Chemical bonds - American Chemical Society

Chemical bonds - American Chemical LE Strong - ‎1958 - ‎Cited by...

0 downloads 169 Views 3MB Size



M. KENT WILSON Tufts University, Medford, Massachusetts

INMUCH that has been written in recent years concerning the effective relationship between high school and college chemistry courses' two comments recur: (1) care should be taken to avoid wasteful repetition between the two courses, and (2) performance in freshman college chemistry appears to be little influenced by whether or not the student has had high school chemistry. This latter statement seems to imply that the standard high school chemistry course is ineffective as a basis for more advanced work. Furthermore it borders on a gratuitous insult in the face of the fact that most college chemistry students say they found their original interest in chemistry during their years in high school. The New England Association of Chemistry Teachers has published from time to time a list of topics which ought to he in any introductory chemistry c o u r ~ e . ~ Ordinarily these lists are prefaced by a statement that the actual sequence of topics is left to the individual teacher. Such a statement implies that there is no logical basis for specifying the best succession of material. It tends to create the impression that the course is only a collection of loosely related topics. Yet modern chemistry has developed considerable internal consistency and the process of fitting the details into the developing concepts is one of the more exciting parts of chemistry. At a conferencesponsored by the Division of Chemical Education of the American Chemical Society and the Crown-Zellerbach Foundation held a t Reed College, Portland, Oregon, in June, 1957,sa group of high school and college teachers again discussed the integration of high school and college courses. It was agreed that repetition should not be ruled out completely for, at the right point, repetition is a most useful aid to learning. The problem is one of judgment as to the material to be included at each level. The group at Reed College did not arrive at an answer to this problem, but one proposal emerged from the discussions which offers a way to move forward. It was agreed that a good high school chemistry course ought to have a quality of intellectual integrity that can he communicated to the student and that this could be achieved by having a focus toward which most of the discussion could be directed. If a course for high school students could be devised with a central theme less broad than the whole of chemistry, 'See, for example, CLAW, L. B.,J. CEEM.Eonc., 32, 141 (1955). ' J. CHEM.EDUC.,34.307 (1957). J. CUEM.EDUC.,35, 54 (1958). 56

but including the major paths by which a chemist proceeds in his dealings with chemical phenomena, then it ought to be possible to produce a reasoned argument for the topics to be included or excluded, the order of presentation, and the points at which individual variation might most readily be introduced. A major differentiating aspect between chemistry and other branches of natural philosophy is the concept of chemical bonds. Indeed, the making and hreaking of these ties between atoms is chemistry. Our proposal is that "Chemical Bonds" is the logical central theme' for a meaningful high school course. It is a theme large enough to include a great amount of descriptive chemistry and at the same time to serve as a guide to the items which can best be included in the course itself. If the high school student can be brought to some understanding of the way in which atoms are held together and to the idea that many of the properties which a substance exhibits are determined by these bonds between atoms rather than by the specificproperties of the atoms themselves, he is a long way toward being able to think about phenomena in chemical terms. Thus, if a chemist wishes to account for the observation that some molecular material has a red color, he does not conclude that the atoms in the molecule have colors which add up to red, but rather that the redness is a property of the entire molecule largely determined by the nature of the chemical bonds which link the atoms together. When the particular set of links is altered to some other set, even though in a sense the atoms remain the same, the color changes. This is not to say that each atom contributes nothing by itself to a compound, for clearly steric phenomena are due to atomic volumes as well as molecular geometry. To achieve clarity and simplicity in high school chemistry one could well concentrate on a few prototype bonds without serious loss of accuracy. A wide variety of compounds and related physical and chemical phenomena can then be discussed with reference to the main features of the bond types involved. It is suggested that three bond types be presented: ionic or electrovalent bonds in which the bonding forces are the result of electrostatic attraction, covalent bonds in which the bonding forces arise from the mutual attraction of two nuclei for a pair of electrons, and metallic bonds in which the bonding arises from the mutual attraction of many nuclei for many electrons. Since these are 'H~~cKEL, W., "Structurd Chemi~tryof Inorganic Compounds," Elsevier Publishing Co., Inc., Amsterdam, 1950, Vol. I, p. 44. JOURNAL OF CHEMICAL EDUCATION

extreme types and most real substances do not have linkages which correspond perfectly to any one bond type, the student would be put on his guard by a later discussion of polar covalent and multiple bonds as indicated in the following outline. Proposed Outline for High School Chemistry Course Based on Chemical Bonds as the Central Theme

