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THEORETICAL CHEMISTRY AND DESCRIPTIVE CHEMISTRY IN THE GENERAL CHEMISTRY COURSE' NORMAN DAVIDSON California Institute of Technology, Pasadena, California
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MAIN Object is to describe the freshman course in genera' chemistry at the InstituteOf Technology and to consider especially the interplay between descriptive chemistry and theoretical chemistry in our presentation of the subject. 1 shall also very briefly describe those points in the undergraduate curriculum a t which chemistry majors receive instruction in descriptive inorganic chemistry. Finally, 1 would like to express my opinions about the general problem of this Symposium. The genera' chemistry course is taken by Our entire freshman class, i. % by a group of students who are preparing to specialize in somc branch of science or engineering. The text used is "General Chemistry1' by ~i~~~ pauling and D ~pauling , has a major role in shaping the present character of the coure. I t will be convenient to quote in part, from a previous description of our freshman course.?
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The course. .covers essentially the topios customary in firstyear college ohemistry. An effort is made, however to present the material in as unified and logics1 a manner m is possible at the of development of science. I,, particular, there is emphasis from the beginning cn the atomic and molecuIm structure of matter and the explanation of the p!lysical and chemical properties of substances in t e r m of this cona.:p%. The first few lectures axe devoted to the atomic and molecular structure of crystals, liquids, and gases, with emphasis on q,alitstive rather than quantitative considerations. A brief survey of desoriptive chemistry in relation to the position of the elements in the periodic table is then presented, followed by the discussion of weight relations in chemical reactions. Atomic weights and stoichiometric oalcul&ms are not treated by the method of historical development, but rather in the simplest possible way, on the basis of atomic theory. The electronic structure of atoms and molecules and the nature of valence is taken up next. This subject is treated in detail, first for ionic compounds, together with a discussion of the phenomenon of electrolysis of fused salts and of s d t solutions. Covalenoe is then discussed, fallowed by the definition and use of oxidation number and the discussion of oxidation-reduction reactions. The policy adopted in treating these theoretical subjects is introduced are to be treated thoroughly that the topics so as to he understandable by the student. It is assumed that a freshman student has on the average as much ability to understand theoretical concepts as an upper classman or graduate Contribution No. 1353 from the Gates and Crellin Lahoratories of Chemistry, California Institub of Technology, Pasadena, California. s ~ -r,, H,, ~ J,, cmM,E~UC,, 24, 574 (19471, ~h~ description quoted here is essentidly due to Dr. Pauling.
student, although his experience and training have been less extensive. In accordance with this assumption, the detailed prooesses of the conduction of electricity through a molten salt or an electrolytic solution and the accompanying electrode reactions are treated in essentially the same way as in a course in physical chemistry, but necessarily somewhat more slowly and with great emphasis on the experiment$ facts. One consequence of this thorough treatment of certain theoretical subjects in the course is *hat the number of topias during the year is necessarily somewhat smaller than is usually the case and the treatment of the descriptive material consists of an intensive study of selected representative topics rather than a more general survey of the entire field. The first set of elements selected for detailed descriptive discussion comprises chromium and manganese and their congeners. This departure from the conventional order has been adopted because the chemistry of chromium and manganese is especially ~nterestiug,and most students are not very familiar with it. These elements also provide many examples of oxidation-reduction reactions permitting the application of the general principles which have just been discussed.
We use our own set of mimeographed notes for the laboratory work. In with the lecture material described above there are experiments on simple inorganic preparations, on weight changes in chemical reactions, on ionic conductance and electrolysis, and on the chemistry of chromium and manganese. The lecture work in chemistry is correlated with laboratory exercises in which the students individually assemble cork ball models of some of the simple mobular and crystalline structures and then study the properties of these During the remainder Of the year, the lectures Of S U C C ~ S Sweeks ~ ~ ~ alternate between theoretical topics andtopics in descriptive chemistry such as the halogens, sulfur and its nitrogen, phosphorus, and other nonmetals, and the various metals. The theoretical work of the later part of the course is largely a relatively thorough study of chemical equilibrium. Both heterogeneous and homogeneous equilibria are considered and, of course, we pay especial attention to equilibria in aqueous solutions-acid-base reactions, solubilities, complex ions, and oxidationreduction equilibria and potentials. This work goes hand in hand with the laboratory study of solu+,ion chemistry which culminates in qualitative analysis for small selected of elements, There is a careful application of equilibrium theory to the control of the conditions of analytical separations. Emphasis is placed on the principle that to understand a particular 445
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system one must first understand what are the main over-all reactions that determine the concentrations of the major species present, and then use this information to calculate the concentration of those ions and molecules that are present in low concentrations. I have already stated our belief that the presentation of descriptive chemistry should be limited to an iutensive study of selected representative topics rather than a more general survey of the field. Every effort is made to correlate the descriptive material with the theoretical work and to consider the chemical properties of the elements in the light of their position in the periodic table and their electronic structures. At the same time it is emphasized that chemistry is not a purely deductive science, that there are many facts that are not adequately explained by modern theory but must be memorized, and that newer and more powerful theories are required to explain more chemical phenomena. Our experience is that freshmen are interested by the presentation of structural theory in their chemistry course, that they appreciate the contributions which this theory has made in inorganic chemistry, and that they are stimulated rather than disillusioned because there are many facts which this theory