'bescriptive Chemistry Henry A. Bent North Carolina State University, Raleigh. NC 27650 Brian E. Bent1 University of California at Berkeley, Berkeley, CA 94720
"Must a name mean smwlhing'!" Alice asked doubtfully. "Of ;oursc it m u i t , " Humpry Dumpty said. I.ewis Carroll. 'Through the Lmking Glarr" lntroductlon and Synopsis Pauli's Remark Chemists usually draw a distinction between theoretical chemistry and descriptive chemistry. It's a distinction that irked Pauli. Theoretical chemistry wasn't very theoretical in Pauli's view. "You have to know too much chemistry to use it," he said. Pauli's remark comes close t o the main point of this paper. You haue to know much descriptive chemistry to use chemical theory correctly. Conversely, descriptive chemistry isn't very pure, either. You have to know much theoretical chemistry to describe chemistry chemically. Whewell's Thesis Efforts to draw distinctions between theoretical chemistry and descriptive chemistry drive education in chemistry ti two extremes: A Prevalent Thesis: Theoretical chemistry is the pinnacle of chemical thought; therefore, theoretical chemistry is the most important chemistry to teach. A llrrmrrging Ant:thesis: Descriptive ch~miatryis the foundatron of chrmiznl thought, thcrrture, der;criptwechemistry ir the must important chemistry to teach.
Our Prooosed Svnthesis: Theoretical chemistrv and descriptiue chemistry are, a t the bottom, the same tiing. Both describe chemistrv. Both use theoretical terms. The difference between the two lies in a degree of familiarity. A fact, observed Whewell many years ago, is a familiar theory ( I ) . I t is regarded as a fact today that the substance chemists call "chlorine" is an element. But a t one time that thought was regarded as an unlikely conjecture seriously supported by only one chemist, Sir Humphry Davy. Chemists' Snobbery "Descriptive chemistry" uses familiar theoretical terms: solid, liquid; colored, colorless; reactive, unreactive. "TheoA reolv to the ouestion What is Descriotive Chemistrv?. raised after a ralk dy bne of rhe authors IHAB). on .let s Teach ~h;mistry," given at a 2YC, Conference on Wnat Are We Trying to Teach in Chemistry?." honhern V r g n a Community College. Alexandr a. VA. December 3, 1983. Present address: ATaT Bell Laboratqries. 600 Mountain Ave.. '
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retical chemistrv" uses less familiar descriotive terms: unit cell, radial distribution function; absorption spectrum, transilion prohnhility; activation energy, Gihbs e n e r a . l'nfortunately, familiarity hrecds contempt. Descriptive chemistry in current general chemistry texts is generally relegated to the back of the book. More highly regarded, less familiar, theoretical chemistry is put up front. The result is too much uufamiliar chemistry too soon too fast for many students. True. readers should not mistake unfamiliaritv for inherent difficulty advised Eyring, Walter, and ~iml;allin their introduction to ouantum mechanics. Teachers. on the other hand, should heAadvisednot to mistake familiarity for the inherentlv. easv. and unsoohisticated. The pnhlcm is, the spectrum of familiarity is nearly continuous: acid. l~usc:rlectro~hile.nucleoohile: coordinawlv unsaturated, highest occupied molecula; orbkal. Easily, ih unnoticed steps, the theoretical chemistrv vesterdav he. of . comes for chkmists-hut not for beginning students of chemistry-the descriptive chemistry of today. Easily forgotten is that the familiar was once uufamiliar. That synoptic view of chemical thought clarifies only a part, however, of the question, What is descriptive chemistry? Hanson's "Seeing" Any particular part of chemistry, however viewed, as descriptive chemistry or as theoretical chemistry, as elementary chemistry or as advanced chemistry, is always, as chemists see it, a union of two thines: thouehts and sensations. ~hemistr;to chemists is not merely veibal combinations of familiar terms, or mathematical manipulations of abstract terms. Nor, on the other hand, is chemistry merely sensational hangs and sudden color changes. Chemistry qua chemistry is ch&ical phenomena described chemically. Descriptive chemistry, Hanson would say, is seeing chemicals a i d chemical tr&sformations as chemists see them-optically, of course, and, beyond that, theoreticallv. .. in the mind's eve (2). . . T o describe the theorvladen acrivity of seeing und describing chemistry as chemists ser it, we shall use the phrase Desrrrptiuc Chwnisrr)
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Remsen's Reaction Copper's Behavior Big-D Chemistry-Descriptive Chemistry with a capital D-is often best begun with a demonstration of Nature's nature-in Heidegger's words, with "an unconcealing of being" (3).Descriptive Chemistry is seeing that, in Remsen's words, "nitric acid acts upon copper." Descriptive Chemistry is seeing, even before the copper Volume 64
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and the nitric acid are allowed to interact, that there is no reaction. There is no visible reaction, a t room temperature, between bulk "coooer" and "air." Descriptive ~ h & n i s t ris~seeing that "copper" is a "noble metal." It's seeine. -. as chemists sav. .. that "coooer .. (in bulk) is not readily oxidized by molecular oxygen." (Few beginning students of chemistry, seeing a penny, would think to say t-.h.-d..\,