I. Introduction Metric system 11. Elements and atoms Laws of chemical combination Atomic weights and symbols Atomic structure Electrons ' Electronic forces: coulombic, exchange Atomic numbers: protons and neutrons Periodic table 111. Chemical bonds-discontinuity of chemical change Bond types: ionic, cavelent, metallic Physical properties of substances Gases: gas laws, kinetic moleculsr theory Liquids Solids: crystals, e.g., diamond, sugar, sodium ohloride Physical transformations and temperature Gas to liquid Liquid to solid Relation of mass to properties Relation of transformations to bond types Clas~ificationof matter and physical transformations: mixtures, solutions, compounds, elements Purification procedures Discontinuities between elements and compounds IV. Chemical change and cavctlent chemical bonds Reactive systems go to unreactive systems Inert gases Reactivity and structure Methane, hydrogen, chlorine, hydrogen chloride Physical properties Substitution reactions: formulas, equations, calcula.tions Chloromethanes Oxygen, water, and carbon dioxide Combustion Chemical energy Chemical geometry V. Chemical change involving metallic and ionic bonds Atomic structure of metals Oxidation and reduction (metals plus non-metals yield ions) NeCI, MgCla, KCI,,MgO Physical properties Simple chemistry Electrolysis to produce Na, Cb, Mg Mein chemistry of electrolysis VI. Periodic table VII. Hydrogen, chlorine, hydrogen chloride Relative attraction for electrons, e.g., stabilities of NeH, NaCl and HC1 Poler covalent bonds Properties of HCI VIII. Pro~ertiesof H.0 physical properties Reaction with HCI Reaction with Na IX. Acids and bases Stoichiometry Titrstion X. Nitrogen and NH~system Equilibrium XI. Polyatomic ions Oxidation of NHa to yield NOaSulfuric acid XII. Bonds between like atoms Carbon chains Multiple bonds Functional groups

VOLUME 35, NO. 2, FEBRUARY, 1958

It is intended that the major emphasis in the course be placed on the problem of how the properties of a compound can be understood in relation to the properties of the elements from which it is constructed. This is one of the major problems of chemistry. Although it has been thought about and worked on for several thousand years, it is only in the present century that any effective insight has been achieved. The road to our present concepts has been through detailed experimental consideration of weight relations of the elements in compounds as begun by Lavoisier rather than the more sophisticated logical analysis of Aristotle. But the considerations of gravimetric composition which led Dalton to the atomic theory were in a sense a detour around the difficulties of translating the ideas of Aristotle and Paracelsus into meaningful experiments. Thus it is intended that only a brief presentation of chemical stoichiometry be made in Part 11. No attempt would he made to give a detailed proof either by physical or chemical methods for the existence of atoms. Since every student is abundantly aware of atoms, nuclei, etc., the presentation would he confined to a discussion of the concept of atomic structure and the definition of terms. Although it is presumed that atomic structure would be presented largely in terms of the "Bohr atom," the strict analogy to a solar system would be avoided. Part I11 includes an elementary presentation of kinetic molecular theory and its application to physical changes of matter. The various bond types are defined and the properties of matter are shown to be a reflection of bond type; The physical changes which matter undergoes are seen to be useful in distinguishmg mixtures, solutions, and pure substances. I t could then be pointed out that this is still quite inadequate for understanding the relation between the properties of even a simple molecule and its elements. It is precisely here in fact that some of the striking features of chemical change can be presented. When, for instance, carbon combines with hydrogen to produce methane why is the compound a gas at room temperature like hydrogen rather than a solid like carbon or when bromine combines with sodium why is the product a solid rather than a liquid and what has happened to the red color of the bromine? Surely such characteristics of chemical transformations have a dramatic quality that can be used t o stimulate student thought and inquiry along the lines which a chemist channels much of his own productive thinking. The major work of the course then follows in Part I V and subsequently with considerations of the role played by various bond types in chemical change. It seems simplest at this juncture to consider covalent bonds first, as compounds with these bonds can be made from starting materials which themselves are covalently bonded. The choice of illustrative material is largely a matter of individual preference. Here methane is used for illustration as it is a simple, naturally occurring compound which lends itself to substitution reactions. Ionic bonds are formed, in general, by reactions of covalent compounds with metals. Thus in this system one is compelled to discuss the metallic bond along with the ionic bond. Again what compounds are considered is largely a matter of taste. The outline suggests substances of considerable industrial impor57

