Prospectus: Concepts of Chemistry

Nicholas Kildahl

Department of Chemistry and Biochemistry

The Worcester Polytechnic Institute

Never before has it been more evident that chemistry is an evolving, dynamic process rather than a static, stale body of knowledge. Things are changing in chemistry; the science of chemistry is taking new and exciting directions that emphasize its role as the central science. Recent advances in the understanding of molecular recognition, molecular scale mechanical and electrical devices, the details of biological processes, the mechanisms of molecular events on the femtosecond time scale, the electronic structures of molecules revealed by molecular modeling, and the advantages of a combinatorial approach to organic synthesis have been profound. Partly as a result of these marvelous research advances, there is ferment in chemical educ ation, as the realization has dawned that what we teach and how we teach it at the general level is no longer adequate to prepare our students for a modern understanding of chemical principles. A Task Force on the General Chemistry Curriculum, created in 1989, has examined the curriculum of general chemistry at colleges and universities in the United States, and has developed a number of recommendations for curricular reform. At the core of these recommendations is the realization that the teaching of general chemistry must reveal to students the essence of modern chemistry. It is time for a change.

It is in the spirit of change that Concepts of Chemistry has been created. It has been used "in house" at the Worcester Polytechnic Institute since the fall of 1995, and has been refined and improved in response to student and instructor feedback. Although it was originally conceived and written prior to the appearance of Task Force recommendations, it turns out to be entirely consistent with these recommendations. Thus the text is relatively short in comparison with most "traditional" general chemistry texts, consisting of 20 chapters spanning some 500 pages. The first fifteen chapters address the conceptual areas (themes) that, in the author's opinion, are at the heart of chemistry: molecularity (the atomic/molecular view of matter); intramolecular and intermolecular forces; physical and chemical equilibrium; chemical dynamics; and chemical reactivity. Discussions of chemical reactivity are woven throughout this block of chapters, rather than concentrated together in a single chapter or contiguous series of chapters. In this way the concepts and realities of chemistry are intertwined. The last five chapters of the book focus on applications of concepts and principles to significant modern applications: Materials; Biochemistry; Structure-Function in Chemistry; Transition Metals, and Catalysis.

The Author's Vision. Concepts of Chemistry has emerged as the result of a number of factors. I have been long frustrated that textbooks of general chemistry present the subject as a potpourri of 25 or so topics, each awarded its own chapter, that though important, seem completely unrelated to many students. Although the major themes of chemistry are undoubtedly clear to the authors of these texts, the relationships of the topics to the themes are not stressed. The result is that the student completes the course with no appreciation for the relative importance and utility of the multitude of topics to which s/he has been exposed. The need in my own teaching for a text that is clearly and consistently focussed on a set of identified major concepts has been a strong motivation. Second, in my struggle to become a competent teacher, I have continually sought new ways to present old ideas; in particular, I have attempted to substantiate my conviction that the ability to convey the essential simplicity of a scientific idea is a sign of true understanding. I believe that I have had some success in developing new approaches to at least a few important ideas, and the time seems right to move them from the classroom to the textbook format. Third, the exciting developments in chemistry mentioned above, and the emphasis on the molecular view that they entail, place different demands on the instructors who must prepare students to think and function at the molecular level. The early training must be consistent with the projected professional needs.

These factors, then, have been operative in shaping my vision of a textbook. Concepts of Chemistry is my attempt to create a textbook that meets my own teaching needs. It is my hope that it also meets the needs of not only other teachers, but of the students who use it. In it, I have attempted to

If I have been true to my vision, you may find some unusual viewpoints and interpretations in this text--I hope so. I hope that they stimulate you to question and to think about how you teach chemistry.

