edited by: DAVIDA. PHILLIPS Wabash College Crawfordsville. IN 47933
PRUDENCE PHILLIPS Crswfordsville High School Crawfordsville. IN 47933
Should Thermodynamics be X-Rated?
Constant Properties of Systems: A Rationale for the Inclusion of Thermodynamics in a High School Chemistrv Course
Henry A. Bent North Carolina State Univwslty Raleigh, NC 27695
Why teach thermodynamics? To a young instructor in 1952, thermodvnamics seemed to return an enormous fund of assured theory for a trifling investment of fact. It seemed, therefore. like an ideal suhiect for studv for fact-burdened students of chemistry-an: perhaps the sooner begun the better? For high school chemistry was not then overburdened with physical chemistry. And so, fresh from a graduate course in thermodynamics, I began to teach thermodvnamics in a general che&istry courseffor freshmen. Now, sbme 30 years later, I must profess as a professor to feeling uncomfortably well-fit by that definition of a professional as one who's made every possible mistake in some small field. As has been said, We're too soon old and too late smart. One sees today general dislike for "thermogodamnics"; Clapeyron's enuation on Advanced (Mis?)-Placement Exams: and horrendous results on the;mod&mics on ~ r a d u a t eRecord Examinations--even though (or because?) formal study of the subject begins in high school. Emerging from those disasters mav he a simple truth reaardina the formalisms of thermodynamics: ~ e i h a p hette;later,lf s ever. While the logic of thermodynamics has been trickling into beginning chemistry courses, through the ill-logic of logical curriculum development, l o ~ i citself has been trickling into thermodynamics, through advances in its axiomatization. Now, more clearly than in 1952, one can see that formal thermodynamics is merely a set of logical relations. Logic alone does not create new information. I t merely transforms information: for examnle. from the statement that o statement that a a reaction is spontaneous if Q i ~ t the reaction is svontaneous if AG(= R T In QIK) . . < 0.We don't eet something irom nothing, especially in the teaching of th&modynamics-except perhaps disorder in the curriculum. To put the matter slightly differently-Thermodynamics is like an encyclopedia. T o its users, it is asource of information: for example, K = exp(-AGolRT), AG" values being found from tabulated thermodynamic data. T o its creators, however, thermodynamics is merely a summary of information: AGO = -RT In K, K values being determined hy experiment. And what a pale summary of experiment thermodynamics is! What is less vivid than a set of signed numbers? That's chemistry? It's more like minimal art-art without sex. Compare, for example, the thunderous explosion of hydrogen and oxygen with the bald statement that AG;(HzO(g), 25OC, 1atm) = -237.191 kJ1mol. Truly, AGO may have a negative value: i t may force students right out of chemistry. (Continued in Col. 1,page 229)
228
Journal of Chemical Education
Ethel I.. Schultz MarbelheadHigh School Marblehead. MA
The question often arises as to whether it is appropriate to include thermodynamics in an introductory course in high school chemistry. Perhaps the most compelling reason for teaching thermo a t that level is to introduce the concept that systems have chemical properties. The idea of pure substances having characteristic properties is a traditional topic taught a t the very beginning of chemical science. Indeed, a fair amount of time and effort is soent in differentiatine- pure . substances by means of their melting points, boiling points, densities, refractive indices, etc. The concept of characteristic properties of systems, under specific conditions, is not usually stressed as a function of a chemical reaction. Couched in more sophisticated terms, such as the standard enthalpy of water, this concept, which is familiar to chemists. is rarelv recoenized by the no& student of chemistry as a characteriFtic property of a chemical system. The students may use AH; as a "number" in the "solution" of a problem involving Hess' Law hut not connect i t to other situations in which the energy represented by that statement could he useful. While the topic of enthalpy is included in many chemistry courses. the concent of entronv is sometimes included for its interest value as c'onnected a nonchemical system. (The term entropy is often used in discussion of society and its increasing multiplicity of problems.) The concept of free energy is rarely taught. I contend that all three thermodynamic functions should he presented and taught to the level of sophistication appropriate for the students. It is imperative that connections be established between these phenomenological constant properties of pure substances, under varvina conditions oftemperature,-pressure, and c¢ration. ' Here is an example that can he useful throughout the course, the redox reaction of zinc and copper:
to
Zds) + CuZ+ (aq)
-
Cub)
+ Zn2+ (aq)
AHD = -52.1 keal' AGO = -50.7 kcal ASo = 4.7 X 10-3
keal/K This system offers an excellent example of a reaction that can he repeated a t many points in the course with different applications. Students may studythe stoichiometry of the system. The definite properties of mass and reactants, color of filtrates, and temperature changes all can be used to determine the optimum ratio of reactants (the stoichiometric
'
All functions are expressed in k c a l since this is still the most commonly used unit in high schwl texts. (1 kcal = 4.184 kJ.) (Continued in Col. 2, page 229)