tance. Now that the main bond types have been illustrated, the periodic table can be surveyed with profit. It is suggested that this survey be primarily concerned with the first twenty elements to avoid the complications encountered with the transition elements. The next four sections extend the concept of the covalent bond to include those circumstances where the electrons are not equally shared by the two nuclei. It would seem possible to present acids and bases from the standpoint of the Lewis theory but the principal stress would be on the experimental properties of acids and bases. As further examples of polar bonds some aspects of the chemistry of nitrogen are introduced. Here the student can be confronted with non-mathematical aspects of chemical equilibrium as well as an illustration of solvent systems other than water. Oxidation of ammonia to yield nitric acid introduces the concept of polyatomic ions and those substances whose atoms are linked both by covalent bonds and by ionic bonds. Further examples can be obtained by a consideration of sulfuric acid and its commercial production. A preliminary estimate of the amount of time required to present the material included in the outline indicates that there would be room for additional material. Some teachers might than choose to expand one or another of the listed topics by adding additional descriptive material of particular interest. In other situations it might prove desirable to add other topics. For example, nuclear transformations although not directly involved with chemical bonds are of great interest to many students. Certainly there are a variety of industrial processes and consumer goods that can provide additional insights into chemical phenomena and may prove useful in certain classrooms. Most high school chemistry courses provide some facilities for lecture demonstrations and for laboratory work: These activities are to be encouraged. We do not, however, propose to discuss these important matters at any length in this paper. It seems reasonable to believe that experiments appropriate to the course outlined can be devised or secured. Each section of the outline suggests by implication a variety of possibilities.

Not every student taking high school chemistry will go to college nor will all of those who do go to college take further work in chemistry. Thus an effective high school course must be one in which the teacher can assume responsibility for students of different interests. The great power of physical science lies in the fertile combination of conceptual schemes with observation and experiment. With the concept of chemical bonds linked to a rich variety of descriptive chemistry, the teacher should have an opportunity to s h o the ~ student the scientific operation in as true a perspective as possible.. Although the proposed course may represent an increased emphasis on abstract ideas, this is the result of limiting major attention to a single concept with varied illustrations. This should give greater possibilities for interpretation and clarification to aid the student whose main interest may be centered outside the physical sciences. A course such as outlined here should give the student a better insight into what Conant calls "the tactics and strategy of science" than the present courses based on typical textbooks so far available to the high school teacher. ACKNOWLEDGMENTS The ideas presented in this paper and many of the details of the course outline were first discussed in Subcommittee "C" of the Reed College Conference. The participants in the subcommittee were: Richard Miller, Los Angeles High School, Los Angeles, California Harley Reifsnyder, Portland High School, Portland, Oregon Robert D. Sprenger, College of Puget Sound, Tacoma, Washington Violet Strahler, Stivers High School, Dayton, Ohio Laurence E. Strong, Earlham College, Riohmond, Indiana Donald Swinehart, University of Oregon, Eugene, Oregon Dorothy Tryon, Redfard High School, Detroit, Michigan Harold V. Wik, Union High School Dist. No. 10 Joint Washington and Multnomah Counties, Beaverton, Oregon M. Kent Wilson, Tufts University, Mediard, hIessachusetts

In addition, the authors were given valuable assistance through discussions with William Kieffer, College of Wooster, Wooster, Ohio and Ralph Beebe, Amherst College, Amherst, Massachusetts.


IMPROVING THE QUALITY AND EPFECTNENESS OF INTRODUCTORY COLLEGE PHYSICS COURSES A thorough and rigorous coverage of 5 limited number of topics is more effective than an encyclopedic and showy introduction to a wide range of subject matter. I t is ~robsblethat no course of less than six semester hours can mesent adeaustelv the hasio

structure of the stam. Physics should be taught as a. growing subject and the student should be given illustrations of problems on present frontiers. Introductory physics courses should be ~vailsbleto freshmen. This may make it necessary for the instructor to introduce mathematical ideas. such as those of the calculus. in order that the subject be developed with the desired intellec&l rigor. Senior and experienced staff members should engage in the teaching of introductory physics courses, in the training of teaching assistants, and in experimentation directed at the improved teaching of physics. -From

the Report of the Amerioan Association of Physic8 Teaohera: American Joumol of Phyaics, P6, 417 (Oot.,1957)