Organization. In creating Concepts of Chemistry, I have abandoned the almost universal organizational scheme that has dominated general chemistry texts for a number of years in favor of an approach that puts the atomic-molecular viewpoint of the chemist "up front." Indeed, all of the modern research areas mentioned above focus on chemistry at the molecular level, rather than the macroscopic level. An early, steady exposure to the manner in which chemists think about molecules is, perhaps as never before, essential. Thus the quantum nature of the atom appears in Chapter 2 of Concepts, and ideas of molecular structure and stereochemisty in Chapter 3. This is to be contrasted with the traditional placement of atomic and molecular structure no earlier than Chapters 6-8 of a 25 chapter text. Molecular stereochemistry is given particularly thorough treatment in Chapter 3 in the context of the VSEPR theory of Gillespie and Nyholm. After much thought, I have opted to retain the electron group approach to shape, rather than adopt the newer "electron domain" model advocated by Gillespie and others. My justification for this is that thinking in terms of electron groups is conceptually no more difficult for students than thinking in terms of domains, which were originally offered to avoid the necessity to talk about atomic orbitals. Atomic orbitals are, to the best of our current knowledge, real. Their significance in covalent and metallic bonding must inevitably be discussed. There is no doubt about it, nature is strange! But we must not protect our students from its strangeness. Chapter 4 of Concepts is as far as I know unique to this book. In this chapter, spectroscopic methods are introduced and their central role in molecular structure elucidation is described and illustrated. The physical aspects of the interaction of light and matter are framed in terms of the matching of frequencies of electromagnetic radiation and molecular motions, and analogies with familiar macroscopic systems are drawn. By the close of Chapter 4, the foundations of the molecular view have been laid. The structure of the atom, basic aspects of the bonding in and stereochemistry of covalent molecules, and the experimental methods that enable us to "know what we know" about these unimaginably tiny systems have been discussed "up front."

Chapter 5, dealing with the properties of gases and with the kinetic molecular theory, begins a series of chapters in which the ideas of intramolecular and intermolecular forces and bonding are developed. An understanding of the relationship between temperature and molecular kinetic energy developed in Chapter 5 prepares the student for the discussion of forces and potential energy wells in Chapter 6. Coulomb's law and the potential energy well are central to an understanding of all attractive forces between atoms and molecules; their importance is given due emphasis in Chapter 6. Chapter 7 is devoted to a treatment of the condensed phases of matter and to an introduction to phase transformations.

The first law of thermodynamics is developed and applied to phase transformations and chemical reactions in Chapter 9. This chapter, while reinforcing the concepts of forces and bonding, is the first in a series of chapters intended to reveal the essence of physical and chemical equilibrium. Chapter 10 is devoted to a thorough discussion of phase equilibrium. The macroscopic aspects of phase equilibrium are rationalized in terms of the dynamic aspects of the equilibrium at the molecular level. The entropy concept is introduced in a qualitative manner in the context of phase transformations, then in a more thorough quantitative manner in Chapter 11. Entropy is inevitably a difficult concept for undergraduate students; it is made more difficult when introduced in the traditional context of ideal gas expansions. In Concepts of Chemistry, the physical significance of the quantitative definition of entropy in terms of heat and temperature is clearly delineated. Absolute entropies are presented and discussed in a common sense manner, then utilized to determine changes in entropy in chemical and physical processes. The link with disorder is continually emphasized to assist the student in making sense of entropy changes. The Gibbs Free Energy is introduced, but the usual (and questionable) "derivations" are avoided. Chemical equilibrium is introduced in the context of gas phase processes in Chapter 12, with emphasis placed on a graphical interpretation of equilibrium. A bonus of this graphical approach is that it subtly introduces the dynamic aspects of reactions, which are treated thoroughly in Chapter 15. Chapter 13 extends the ideas of equilibrium to acid/base systems, and (qualitatively) to solid-liquid and gas-liquid phase equilibria. Finally, in Chapter 14, electron transfer processes are considered, and their importance in biological systems is emphasized.

