give us sadly little help. Suggest that xy = a constant hears some relationship to PV = a constant and your average student is immediately suspicious. Little x's and y's are not meant to represent physical quantities hut to he their ineffable and sexless selves. Give students a J-tube or a hvpodermic syringe and textbooks-or failing that exhume Boyle's original measurements-and let them come up with their own data and plot them: P versus V , 1IP versus 1/V, P versus 1/V, V versus 11P or whatever. Are the plots straight or curved? If the former, what is the magnitude and meaning of the slope? If a hypodermic syringe is used what is the magnitude and meaning of the slope and intercept in the P versus 1IV plot? Sneak in a hack reference to their beloved y = mx b. I t is astonishing how uneasy even capable students can he about the interpretation of simple graphs and
Everyone-whether scientist, engineer, or nonscientist-interprets in terms of the kinetic theoryphenomena involving temperature,heat, meltine and freezine. eva~orationand boiling: -. it is basic to the design and operation of heat kngines-whether steam, gasoline, jet o r rocket-refrigeration and air conditioning.All professional persons concerned with these phenomena need kinetic theory as part of their stock-in-trade.Furthermore, everyone needs some familiaritywith kinetic thwry in order to experience the satisfactionsthat come from feeling at home in his environment. The last point made by Hildebrand is, I think, the key to answering the question, "Why do we teach gas laws?", but i t is the key to answering questions asked about our reasons for teaching other scientific topics as well. The hasic assumption we make as science teachers is that what we teach will help students understand ni~turnlphysical laws and how reliablr knowledac is obtained. I believe that students can he led to discove; the gas laws, and that this serves to teach students how man acquires reliable knowledge. Students easily grasp the qualitative relationships between P, V , T, and n in the gas laws. For this reason, the gas laws become a means to introduce the use of mathematics and the logic employed in explaining the behavior of gases. Since the observable and verifiable properties of gas are conveniently studied in the high school laboratory, the student learns to annreciate the im~ortanceof the lahoratorv as a olace to in&;tigate nature. i n our attempts to conve; to our students the idea that science Dresents a consistent view of nature and reliahle methods for learning what is true, the gas laws and the oredictable ~rooertiesof eases based on these laws serve today's students as they ha;.e served past generations. Perhaps we should ask ourselves the question, "Is the way
' Hildebrand. J. H.. ' An Introduction to Molecular Kinetic Theory."
1st ed., Reinhold Publishing Corp.. New York. 1963, p. 1
(Continued on page 506, Col. 1 )
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(Continued on page 506, middle of Col. 2) Volume 62
Number 6 June 1985
505
(Continued from page 505, Col. I ) we are teaching the gas laws the hest way for achieving our eoalsof helpine thestudenrs become scientificallv literate?" By scientif;calG literate, I mean that a student should be able to understand how laws and theories were developed as well as how to apply scientific laws and theories in everyday life. An historical study of the gas laws from Torricelli, Boyle, Charles and Avogadro, along with an experimental study of the properties of eases, can aid the student in understanding the kinetic molecular model. A student who gains an understanding of the connection between temperature and kinetic energy and the properties of gases through discovery will have confidence in the kinetic molecular model. Students who understand this model are able to work gas law hehavior problems based on understanding rather than on mechanically solving gas law equations.
