JOURNAL OF CHEMICAL EDUCATION
630
INTRODUCTORY LECTURE IN PHYSICAL CHEMISTRY LATIMER R. EVANS New Mexico College of Agriculture and Mechanic Arts, State College, New Mexico
IT
IS the unfortunate opinion of many beginning physical chemistry students that they are being confronted with a hodgepodge of unrelated, highly theoretical material. This impression tends to increase the diiculty which most students have in understanding the subject matter of a typical undergraduate physical chemistry course. In an article on the subject of teaching physical chemistry,' Kieffer stated that "the greatest challenge to the professor of physical chemistry is to establish a solid relationship in the student's mind between the abstractions themselves and the phenomena upon which they are based." This objective may be achieved, a t least partially, if the course is designed to emphasize the continuity and usefulness of the topics to be covered. The purpose of this paper is to present an idea aimed a t the accomplishment of this purpose. There are admittedly several difficulties which hamper some students in acquiring an understanding of the material in physical chemistry. Some of the irnpediments to leaning which are offered as reasons or excuses by students are the mathematics involved, the lack of adequate preparation in other chemistry or physics courses, and the difficulty of the course itself. Some instructors also feel that inadequate mathematical background is the principal barrier.% It is the opinion of the author, however, that perhaps the major obstacle which prevents students from understanding the material is simply a lack of enthusiasm for the subject. There is some accord with this opinion as evidenced by the number of articles on lecture demoustrations which serve principally to increase interest by augmenting the written and the spoken word with a visual observation and thus simplify the learning ' XIEFFER, W.F., J.CHEW.EDUC., 29,226 (1952). 'WILLIAMS, H. B.,ibid., p. 77.
process. Also, a recent article indicates that the salesmanship of the instructor is of prime importan~e.~ The suggestion offered in this paper is another device to arouse more enthusiasm for the course in the minds of the students. Furthermore, it is suggested that the fostering of this interest be initiated a t the first lecture. It is the author's opinion that enthusiasm will increase if the student can be made to appreciate the importance and significance of each of the topics covered. The suggested approach toward this aim is simply to devote a portion or all of the initial lecture to a study of some simple chemical process. By this method the student is impressed a t the outset by the importance of many of the topics to be covered. He readily realizes how many questions which naturally arise concerning a simple chemical reaction may be answered by a knowledge of the fundamentals of physical chemistry. Hi interest in the course material is thus heightened from the beginning. Although only a few of the many topics to be covered are introduced in this way, once the student is convinced of the importance of these few, he is more willing to accept the instructor's assurance that all of the material is valuable. One good reaction for this purpose is the Haber process for the fixation of nitrogen. To illustrat,e still further subjects covered in physical chemistry, it is helpful to add that the product, ammonia, is to be dissolved in water after i t is made. The reactions then would he: NHs
+
N1 3Ha e 2NHa H 2 0 -+ NHdOH (sq.)
+
It is now helpful if the class pretends that it knows nothing at all about this reaction (often not too much of a nretense!) but is eoine to exnlore its nossibilities bv -a SCHWENCK, J. R.,ibid.. p. 368. ~
DECEMBER, 1953
631
doing some research with the eventual plan to erect a capacities of gases and to determine the work and heat plant to market aqueous ammonia. In this case some involved in isothermal and adiabatic compressions or of the questions which naturally arise and which can be expansions of gases. answered by physical chemistry are as follows: (7) What will be the heat effects when NH, is dissolved in HaO? (1) Is it plausible t o expect the reaction t o occur? Heats of solution and solubilities are not so well Thermodynamics shows that the change in free energy for this reaction is -3.94 kg.-cal./mol of NH3 understood as some other physical and chemical and so it should proceed. Thermodynamics tells us phenomena, but methods of calculating and handling that it is plausible from an energy standpoint but it does integral and differential heats of solution will be disnot tell us how t o make the reaction go. cussed. Also, based on molecular structure, which is studied in physical chemistry, intelligent guesses con(2) How much NH3 will a mol of Nz give? The equilibrium constant for this reaction a t a given cerning solubilities may sometimes be made. For extemperature and pressure shows exactly how much NH3 ample, NH3, with an unshared pair of electrons on a is in equilibrium with any given aniounts of Nzand Hz. reasonably eleet~negativeelement, would be expected As a corollary to this, it might be mentioned that the t o be quite soluble in water, owing t o the formation of specific effects of changes in temperature and/or H-bonds, and t o evolve heat upon solution. pressure can be calculated. (8) Will there be storage problems? How basic are (3) How long will be required for the reactants to solutions of NH,OH? The discussion of ionic equilibria teaches the student produce the equilibrium concentration of NHa? A study of chemical kinetics in physical chemistry to calculate readily the pH of aqueous solutions of any will show how these velocity constants may be deter- reasonable strength. Conductance measurements of mined and how they may be used to determine the aqueous solutions permit ready determination of the length of time required for any given portion of the degree of dissociation and thus the ionization constant. reactants to be consumed. These constants may also (9) What are the possibilities of loss due to evaporashow the effects which altering the stoichiometric ratio tion during handling? Means of determining the vapor pressure of a liquid or of reactants may have on the rate of reaction. Furthermore, the role of catalysts, which alter the rate of the partial pressures of components of-a solution a t various temperatures or pressures will he studied. chemical reactions, will be discussed. (4) Since they are gases, how may the quantities of (10) Will there be any difficulties in handling the the various materials be readily determined? liquid product? The simple gas laws or compressibility diagrams allow The physical properties of liquids, such as viscosity the amounts of gases, even up to moderately high pres- and surface tension and their variation with temperasures and down t o reasonably low temperatures, to be ture, will be discussed. These and other properties are calculated from a knowledge of the volume, tempera- useful in connection with pumping,, pouring, and other handling operations with the product. ture, and pressure or the critical constants. All of these concepts are sufficiently simple that the Will the reaction be exothermic or endothermic? (5) Methods for determining not only whether the re- student of elementary physical chemistry can grasp action will ahsorb or evolve heat but how much heat will their significance, and there are undoubtedly many be discussed when the subject of thermochemistry is other points which can be made to illustrate still other covered. Also the change in the amount of heat topics which are to be covered in the course. The absorbed with a change in temperature may he deter- student is thus able to visualize the value of physical chemistry before starting the course and possibly his mined. interest in the subject matter is stimulated from the How much work will he required t o heat the (6) reactants to the optimum temperature and to compress beginning. It might also be helpful in pointing out the them to the optimum pressure? value of certain sections as they are taken up in the I n the basic study of chemical thermodynamics, the course to refer occasionally to this introductory lecture student will learn t o handIe expressions for the heat to refresh the student's memory.