In my experience, reaction stoichiometry is perennially difficult for many students. Yet it is of fundamental importance in the development of an understanding of concepts of equilibrium and reactivity. In Concepts of Chemistry , stoichiometry is treated in three different chapters, with the main thrust postponed until Chapter 8. Chapter 1 introduces the basis for stoichiometric relationships, which is the idea of conservation of atoms (moles of atoms). This idea is applied to chemical reactions in atom and mole terms only in Chapter 1, and again in Chapter 5, where reactions producing gaseous products are discussed. Finally in Chapter 8, the full power of the mole concept in dealing with reaction stoichiometry is developed. Despite the fact that reaction stoichiometry involves little more than simple counting, I believe that students have difficulty with it because they do not have a concrete grasp of what they are counting. Chemical equations involve abstract symbols (formulas) for abstract entities (atoms and molecules). This stratification of abstraction is too much for many students. In an attempt to concretize the ideas of stoichiometry, I have implemented a teaching approach that I have used for many years in the classroom: that the chemical equation is in many respects similar to a recipe. In my experience, students can effortlessly solve limiting reactant problems formulated in terms of a recipe, because they have concrete experience with recipes. By stressing (in Chapter 1) and restressing (in Chapter 8) the parallel between chemical equations and recipes, I attempt to make more sense of stoichiometry for the student. I hope that this attempt is successful.

In addition to the primary conceptual areas (atoms and molecules, forces and bonding, equilibrium, dynamics, and chemical reactivity), there are a number of what might be called "nuts and bolts" aspects of science that are fundamental to a well-rounded understanding of the scientific process. A number of these are bulleted below.

  • An appreciation for the significance of measurement in science.
  • An appreciation for what to do with data once you have it. In this context, tabular and graphical methods of presentation are important, as is the ability to "make sense" of tabulated and graphically presented data.
  • An appreciation for the use of modern technology in asking and answering questions. This ranges from the relatively simple applications of spreadsheet software in data treatment and presentation, to sophisticated molecular modeling software and mathematical and statistical packages.

In Concepts of Chemistry, I attempt to involve students with computer applications in chemistry. Thus sections devoted to the use of spreadsheets in equilibrium calculations and kinetics analyses are included in Chapters 12 and 15, respectively. Special Spreadsheet Applications are included with the problems at the end of each chapter. And output from molecular modeling software appears in a number of figures throughout the text.

Using the Text/Flexibility. Instructors are quite naturally interested in knowing whether or not a new text is adaptable to the variety of subject matter orderings that exists in general chemistry courses. Is the book flexible with respect to order of coverage, or does it need to be taken in sequence? I offer two thoughts before addressing this matter. First, chemistry (or any science) is by its cumulative nature inherently inflexible. Certain ideas must be in hand before others can be grasped. Thus one cannot expect to succeed in a junior level course in physical or inorganic chemistry if one has not learned the basics of chemistry in earlier courses. In writing a textbook, one builds a structure, with each successive brick laid on and interlocked with those laid previously. All texts in general chemistry are thus by necessity cumulative, and are therefore, also of necessity, only minimally flexible. An attempt to offer flexibility often suffers from lack of coherence, and so exacerbates the problem of perceived unrelatedness of subject matter. In my opinion, "flexible chemistry text" is almost a contradiction in terms! Second, Concepts of Chemistry offers a new organization to address modern needs. To use it to maximum effectiveness will require some course restructuring. Having made these comments, I will point out the flexibility that does exist. I believe that the first 3 chapters should be covered first, in order, because they bring the student to grips with the molecular view at the outset. This chemist's viewpoint can then be built upon and relied upon throughout the course. Following this though, it is possible to address the main thrust of stoichiometry in Chapter 8; or to proceed to gases in Chapter 5; or even to proceed to energy in Chapter 9; while postponing Chapter 4 until later in the course, or even omitting it altogether. Should the choice be made to proceed to Chapter 9 following Chapter 3, the instructor must bear in mind that many of the ideas in Chapter 9 depend, directly or indirectly, on the ideas in Chapter 6 (molecular forces and bonding). Thus student understanding of energy ideas may not be as clear as if Chapter 6 had been read first. In my opinion, it is necessary to talk about gases and molecular forces (Chapters 5 and 6) before discussing the condensed phases (Chapter 7) and phase equilibrium (chapter 10). These chapters, then--5, 6, 7, and 10--should probably be taken in order. Chemical equilibrium is normally covered no earlier than the midpoint of a year-long course, and is about the same place in Concepts as in most texts. Similarly, the chapters dealing with acids, bases, and salts (Chapter 13), electron transfer processes (Chapter 14), and chemical dynamics (Chapter 15) are not unusually placed. Chapter 15 can probably be taught before equilibrium if desired, although the two concepts are intertwined. Finally, Chapter 11 dealing with entropy and free energy, may be taught prior to or following equilibrium, with appropriate adjustments in the coverage of both.