= nRT is a noble equation. The fact that all gases have a molar volume of 22.4 L should be a source of wonder. The basic postulates of the kinetic molecular theory have both elegance and simplicity; their mathematical elaboration is straightforward and compelling; their consequences are revelatory. In no other instinee a;e the marros;.opir and microsco& domains ol'the chemists' somewhat schizophrenic mind-set so closely yoked together and so visible to the mind's eye. If I were asked to choose one experiment to demonstrate the power and majesty of science I would not hesitate in choosing that of Torricelli. By simple and not unreasonable arguments a beginner can be led to accept that a column of mercury approximately 76 cm high counterbalances a column of air stretching from the surface of the earth to the outerreaches of the &mosphere. Since the mass of the column of mercury is readily measured the mass of the counterbalancine rolumnof suhtended air is indirectly known. All that is then needed is to scale this to the total surface area of the earth
Robert Roe, Jr. Highland Park High School Dallas. TX (Continued from page 505, Col. 2 ) the manipulation of simple algebraic formulas when real live quantities invade the abstracted world of algebra. Stoichiometry provides similar opportunities for driving home the utility of simple algehra and graphs, particularly if a continuous variation approach is used. Neither our students nor our colleagues in enpineering, statistics, economics, and the like, for these basic lessons so patever show sufficient iently, painfully, even grimly, taught. Practicallv it is alwavs a orohlem what to set the student to doing in t i e laboratory during the opening weeks of the first semester. Here again the gas laws come to our aid. Boyle's Law can be quantitatively verified for several gases, absolute zero can be estimated, the molecular weight of butane measured, and that uniquely memorable number 22.4 L obtained for the molar volume of oxygen a t STP. Alas, the extraordinary fact (students must somehow be made to realize that i t is extraordinaw) that the molar volume of all common eases is also 22.4 L is nbt so easily demonstrated. At the very c a s t we can give them measured densities for 20 or so solids, liquids, and gases a t STP and let them make their own discovery. Early experiments in stoichiometry are similarly hard to come by. Burning magnesium eats upcrucihles, copper sulfide is n o i scoichiometric, molarity has not yet been defined, silver is too expensive, bromine hurns. Once again we turn to gases. With the help of an expvnsive eudiomrtt:r tin high school) or of a cheap hahy hottle (in college) thc stoichiometry of the reaction of metals with acids and even bases to produce hydrogen can be studied with arreptable prerision. Hut this rannot be achieved witltout prior knowledge of the clasitcal gns laws inrludinc- Dalton's Law of Partial Pressures. As far aa herinning chemistry is concerned it would appear one can never suffer from excess gas. Finally we cometo aesthetic reasons for teaching the gas laws to beginning students. In its compactness and power P V "
506
A
Journal of Chemical Education
Thus by filling an open-ended tube with mercury, feeling the mercury pressing on the finger as the tube is inverted, and watching the mercury fall to its preordained height, the mass of the earth's atmosphere can be determined. How marvellous to "weigh the insubstantial air" using but one finger! Derek A. Davenport Purdue University West Lafayette, IN 47907 Derek Davenpat is Wofeswxof Chemisby at atUniversity. Born and raised in England, he earned his BSc and PhD degrees at University Collem of lhe Universitv of London. A freouent conhibutor to mls J ~ N A . .Davenpon r p&aps best knownfor n s lectureson Chemcai Eobcatlonand the Htrtory ol Chemistry A past chairman of tneDlvlslon ot Chemical Educaitm, this year he will begin a term as Chairman of Division of History of ChemisQ. Among his honors are the Standard Oil UndergraduateTeaching Award, the Manufacturing Chemists Award in Chemical Education. andthe ACS Award in Chemical Education. Robert Roe. Jr. teaches chemistry and physics at Highland Park High SChmi in Dallas, Texas. He earned a BS hom Southern Methodist University, an MED from East Texas State University, an MS from New Mexico Highlands University, and a PhD in organic chemistry from Novih Texas Stale University. An active peRicipant in a number of professional societies. Rae has been ahequent contributor at regional and national scientific meetinas. Amom his honorsare Perm FoundationAward for Excslence m Science Teachmg, the S gma XI Award as an Outstandmg SCence Teacher. and Uv2 ACS J a m Bryant Conant Avaro for Teach ng Excellence and a Prerldental Award tor Excelence m Soence Teaching
In planning a chemistry muse, wrme of Uv2 moat impatant decisions a teacher mwt make involve the selection of mieriai to be covered and the t me to 0s aevotea to each top c For each column m l h s sere$, a hlgh ~ h wand l a wliege teacher have own lnvned to dlsc~sswny m e y feel a panlcuiar mp c s Imponant and how d m odes to me shdms' understanding of chemistry
