provocative opinion The Changing Roles of Descriptive Chemistry Integrating Reactions and Properties with Theories and Principles of Inorganic and Organic Chemistry Ronald J. Gillespie McMaster University, Hamilton, Ontario L8S 4M1 In the context of the introductory (general) chemistry course "descriptive chemistry" usually means the discussion of the ~ r e ~ a r a t i o. .properties, n. . and reactions of substances, pa&;larly inorganic sub&ances. But chemistry has lonp since ceased to be a ~ u r e lor s even lanzels descriptive subject. In all areas ofchemiitry we attempt to interpret the properties and reactions of substances in t e r n of models, theories and general principles, and the discussion of the facts concerning the properties of substances and their reactions is inextricably mixed with the discussion of these theories and principles. Not surprisingly the term d the con"descriotive cbemistrv" has d i s a ~ ~ e a r eoutside text of 'the general chemistry course. There is no ACS division for "Descriotive Chemistrv" and there are no courses in descriptive chemistry in the chemistry major program. But the term has lingered on in discussions concerning the content of the introductory (general) chemistry course, where it used to describe the reaction chemistry of inorganic substances. Forty years ago the description of inorganic substances and their reactions was a major part of the introductory course. From around 1960 onward, however, increasing amounts of this material were eliminated from the course and replaced by theory and principles, such as more indepth treatments of bonding and thermodynamics. However, beginning in the 1980's it was realized that many students, even those graduating from a chemistry majors oromam. had a verv sketchv knowledm of even the most La& facts about common su"bstances. consequently, there was a move to return some of this "descri~tive"material to the course. Typically, this has been done by adding additional chapters to the textbooks, either at the end or sometimes dispersed between the conventional principles chapters. This resulted inevitably in the increased size of textbooks to over 1000 pages. Since the beginning of the present decade there has been a growing realization that these books now contain considerably more material than can be covered in the usual two-semester course. Not ody are these books too heavy to carry to class and too expensive for some students to buy, but also the thought of having to work through these enormous books during the short academic year is overwhelming for many students. Moreover, because all the material in these books cannot be covered, it is most often the last chapters in the book that are omitted. So, despite the demand for the inclusion of "descriptive chemistry" in textbooks, it frequently does not form a significant part of the course that the student actually receives. It also is often omitted because when it is presented as a rapid survey of the groups of the periodic table, more or less independently of the other topics in the course, it is regarded as dull and boring both by students and by instructors, and as an exercise in memorization
rather than understanding. The solution to this problem is to recognize that there is no subject " Descriptive Chemistry" and to integrate the discussion of the properties and reactions of substances, both inorganic and organic, into the introductory course, just as is done in modem presentations of inorganic and organic chemistry. Models and theories have been developed to explain the ~ r o ~ e r t i of e s substances. Students should have some bdwledge ofthe fads that led to the development ofa particular model or theory. They can then see the need for the model and better understand that new observations can cause a theory to be modified or abandoned. Despite the good discussions of the nature of thr scientific method and of theories and models that arc found in m3n.y texthooks, theories and models frequently are presented in an authoritative manner that suggests that they are immutable laws of nature. Such a presentation does not give students a correct impression of how science is really done. For example, acids and bases frequently are defined first as proton donors and acceptors without any prior introducof substances tion to the characteristics and properties . . that have come to be called ands and bases. Simllarl.v,0x1dation and reduction are defined in terms of electron loss and gain without any prior introduction to the reactions that led to these definitions. Only after the terms have been defined are a few reactions given as illustrations, but the emphasis remains much on the theory. It is better science and better pedagogy to put fads before theories and to put the concrete before the abstract. The need for the theory becomes apparent, and this leads to a better understanding of the theory and a better retention of the facts. We should, therefore, abandon "descriptive chemist$ as a topic in the introductory course and integrate the reaction chemistry of inorganic and organic substances with d Doing this would the appropriate t h e o ~ ~ a npnncipies. enable us to lead into models and theories on the basis of the observed ~ r o ~ e r t i and e s substances and their reactions. Apart fiom'the pedagogical advantages of this type of oresrntation i t also is more efficient and time swine because having used descriptive chemistry to introduce keories and principles it is not necessary then to repeat the same facts in a survey of the groups of the periodic table. Indeed, there is no need in an introductory course to do a group by group discussion of the periodic table. This should be part of a following inorganic course for chemistry majors. It is better to use the chemistry of a relatively few of the more common elements to introduce theories and principles. The choice of the elements that should be discussed is to some extent arbitrary, but if it they are chosen on the basis of their abundance, their occurrence in common substances, and in life the choice becomes fairly obvious and would include, for example, the nonmetals H, C, Volume 71 Number 8 August 1994
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N, 0, ,'F S, and the halogens, and among the metals Na, K, Mg, Ca, Al, Cu, and Fe. A vast amount of important and relevant chemistry can be based on the properties and reactions of just these elements and their compounds. By limiting the "descriptive" chemistry to such a short list of elements and using the same or similar reactions to introduce and to illustrate different theories and principles, it bewmes possible for the student to digest and remember some ofthe common fads about substances. This wnsiderably reduces the chances of students being overwhelmed, as many are, by the wide variety of different reactions of many different elements that often are used to illustrate theories and principles. The following are some examples of how inorganic and organic reaction chemistry can be used to introduce theory and principles: The elementary chemistry of nitrogen, oxygen, and hydrogen provides enough example of simple molecules such as HzO, NH8,CHs and COz that the wncept of molecular geometry and the VSEPR model can be introduced. Some simple chemistry of the halogens can, for example, be used to introduce three important reaction types: acidbase, redox, and precipitation. The chemistry of elements such as sulfur and phosphorus that have several different oxidation states is useful particularly for introducing the ideas of oxidation state and oxidation number and expanding the discussion of acids and bases to include the oxoacids.
666
Journal of Chemical Education
The chemistry of the hydrocarbons, and particularly their combustion mactions, leads naturally to a discussion of energy changes in reactions. The properties of some common metals such those mentioned above can lead to a discussion of the metallic state and the metallic bond. The chemistry of aluminum can be used to introduce Lewis acid-base theory. This seems preferable to using BF3 as an example of a Lewis acid, because this substance and its complex with ammonia BF3.NH3are not met by the student in any other context. The chemistrv of iron and of w ~ ~can e be r used to revisit the concept of kdox and to lead kturaIly to the discussion of com~lexions and to the t o ~ i of c electrochemistrv(auanLet us teach chemistry as it really is and not as it has been artificially subdivided into theory and principles on one side and "descri~tive"chemistrv on the other. Let us show students that'the need to explain and rationalize facts leads tn the develo~mentof theories and not eive the impression that theories dictate facts. Let us shiw students that a knowledge of theories and principles is useless ifthey do not have a basic background of facts that can be understood and rationalized in terms of these theories and principles. Let us stop boring students with "descriptive" chemistry. Let us integrate the reactions and properties of inorganic and organic substances with the theories and principles. In other words, let us teach modern inorganic and organic chemistry in the introductory course.