My intent in the particular organization that I have chosen is that the foundation chapters 1-15 be read first, so that the conceptual framework developed by the student can then be brought to bear on some of the exciting modern areas addressed in Chapters 16-20. It is possible, however, to work a couple of these chapters into earlier slots if desired. For example, Chapter 19 (Materials) can be understood with the ideas of Chapters 3, 6, and 7 in hand. Chapter 18 (Biochemistry) can be inserted once energy concepts from Chapter 9 have been learned, omitting of course the references to kinetics.

Intended Audience. Concepts of Chemistry is intended for well-prepared students. It is written at a high level, involves frequent use of calculus, and incorporates very few "frills." Important ideas are presented, explained, and amply illustrated; but the technique of stating an idea over and over again, common in traditional texts, is not used. There are no marginal notes, no "boxes" dealing with modern research topics or applications (these are mentioned in the flow of the narrative), no key terms and end-of-chapter summaries, and there is no battalion of supplementary materials. Although there are numerous figures, they are drawn with economy of style. There are no photographs used in the book. Concepts is a simple, straightforwardly formatted textbook, written with economy of style. I expect that it will be most suitable for use in advanced or accelerated general chemistry courses, populated by students with an interest in and intention to major in a scientific or engineering area. At the Worcester Polytechnic Institute, it has been used for two years in a 1 semester general chemistry course for students intending to major in chemistry, biochemistry, chemical engineering, and biology and biotechnology. In this context, it has been successful.

It is very clear that the personal computer is rapidly assuming the central role in education. Exciting visual and textual material useful in the teaching of chemistry is proliferating on the World Wide Web and in the compact disc format. Video segments illustrating reaction dynamics and molecular motions are readily available. In the opinion of the author, the extensive, expensive, 3-color, multi-supplemented textbook of general chemistry is becoming superfluous. Instead, what is appropriate in computer-centered education is a basic, inexpensive, conceptually oriented reference text in which students can read a clear, concise exposition of fundamental chemical concepts. Such a text should be easy and economical to produce, and should put a minimum burden on the student pocketbook. It should contain essential diagrams and structures, but these need not be 3-dimensional or even colored. It should contain a few well-designed problems to supplement problems supplied via computer format. The fancy stuff comes from the computer; there is no longer a need for it in the textbook. Concepts of Chemistry is a text of this type. It is a text for the future.

Outstanding Features. A number of interesting and/or unique features characterize Concepts of Chemistry.

  • The organization stresses the major conceptual areas--themes--of chemistry:

    The atomic/molecular view (i.e., Molecularity)
    Intramolecular and intermolecular forces
    Equilibrium
    Dynamics
    Reactivity
  • Conceptual understanding, rather than mechanical algorithmical performance, is emphasized.
  • Each chapter begins with the major concept area addressed, and a listing of specific subconcepts dealt with in the chapter.
  • The format is simple (no marginal notes; no "boxes").
  • A pictorial/graphical approach to concepts is used as much as possible.
  • The book is consistent with task force recommendations: core concepts, modular concept applications.
  • Concepts are applied to exciting current areas of research.
  • It is written at a high level, intended for an accelerated track.
  • Supplements at the ends of several chapters provide optional approaches to material, or interesting applications of concepts.
  • Each chapter contains several worked-out examples. However, these are not excessively numerous.
  • End of chapter exercises are in two categories: Applications, which are essentially problems of the typical type; and Spreadsheet Applications, which encourage the student to explore the uses of spreadsheets in data analysis. Applications are designed to give the student a work-out on concepts discussed in the chapter. They are in no particular order of degree of difficulty, and are not grouped by concept. In most chapters, they encompass both conceptual and numerical problems.
  • Traditional topics that have proven of little use in the professional careers of students have been deemphasized or omitted:

    nApplications of the first law of thermodynamics to gas expansions have been retained, but their extreme importance in environmental and technological contexts has been stressed.

