mthe forum aa0
Descriptive Chemistry in the General Chemistry Course A New Approach Ronald J. Gillespie and David A. Humphreys McMaster University, Hamilton, Ontario, Canada
Why is descriptive chemistry widely believed to be a dull catalogue of unrelated facts? Why are many instructors not enthusiastic about teaching it? Why, in the textbooks, is it often relegated to the back or to interchapters? Why, nevertheless, is it believed by many instructors and textbook authors that descriptive chemistry should be included in the course? What can be done to make descriptive chemistry a mow interesting part ofthe course'!What, in fact. is descriotlve chemistrv' I n thisarticle. we attempt to answer someof these quesGons. We believe that descriptive chemistry, far from being dull and irrelevant to general chemistry, can be an interesting, important, and integral part of the course that provides essential background for the theory and principles, and provides students with basic knowledge that will be useful to them in other medical, scientific, and engineering studies and in evervdav . life. In this article we discuss how we have introduced descriptive chemistry into our course so that it is relevant to the theorv and principles that are an important part of the courseBnd relevkt to the students in their further studies and in their everyday lives.
-
Descriptive Chemistry What do we mean by "descriptive" chemistry? The use of this term is largely confined to the general chemistry course where it is used as a synonym for inorganic chemistry, even though organic chemistry is equally descriptive. In a wider context the term has little meaning. Descriptive chemistry is not a recognized area of chemistry, like physical. analvtical. inoreanic. or oreanic chemistrv. The term descriptive chemistry is dften t&en to signif; the purely descriptive aspects of inoreanic chemistrv. But it is very diffidt, and certainly nocdesirable, to separate the discriptive and theoretical aspects of inorganic or organic chemistry. The term has little useful meaning and what we should be discussing is the role of inorganic and organic chemistry in the freshman course. Inorganic Chemistry in an Integrated General Chemistry Course The logic behind placing inorganic chemistry a t the end of a freshman text, as often has been done, is that the principles that have been discussed earlier can be used to rationalize the facts of inorganic chemistry However, the usual "Group by Group" treatment does little to counteract the widespread opinion among students and instructors that this part of the course is a "dull catalogue of facts". Consequently, partly because it often is viewed in this way, partly because of the shortage of time in an already overloaded course, and partly because of a perceived dimculty of testing this "factual" material, this part of the course often gets a rushed and inadequate treatment, that is not
528
Journal of Chemical Education
inspiring or useful for the students. There is, however, no need in the general chemistry course to range right across the periodic table in a m o u. ~by - -mu^ treatment, as one wouid in an inorganic &emistry course. ~everthefess,any eeneral introduction to chemistry should include a n introiuction to inorganic chemistry &d to organic chemistry as well as to physical chemistry, by which we mean the theory and principles such as equilibrium, thermodynamics, structure and bonding that usually are discussed in the general chemistry course. In order to discuss the principles, for example, of redox or acid-base reactions, or of chemical equilibrium, we need some facts of "descriptive" chemistry to provide a context for the discussion. In a conventional principles based approach, example reactions often areused to illustrate-a concept or principle, but they are presented without any prior introduction to the chemistry of these reactions. Examples are drawn from all over the periodic table, the criterion for choice beine onlv " that thev illustrate the ~rinciple under discussion. The emphasis is on the principle and students learn little about inorganic chemistrv. -. because the properties and reactions of substances are not presented in any organized or coherent manner. Since we need examples to discuss principles, in other words. we need facts before theorv, why not first discuss some descriptive chemistry which we can then use to introduce some theorv or princi~le?Not onlv does this order refled the way in khich a science develops, but in our experience it has important pedagozical advanta~es.We are not suggesting that a detailed Goup-by-group discussion of inorganic chemistry should precede any discussion of principles and theories. This would be worse than discussing inorgank chemistry a t the end of the course. In our experience a "little by little" approach is very effective; some descriptive chemistry followed by some appropriate theorv and nrinci~lesleadim to some more "descriotive" ~ - - ~ chemistry and so on. Using this approach we can illustrate theories and nrinci~lesin terms of "descriotive" chemistrv that the stuient h i s encountered previously so both the descriptive and theoretical aspects are made more understandable and more interesting for the student. At the same time as eainine a n understandine of the principles. the student gradualli builds up a knowredge of some casic facts of inorganic chemistry. Throughout our course, we use only examples of reactions or properties that have been described earlier. So students are not triine - - to understand some new concept or principle in unfamiliar terms. As the course proceeds the students acquire an increasing knowledge of the properties and reactions of substances that can be used to give a proper context to theory and principles. They have time to assimilate the descriptive material. The use of the same properties and reactions in A
-
. ~.