cannot at present explain. They enjoy learning the structures of, say, the oxyhalogen acids or some of the sulfur-containing acids. They find the structural information an aid in remembering the formulas of these substances and in thinking about their properties, even though a knowledge of the structures does not enable them to predict all of the reactions of these suhstances. In lectures on special topics one can illuqtrate the utility of structural considerations in explaining the properties of substances. For example, the complicated formulas and the mechanical properties of the silicate minerals may be understood in terms of their structures. Similarly, the structures of the silicone polymers may be correlated with the interesting properties of these substances. In a lecture on iron and steel, it is possible to point out, at least in a general way, how the mechanical properties of these materials depend on the chemical properties of the iron-carbon system. All of our undergraduate chemists have further contact with factual inorganic chemistry in a one-year sophomore course in analytical chemistry and in a twoterm senior course. Without describing these courses in detail, I shall mention that in their analytical work, the students obtain further experience in the application and limitations of equilibrium theory in qualitative and quantitative procedures. They learn a great deal of descriptive chemistry in connection with the laboratory work and the lecture work o n a qualitative and semiquantitative system of chemical analysis, and also in connection with a critical presentation of many quantitative procedures other than those that they actually study in the laboratory. The philosophy of this course is not the teaching of analytical chemistry, pw se, but use is made of this subject to develop a coordinated background of factual and theoretical mat-
JOURNAL OF CHEMICAL EDUCATION
ter in inorganic chemistry. The senior course in inorganic chemistry considers selected topics, such as the rare earths, fluorine and the fluorides, the fifth and sixth group nonmetallic elements, solutions in liquid ammonia, and complex ions. Emphasis is placed on both the descriptive chemistry of these substances and on a study of their properties from a modern physicochemical viewpoint, in which their thermodynamic constants, their rates of conversion, their magnetic properties, and their valence relations are qonsidered. The course requires abundant reference to the chemical literature. We note here, too, the emphasis on the intensive study of selected topics rather than on a general survey. Now I would like to express my opinions about the general problem of this Symposium-about the relative emphasis in the formal education of a chemist on descriptive factual material and on more general theoretical topics. I think that the best formal preparation for chemical research is one that emphasizes broad general principles and general methods, and is one in which the presentation of descriptive material is closely correlated with the presentation of theoretical topics. (Personally, I find a fact interesting and easy to remember when I consider it as something to hang on to a theory--either as a harmonious adornment or as a disfiguring, but stimulating, misfit.) Let me cite some recent chemical history to illustrate my point of view. During the war, many young chemdts on the atomic energy projects were assigned to study the chemistry of the rare earths or the chemistry of what we now call the actinon elements. The chemistry of neptunium and plutonium is a sort of a hybrid of uranium chemistry and rare earth chemistry. Most of these people had had no previous experience and no undergraduate instruction in either of these fields. But they did a good job because they had had a good basic training in chemical principles. They understood how to apply ideas about oxidation potentials, about equilibria involving complex ions, about rates of reaction, and about other topics to the separations they were studying. Thermodynamic reasoning and an understanding of the effects of ionic radius and charge on the stabilities of substances were of great value in anticipating what solid compounds of the new elements could be prepared and how. In the rare earth field, one of the outstanding advances of recent years has been the development of the ion exchange method of separation of the rare earths. I believe that the people who worked this out were able to do so, not because of previous instruction in the properties of the rare earths, but because they had a basic training which enabled them to understand and apply the principles of ion exchange separations. The amount of factual information about inorganic compounds is growing greater and greater-and of course nobody tries to teach it all. I judge that there is nothing sacred about the particular descriptive chemistry that we do teach. Most of us in qualitative analysis pay particular attention to the properties of a
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selected group of ions of the metals in aqueous media as a function of pH, in the pfesence and absence of H,S and SH- especially. This is good factual material. It can be used to give students a feeling for the value of the periodic table as a tool in correlating properties. and it can be used for instruction in the principles of solution chemistry. But there are other interesting topics which could be used in the same way-the chemistry of the fluorides, for example. If partition chromatography is developed into an effect,iveand general method for inorganic analysis, the descriptive chemistry of some future qualitative course may be organized around the solubility behavior of inorganic ions in organic solvents as a function of the complexing agents present. The important thing is not just what or how many facts we teach but that we stimulate our students to have a healthy interest in facts. This, I think, is most effectively achieved by an intensive study of selected
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topics in descriptive chemistry, and by constant reference to actual experimental material and its interpretation in connection with the presentation of theoretical material. If students can be taught the habit of referring to and browsing in the original literature and the compendia they will be prepared to learn descriptive chemistry a5 they mature professionally. In conclusion, then, I think that in spite of an ever. increasing amount of factual information, chemistry is not becoming more and more fragmented. It is becoming more and more integrated and unified by.genera1 theories and by general methods of investigation which are applicable to a variety of fields. More and more, the biochemist, the organic chemist, the physical chemist, and the inorganic chemist speak a common language. To participate in this synthesis, the student must have training in both descriptive and theoretical chemistry, and I believe there will be a greater emphasis on the correlation of the descriptive and the theoretical topics.