    nRaoult's Law is included, but it is made very clear that almost no real solutions obey it.

    nColligative properties, which are no longer much used, have been deemphasized.

    nQualitative aspects of entropy have been given precedence over quantitative aspects. In particular, entropy changes of gas expansions have not been treated; absolute entropies have been treated in a common sense way that omits discussion of how they are measured.

    nThe Arrhenius view of acids and bases is omitted.

    nA different approach to acid-base equilibrium emphasizes distribution diagrams rather than calculation of pH in a solution of a weak acid/conjugate base.

    nKsp calculations have been deemphasized, because they vastly oversimplify reality
  • Applications of computers in data handling and presentation are illustrated. In paricular, use of spreadsheets in the tabulation, manipulation, and presentation of data, and in solving problems in equilibrium and kinetics, is emphasized.
  • The book offers simple interpretations of complex ideas:

    nStoichiometry is concretized by presenting the chemical equation as a recipe.

    nThe strangeness of quantum ideas is stressed rather than camouflaged, and the point is made that the quantum behavior of matter and energy is something one gets used to, not something one understands.

    nThe shell structure of the atom is grounded in photoelectron spectroscopy data.

    nMolecular structures are interrelated by the isovalent fragment idea (Chapter 3), allowing simple prediction of reasonable molecular formulas.

    nThe treatment of molecular dipole moment is expanded, using simple symmetry ideas and the vector addition of bond dipoles. The treatment emphasizes the importance of this extremely important but neglected topic.

    nMolecular spectroscopy (IR, NMR, UV-Vis) is introduced in Chapter 4. The coupling of light oscillations with molecular motions and the concept of frequency matching are stressed.

    nThe importance of gas expansion processes is explicitly acknowledged and discussed.

    nThe origin of the metallic bond is presented in terms of exposed core charge.

    nThe structures of solids are clearly discussed.

    nThe molecular view of phase changes, phase equilibrium, and chemical equilibrium is clearly presented.

    nOsmotic pressure is interpreted physically as the force required to offset the tendency of a solute to expand in a solution, just as pressure offsets the tendency of a gas to expand.

    nEmphasis is placed on the qualitative aspects of entropy, and in particular on the relationship between entropy and the extent of aggregation.

    nThe underlying rationale for acidity in aqueous solution in terms of Lewis acidity of cations is revealed.

    nThe approach to buffers is qualitative, emphasizing understanding of how they work rather than calculations.

    nThe strangeness of oxoacid formulas is confronted and demystified.

    nThe oxidation levels of the elements are discussed in terms of Pourbaix Diagrams.

    nElectron transfer and proton transfer reactivities are presented in terms of predominance diagrams.
  • Throughout the text, a graphical, pictorial approach to ideas is emphasized:

    stoichiometry
    thermodynamics
    equilibrium
    acids/bases
    solubility
    kinetics
    electrochemistry
  • Finite difference methods are applied to kinetics analysis.

Acknowledgements. I am extremely grateful to a number of colleagues and students who have provided me with comments and suggestions for improving Concepts of Chemistry: Ladislav Berka, for reading and critiquing the manuscript; Ken Balkus, for encouraging me to finish the book when my energy flagged; two generations of students of CH101X and CH102X, a sequence of two experimental courses developed with funding from the Pfizer Foundation, who read and used the book, discovered errors in it, and hopefully learned from it; and a group of very special high school students, to whom I taught a 7-week course in chemistry in the Spring of 1996, and who gave me very enthusiastic and encouraging feedback on Concepts. Thanks go to Richard Herrick, College of the Holy Cross, for the opportunity to spend a sabbatical semester at HC, observing the Discovery Chemistry program in action and working on Concepts. In addition, I thank Rick for many interesting and stimulating discussions on teaching chemistry. Finally, I thank my wife Cathryn, whose warm and loving support at home has given me the strength to put my Concepts of Chemistry down on paper.