630 JOURNAL OF CHEMICAL EDUCATION

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 con- fronted with a hodgepodge of unrelated, highly theoreti- cal 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 teach- ing 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 phenom- ena 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 accomplish- ment 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 irn- pediments 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 under- standing 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 demou- strations 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 sales- manship 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:

N1 + 3Ha e 2NHa NHs + 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 plant to market aqueous ammonia. In this case some of the questions which naturally arise and which can be answered by physical chemistry are as follows:

(1) Is it plausible to expect the reaction to occur? Thermodynamics shows that the change in free

energy for this reaction is -3.94 kg.-cal./mol of NH3 and so it should proceed. Thermodynamics tells us that it is plausible from an energy standpoint but it does not tell us how to make the reaction go.

(2) How much NH3 will a mol of Nz give? The equilibrium constant for this reaction a t a given

temperature and pressure shows exactly how much NH3 is in equilibrium with any given aniounts of Nz and Hz. As a corollary to this, it might be mentioned that the specific effects of changes in temperature and/or pressure can be calculated.

(3) How long will be required for the reactants to produce the equilibrium concentration of NHa?

A study of chemical kinetics in physical chemistry will show how these velocity constants may be deter- mined and how they may be used to determine the length of time required for any given portion of the reactants to be consumed. These constants may also show the effects which altering the stoichiometric ratio of reactants may have on the rate of reaction. Further- more, the role of catalysts, which alter the rate of chemical reactions, will be discussed.

(4) Since they are gases, how may the quantities of the various materials be readily determined?

The simple gas laws or compressibility diagrams allow the amounts of gases, even up to moderately high pres- sures and down to reasonably low temperatures, to be calculated from a knowledge of the volume, tempera- ture, and pressure or the critical constants.

(5 ) Will the reaction be exothermic or endothermic? Methods for determining not only whether the re-

action will ahsorb or evolve heat but how much heat will be discussed when the subject of thermochemistry is covered. Also the change in the amount of heat absorbed with a change in temperature may he deter- mined.

(6) How much work will he required to heat the reactants to the optimum temperature and to compress them to the optimum pressure?

In the basic study of chemical thermodynamics, the student will learn to handIe expressions for the heat

capacities of gases and to determine the work and heat involved in isothermal and adiabatic compressions or expansions of gases.

(7) What will be the heat effects when NH, is dis- solved in HaO?

Heats of solution and solubilities are not so well understood as some other physical and chemical phenomena, but methods of calculating and handling integral and differential heats of solution will be dis- cussed. Also, based on molecular structure, which is studied in physical chemistry, intelligent guesses con- cerning solubilities may sometimes be made. For ex- ample, NH3, with an unshared pair of electrons on a reasonably eleet~negative element, would be expected to be quite soluble in water, owing to the formation of H-bonds, and to evolve heat upon solution.

(8) Will there be storage problems? How basic are solutions of NH,OH?

The discussion of ionic equilibria teaches the student to calculate readily the pH of aqueous solutions of any reasonable strength. Conductance measurements of aqueous solutions permit ready determination of the degree of dissociation and thus the ionization constant.

(9) What are the possibilities of loss due to evapora- tion during handling?

Means of determining the vapor pressure of a liquid or the partial pressures of components of- a solution a t various temperatures or pressures will he studied.

(10) Will there be any difficulties in handling the liquid product?

The physical properties of liquids, such as viscosity and surface tension and their variation with tempera- ture, will be discussed. These and other properties are useful in connection with pumping,, pouring, and other handling operations with the product.

All of these concepts are sufficiently simple that the student of elementary physical chemistry can grasp their significance, and there are undoubtedly many other points which can be made to illustrate still other topics which are to be covered in the course. The student is thus able to visualize the value of physical chemistry before starting the course and possibly his interest in the subject matter is stimulated from the beginning. It might also be helpful in pointing out the value of certain sections as they are taken up in the course to refer occasionally to this introductory lecture to refresh the student's memory.