Descriptive Chemistry is seeing that, chemically, copper resemblrs the othrr coinage metals, "silver" and "gold." It's srrina that, if the heavy metals are listed in vrdrr of increasing dknsity, there is H repetition in chemical properties: copper is followed after a time by silver, then gold. It's seeing, crudely hut shrewdly, that there may he a periodicity to chemical reactivity. It's seeing what chemists saw, well before the discoverv of the electron. auantum ohvsics. and the aufbau principie, that three of the*elementsfirstknown to the ancients belone in the same erouo. of the Periodic Table. Descriptive Chemistry is seeing, long before one gets to physical chemistry and the Gibbs function, that, broadly soeakine. there's a correlation between metal-oxide stahilities a n l d a t e s of discovery of the metals: the less stable the oxide, the earlier the discoverv of the metal. It's seeing, that is, that noble metals have relatively little tendency to donate electrons to molecular oxygen. It's seeing that, as metals go, copper, silver, and gold have relatively high electronegativities. Their cations are relatively good oxidizing agents. They are often electron acceotors in uealvanic cells. Descriptive Chemistry is seeing, in advance of advanced inorganic chemistrv. manifestations of imoortant features of har&oft acid-base theory. It's seeing, later, that, in the case of copper and its congeners, highly polarizable valence-shell electrons of metallic elements are, so to speak, better ligands for "soft" ("humpy") dl0-like atomic cores than are less polarizable, "hard" ("smoother") oxide ions.
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Nitric Acid's Behavior Descriptive Chemistry is seeing, also, that nitric acid is a colorless liquid. That's surprising. Its molecules are relatively small. They must be especially sticky toward each other. Seeing that nitric acid is aliquid is seeing, with achemist's help, the importance of hydrogen bonding. It's seeing, as chemists see, the Lewis acidity and electrophilicity of protons on activated hydroxyl groups and the basicity and nucleophilicity of lone pairs on oxygen. It's seeing, as spectroscopists see, the absence of a visible spectrum. It's swing, i.e., a big energy gap hetween the highest occupied molerular orbitals (HOMO! and the loweit unoccuoied molecular orbitals (LI'MOJ in Octet-Rule-satisfying, Josed-shell molecules of electronegative atoms. It's seeing, as physical chemists see, the difference between kinetic and thermodynamic stability. It's seeing in the passive resistance of air and water to form nitric acid the difference between Gihbs energies of activation and Gibbs energies of reaction. It's seeine. ", as biochemists see. the role of enzvrnes in nitrifying organisms. And it's seeing, as industrial chemists see, the role of homoeeneous and heteroeeneous catdvsts in ni" trogen fixation. Descriptive Chemistry is seeing the Tao of Chemistry in the depths of all things. , All that "seeina" mav seem far-fetched. And it is. There are no crucial experiments in science, emphasized Duhem, onlv more or less sueeestive experiments to the way of scienve is to make tific seeing (4). hea aim of ~ e s c r i ~ t i Chemistry scientific seeing through frequent, .far-fetched, . . familiar . concrete use of scientific terms and images. B I ~ Chemistry D Descriptive Chemistry is seeing what chemicals really 250
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"re: solids, liquids, and gases in stationnry stares of being, with distinctivr modrs of becoming, called "properrirs". It's aeeing the existence of definite, rhar~ctrrizahleslates of things, to which scientists assign unique wavefunctions and distinctive names: "copper", "nitric acid" (5). In the beginning were words, and names for pure suhstances. And that was the beginning of Descriptive Chemistrv. Descriptive Chemistry is seeing that, as Humpty Dumpty said, a name means something definite. "Copper" is a coppery solid a t STP, "nitric acid" a colorless liquid. Descri~tiveChemistrv is a natural history . of. stationary states. ~kthat's not ali. Descriptive Chemistry is more than chemical botanizing. It's more than the naming of pure substances. Descriptive Chemistry is, paramountly, seeing name changes. It's seeing "products" formed from "reactants". It's seeing, in the language of quantum physics, quantum-mechanical transitions between stationary states. It's seeing, in short, chemical reactivity. It's seeing, also, in the blue color of "aquated copper ions" one of the reasons for ligand field theory. It's seeing (once again) effects due to d-electrons on metal atoms and of basic lone pairs on nonmetal atoms. And in the easeous "nitroeen dioxide." it's seeine, given an -. enterprising ~ e m o n ~ t r a t o r , ~many t h e chemical images that can be evoked bv the Brown Bottle Experiment (6). Descriptive &emistry, in summary, is modern language instruction. It's pointing and telling. It's a timely expression of, in Gihbs' words, Nature's "passive resistances" and "active tendencies" (7). I t is, i n a sense, chemical common sense. Copper is a noble metal, oxidizable, however, by nitric acid to blue, aquated ions. Nitric acid is a stable, colorless liquid, reducible. however.,hv.comer .. metal to brown.. easeous nitrogeu dioxide. Descri~tiveChemistrv is simole. declarative chemistrv. declaredat the moment of showing and seeing. It's mo ha!' chemistry. It's being able to say, on witnessing lectures with demonstrations and demonstrations with lectures, "Ah, I see!" Given, however, the aggressive knowledge claims of quantum physics-that all of chemistry is in principle solved (a)-, Descriptive Chemistry may he, in fact, psychologically, the most difficult part of chemistry for chemistry teachers to teach well. Certainlv Descriotive Chemistrv is the most important part of chemistry for students to know well. Practical. aodied. Descriotive Chemistrv is a orereauisite for putting &;roper use ali of the more tgeoretkal parts of chemistrv. One examole mav suffice to illustrate the orimacy of ~ e l c r i ~ t i v~ eh k m i s t ; ~in the hierarchy of chemical thought. ~~~
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Glbbs's Rule The Rule's Roots Descriptive Chemistry starts with simple descriptions of the orooerties of single phases of pure substances. At room copper is a solid, temperature and at&ospheric nitric acid a liquid. Gibbs's Phase Rule is merely a generalization of that familiar Rule of Two: given the temperature and pressure of a homogeneous, pure substance, the substance's other (intensive) properties can be looked up in a handbook of chemistry and physics, or in a chemistry reference hook. Similarly, a two-component, one-phase system-e.g., an aaueous solution of nitric acid: or a cooper-zinc allov-has -. three degrees of freedom: e.g., T, P, and the mole fraction of one of the components must he fixed to fix the system's remaining properties. And so forth. Gibbs considered the rule to be self-evident (7): increase the number of components, C, in a solid, liquid, or gaseous phase and the number
of degrees of freedom, F, increases correspondingly. On the other hand, pure liquid water in equilibrium with. for example, its vapo;has at given temperature a definite pressure (its "vapor pressure"): i.e., a one-component, twophase system bas-onl; one degreeof freedom (e.g., 7'). And so forth. Increase thenumher of phases,P(C constant), and the number of degrees of freedim decreases correspondingly. Briefly put, F = 2 C - P. Gibbs Phase Rule is easy t o state, easy to derive, hut less easy to use. Proper use of chemical theory depends on prior knowledge of basic Descriptiue Chemistry.
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Passive Resistances and Active Tendencies Consider, for example, a gaseous mixture of hydrogen, oxygen, and water. What'sF? According to Gibbs, 2 C - P. And obviously, P = 1.But what's C? Three? Yes, if, experimentally, it's found that the passive resistance to the reaction
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is large; i.e., if the reaction's Gibbs energy of activation is large compared to RT. On the other hand, it might be observed, experimentally, that there is an active tendency for hydrogen and oxygen to form water, and vice versa; i.e., the three species might be in mobile. chemical eauilibrium with each other. owine. ", e.e.. " .to the of a ca'talyst. Then the number of independentIv variable chemical species would be two. not three. - Indeed, the expresGon F = 2 C - P is'mis~eadin~. From an experimental point of view, i t puts matters the wrong way around. I t puts the cart (C) before the horse (F).For, in the first instance, F is not determined from a knowledge of C (andP). Rather, initially, Cis determined from a knowledge of F. Experimental observations of the effects of a system's passive resistances and active tendencies on the system's phase behavior as its temperature, pressure, and chemical composition are varied determine F and, hence, with an experimental value for P, the value of C, from C = F P - 2. Knowledge of a system's passive resistances and active tendencies is essential knowledge in any application of Gibbs Phase Rule. "Passive resistances", remarks Gibhs, "are generally easiIv recoenized upon the most su~erficial . . knowled~e - of. nah e " (7) (emphasis added). A passive resistance, continues Gibhs, is that resistance to change which prevents either of two forms of the same substance which are capable of existine from passine into the other: for example. . . diamonds into graphite;hydrogen and oxygen into water; copper and oxygen into copper oxide; nitrogen, oxygen, and water intonitric acid. Gibbs "superficial knowledge" of Nature's chemical nature is what basic Descriptive Chemistry is about. It's knowledge of Nature's passive resistances and its active chemical tendencies. It's knowledee of chemical reactivitv. It's indispensable knowledge to chemists. It's the knowiedee. for examole. that water is a stable comoound of hvdrogen &d oxyg;n and that mixtures of molkular hydGogen and air mav be ex~losive.It's the knowledee that comer tarnishes in air, i.e.; that copper oxide is s t a k e a t ordinary temperatures, but that copper does not rust like iron. There's a passive resistance to its bulk oxidation. Of course, those facts can be looked up in a book. So can the spellings of the words in that sentence. But to have to do so is to be functionally illiterate. Knowledge of basic Descriptive Chemistry is necessary to be chemically literate.