-
Outline of the Integration of Descriptive and Theoreticalchemistry in a General Chemistry Course (not a complete course summary)
"Descriptive"
"Theoretical"
Gaseous elements - H, N and 0 Gas Laws Periodic Table VSEPR Halogens
Reaction types: acid-base. redox, precipitation (qualitative discussion) Eiectronegativity
Phosphorus and Sulfur
Oxidation states and numbers, oxoacids. Lewis Structures
Carbon and Hydrocarbons
Thermochemistry (combustion of hydrocarbons)
Metals, Na, K, Mg, Ca, Al, Fe, Cu
Metal structure and bonding Electrochemistry
Owanic
Reaction rates and mechanisms
different contexts reinforces this material in the students' minds. Inorganic chemistry is a vast subject, and any comprehensive coverage is impossible. So we limit our discusssion to fewer than 20 common elements, that, with the exception of iodine come from the first four periods. Even then we need some criteria for choosing the material that we can cover in the limited time available. We choose chemical reactions and properties, as far a s possible, because they are already familiar in some way to the student and important in everyday life, or are important in other fields such as environmental science, materials science and biochemistry, a s well a s for their usefulness for illustrating some important concept or principle. The table gives a brief outline of a n organization of some of the material of a general chemistry course that in the authors' experience works well. This is not a complete outline of the course, but is meant only to show what is meant by a n integrated course. No doubt, alternative ways of integrating the inorganic and physical chemistry could be devised. In the data given in the table some simple chemistry of hydrogen, oxygen, and nitrogen mainly in relation the atmosphere is first discussed followed by the gas laws. The student is then familiar enough with a few elements and their compounds that it is appropriate to introduce the periodic table and some basic ideas of bonding, simple Lewis structures, and a n introduction to theVSEPR model of molecular geometry. Next a discussion of the chemistry of the halogens provides a n ideal opportunity to describe three important reaction types, redox, acid-base and precipitation, and to discuss them in a qualitative manner. Electronegativity and polar covalent bonds also can be introduced in the context of halogen chemistry. The chemistry of phosphorus and sulfur can be used tointroduce oxidation states and oxidation numbers. Oxoacids can be introduced with sulfuric and phosphoric acids as examples, and strong and weak acids can be discussed qualitatively. By this time the student is familiar with enough molecules that it makes sense to review the writing of Lewis structures to cover a wider variety of molecules. Any detailed discussion of how to write Lewis structures before the student is familiar with a ranee of molecules tends to be a somewhat meaningless aniboring exercise. Adiscussion of carbon and the hydrocarbonssets the fitage for thennochemist~yfor which
the combustion of hydrocarbons provides useful examples. Having discussed several important nonmetals we turn to metals. Again, we discuss only a few selected examples such as sodium, potassium, calcium, magnesium, and aluminum, and a couple of examples of common transition metals such as copper and iron. In the context of the chemistry of these metals, we can discuss the general properties of metals and the relation of these properties to the structure and bonding of metals. We are then ready for electrochemistrv in which we nut redox reactions on a auantitative basis. At around this stage in the course, the students have enough aualitntivc knowledge of acid-base and other equilibriathai they are ready fora quantitative discussion of equilibrium. We believe that a thorough qualitative understanding of equilibria is essential before a student can tackle numerical problems and that this understanding is more important than doing "chemical arithmetic". Our discussion of inorganic chemistry is, somewhat unconventionally, not tied rigidly to the periodic table. Althoughit is important that students know and understand the periodic table it is not particularly useful at this level to stick rigidly to the periodic table groups in the discussion of inorganic chemistry. For most groups the first two elements are by far the most important a t this level and the similarities between these elements, for example, between carbon and silicon, between nitrogen and phosphorus, and between oxygen and sulfur, are not strong so we discuss sulfur and phosphorus together and separately from oxygen and nitrogen, for example. The few elements on which we place most emphasis : H, Na, K, Mg, Ca, Al, C, N, P, 0 , S, F, C1, Br, I, Cu, Fe are among the most abundant in the earth's crust and the most important elements in living matter. There seems no need a t this level to include As, Sb, and Bi to complete group V or Se and Te to complete Group VI, or to deal with more transition metals, for example. Organic Chemistry in an Integrated General Chemistry Course