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Lavoisier's Recommendation So, in summary, What's Descriptive Chemistry? Well, in fact, What's not Descriptive Chemistry?
All chemistry, "descriptive" and "theoretical", is descriptive, and theoretical. It's all an abstract-i.e., "theoretical"-description of Nature. From naming substances throunh, for example, the Rule of Two and Gibbs Phase ~ u l r , G i h b artivaiion s energies and Gibbs energies of reactlon, it's all Descriptive Chemistrv-or should be. For. if it's not descriptive of Nature's chemical nature, what is it? The question is not so much, What's Descriptive Chemistry? The question is more, How can we teach chemistry so that what we teach is descriptive of what chemistry is? Advised LavoisierStick to the point. Avoid nonseqniturs. Aduance no name, no idea. no theorv but what is necessarv to describe or to explain'an immeiiate experiment or obskruation (9). Do an experiment. Pour a colorless liauid onto a Dennv. Then make an observation. That's aciemical transformation! Repeat the experiment, noting changes of "state", and color. Advance (simultaneously) some names: "copper", "nitric acid", "nitroeen dioxide", "copper ion". Do another experiment. P& anhydrous copper sulfate and copper nitrate into water. Note solutions' colors and electrical conductivities. Introduce some ideas: elements, compounds, ions. Suggest readings in the text, on elements and compounds, copper and nitric acid, ions and electrons, oxidation and reduction. Respond to questions with more experiments, more terminolom, more ideas, more readine. . .and so forth throuehout the&m, and the entire cnrri&lum. Such teaching is not, of course, logically neat and tidy. Nor is i t intended to be. It's intended to show what chemistry is: something exciting, often surprising, highly creative, frequently useful, remarkabIy abstract, sometimes dangerous, constantly changing, always interesting. &
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Conclusions Descriptive Chemistry is more a general style of teaching than a specific curriculum content; more something for the lecture bench than the printed page; more the suggestion of memorable images than the presentation of formal proofs; more a discussion of what's useful for chemistry students to know than what's pleasine for theoretical ~hvsiciststo contemplate; more chemistryihat is psycholo&&y meaningful than chemistry that is logically rigorous; more a language used than a language derived; more the spirit of Faraday's lectures on "The Chemical History of the Candle" than the spirit of modern introductions to chemistry, with their emphasis on chemical arithmetic; more Whitehead's "stage of romance" than his "stage of precision;" more an effort to emphasize the simple-subtle sophistication of elementary chemistry than the mathematical elegance of physical chemistry; more a matter of chemical concepts than physical theories; more a matter of what chemistry is than what quantum physicists believe chemistry should he. Acknowledgment The authors are indebted t o Henry E. Bent for constructive criticisms of an early draft of this paper. Literature Cited
......, 4. Duhem,P. Th.
Aimondstructur. "lPhy8isicolTheory; Wiener. Philip, Trans.:Princeton Univeristy: Princeton, N.1, 1951. 5. Bent, H.A. J. Ch.m.Educ., in press. 6. Bent, H. A : and Bent, H. E. J. Chem Educ. 1980,57,6W. 7. Gibb8, J. W. Collected Works. Volume I Th~rmodynomica:vale univeniqv: Nrn Haven, 1948, pp 58.96. 8. Dirac.P.A. M. Proc RoyolSoc. (London) 1929,A1%7,714. 9. Lavaisier, A. Elements of Cheminry: Kerr, Robert, Tram.: Dover: New York. 1965, P XVI.
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