Traditionally, organic chemistry has played a very minor role in the general chemistry course. Mainly it seems because the general chemistry course is usually followed by a n organic chemistry course for chemistry majors, premeds and some other students. But many students in the general chemistry course do not take the organic course. So they miss a n important p a r t of chemistry t h a t illustrates so well what many chemists spend most of their time doing-namely, synthesizing new substances. This is an aspect of chemistry that appeals to many students, as it does to many chemists. Organic chemistry deals with substances that are of importance to life and, therefore, of interest to many students. Moreover, it does not intimidate the mathematically insecure student who is unlikely to develop any enthusiasm for chemistry in a course that emphasizes the solving of the numerical problems that are usually a n important feature of the conventional principles based course. Moreover, it is surely just as important for the student to know something of the properties of benzene, acetic add, amino acids, and CFC's as it is to know the properties of ammonia, sulfuric acid, sulfur dioxide, and the oxides of nitrogen, or to have some knowledge of the nrincinles of eauilibrium. thermodvnamics. and electrockemis~ry.So of the "descriptiv> chemistry that is intemated into the course should be oreanic chemistrv. We intrGduce hydrocarbons early so that tieir most important reaction, namely combustion, can be used in a discussion of thermochemistry. Functional group chemistry is discussed later. Substitution reactions such a s the hvdrolvsis of alkyl halides then provide useful and intert!;itingex&ples ibr the discussion of reaction rates and of himole~ulur Volume 70 Number 7 July 1993
529
and unimolecular mechanisms. These organic reactions are more useful and more relevant to the-students than, for example, the decomposition of &nitrogen pentaoxide that is otherwise of little importance at this level. Advantages and Disadvantages of an Integrated Course
The principal advantage of the integrated course is that it makes both the theory and principles (physical chemistry) and the "descriptivechemistry" (inorganicand organic chemistrv) easier to understand and more interesting and relevant lor the student to learn, than in the convengonal course. We have found that it also makes the course more interesting for the instructor to teach. Moreover, it emphasizes that experiment and observation come before theory; whereas, the conventional course appears to give the opposite im~ression.It re vents the inoreanic chemistrv from becoming a dull ca&ogue of facts, aid, in general, 6 gives a more balanced introduction to chemistrv. At first sight it may appear that the integrated course has some disadvantapes. The early introduction of inorganic chemistry means that we also-need at the same time some aualitative ideas about equilibria. reaction rates, acids &d bases, and redox reactions. We come back to these topics later to give a more quantitative treatment. This fragmentation of topics that are conventionally treated as one segment of the course, or one chapter of the textbook, might appear to be a disadvantage but in our experience it is an advantage as it allows students more time to become eraduallv comfortable with new conce~tsin a qualitativeway befire having to deal with them q;antitatively. Moreover, this qualitative understanding is an essential prerequisite for solving a numerical problem. Too oRen students do not have this aualitative understandingbefore they begin plugging numders into formulas.
530
Journal of Chemical Education
In our experience integrating a course in this way is not only more interesting for the student and the teacher, and greatly assists the student to acquire a useful knowledge of basic facts, but it is more efficient and allows time for some optional "descriptive" chemistry at the end such as environmental chemistry, polymer chemistry, biochemistry, geochemistry, and materials science. Such material is vastly more interesting and relevant for the student than a group-by-group treatment of inorganic chemistry at this stage in the course, and it gives an opportunity for a review of the principles discussed earlier in a new and relevant context. Of course, teaching a rearranged course like this means discarding some of those old lecture notes, but surely this is necessary in any case if we are to make some real improvements in the general chemistry course. In our opinion, the course should be a truly general introduction to chemistry in which students get an interesting, relevant, and understandable introduction to the three main subdisciplines of chemistry-organic, inorganic, and physical. The type of integrated course outlined here is such a general introduction to chemistry. We believe that our course is just as useful for prospective chemistry majors as it is for the large majority of students who will take only one more chemistry course or in many cases no more chemistry at all. What the students learn in our course will we believe continue to be useful to them in their professional lives as engineers, medical practitioners, geologists, or biologists and as chemically literate citizens. Introducing students to the true scope of modem chemistry, rather than just the physical chemical principles, we believe will, not only ensure a more chemically informed and chemically literate public, but it will create sufficient enthusiasm for chemistry that more students in the introductory course will think of adopting chemistry as a career.
