The Deceptive Difficulty of Descriptive Chemistry and the Chemistry Curriculum Henry A. Bent North Carolina State University, Raleigh, NC 27695 It's easy for chemists to underestimate how difficult descrintive chemistrv is for nonchemists. It's easv to think that it's a trivial matter to go from a verbal description of a chemical reaction to a chemical eauation. T w , how".~er,asking beginning students to write a haianced chemical &+ation for formation of carhon monoxide and molecular hydrogen . . from the action of steam on hot carhon.
It's not easy for nonchemists to fill in the blanks with C, HzO, CO, Hz. Descriptiue chemistry is deceptive. I t looks easy. I t sounds easy. Yet it isn't easy. Saying It In Words True, descriptive chemistry is merely a matter of words. And a t first glance it may not appear to be particularly scientific. Many scientists concur with Kelvin that we haven't genuine, scientific knowledge of something unless we can exnress i t in numbers and mathematical eauations. Yet the opposite of a profound statement may be H profound statement. We haven't genuine undrrsrandinr of somethinr. said Dehye, unless we c& explain it to the m i on the streetxthat is to sav. unless we can sav i t in words. ~ a x i e l lin , his classictreatise on the "Theory of Heat," didn't exnress his celebrated Relations mathematicallv (except in fine print in a footnote). He concentrated, instead, on saying in words what his Relations mean. Saying it in words isn't easy. Word-problems are hard. Genuine understanding isn't quickly acquired. Students need much practice in going (as above) from English to the language of chemistry and (as below) from the language of chemistry t o English. Problem: The following chemical expression C(s) + Odg) = CO&) might express (True or False)Combustion of graphite. Oxidntmn of d~nmmds. Burning of charcoal. Heating with fossil fuel. Gasification of coal. Cleaning of carburized catalysts Reduction of oxygen by coke. A steo of drv ice. . in ~roduction . A step in production of electricity. Fomrntio#lof uilrbuu dioxide irum its elements. Answers: True, in each ease. The expressions are different ways of
saying essentially the same thing: carbon plus oxygen gives carhon dioxide-exothermically. Understanding is seeine that the same thine said in different words is t i e same tiling. No phrase is mire welcomed to readers striving.for understanding- than that friendlv~. phrase "In other words, . . ." Words are fundamental to understanding and essential for thought. "We think only through the medium of words," writes Lavoisier, quoting Condillac, in the Preface of his "Elements of Chemistry."
True Analytical Methods The use of words to think and to communicate thoughts is called language. And languages, continues Condillac, "are true analvtical methods." Anv mathematical ex~ressioncan be " expiessed in English. English is a universal language. I t is a language into which all other languages can be translated. Mathematics, on the other hand. is not a universal lanauaae. Most of descriptive chemistry,'like most of ~hakespeare,cannot he usefully translated into a mathematical idiom. I t cannot he reduced t o mechanical, computational procedures that require, by design, little creative thought. Descriptive chemistry is difficult because i t must be nonmathematical. I t requires special, trans-computational skills: careful observation, a good memory for chemical facts, a lively but disciplined imagination, and a solid understanding of chemical theory. Without, however, direct sensory experience with chemistry's stinks and hangs, an appreciation of descriptive chemistry is impossible. For although the language of descriptive chemistry is, in the words of Lavoisier, "both copious and expressive," the most vivid verhal descriptions and precise chemical statements of the explosion of hydrogen and oxygen, for example, or the aluminum reduction of iron oxide are strikingly inferior to the sensations those transformations produce in most observers.
A Mod Moded Science Chemistry, in other words, is simultaneously an amazing and an amazingly modest science. I t is amazing a t the lab bench and amazingly modest in the lab notebook. The thunderous explosion of hydrogen-oxygen mixtures becomes 0 2 = H20. Seeing those sides of the subject simply Hz simultaneouslv, as two sides of the same thing, .is seeing what descriptive chemistry is. Descriptive chemistry is basic to basic chemical literacy: the ahility to link verbal statements with primordial chemical images--remembered or imagined--combined with the ability to express such thoughts in rhvmiral notation. The purposc of instructional chemical lnhorarories is to provide students with o~oortunitiesto acauire those abilities. Thar is not difdo. Most of the following illustrative examples of ficult descrintive chemistrv are based on inexnensive. short. safe. striking, simple leiture experiments:' usually dropping somethine into somethine. -. or heatine somethina. - Hints for the exercises are given in parentheses. Answers are given at the end of this section.
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1) A white, water-soluhle,sweet-tasting solid heated yields a colorless liquid and a black, insoluble mass! (Remsen's Experiment Number 1) -(8)
sweet
=-(I) colorless
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-(s) black
(From carefully recrystallized reactant, the process yields in high purity a highly refractory element.) Volume 61
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2) A Lustrous, silvery solid heated burns brilliantly in air to a white powder!
12) Twocolorless, suffocating,nornutrients yield with a k the sun edible solids and n life-supportinggas!
(Used in flares and fireworks)
(1) =( 9 )+ (s)+(9) vital am colorless colorless edible (Commonly called photosynthesis)
3) A hot, silvery substance burm brilliantly in carbon dioride producing a white powder and black specks!
9 ) = -(8) + -(9) silvery colorless white black (Metallurm in reverse) .~
-(8) (
4) A soft, lustrous solid added to phenolthalein-spiked water sputters about vigorously with production ofo pink liquid and a flammable gas! -(s) (1) =(as) -(as) (9) +soft colorless cation anion flammable (The pink product is an excellent conductor of electricity.) 5) A soft, highly reactive, lustrous solid burns brilliantly in a pale, green, poisonous gas and yields ordinary table salt! +
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(3) (9) = (8) highly reactive poisonous condiment (Prototypicalexample of a metal reacting with a nonmental) +
6) Sea salt plus a colorless liquid-which when added to water produces much heati-fizzes with evolution of a colorless, highly poisonous gns!
(The toxic gas is produced, also, in building fires that consume polyvinyl chloride.) I ) Two colorless, pungent, highly water-soluble gage-ne tremely toxic-combine to form a harmless white solid!
ex-
(The product often forms on glassware stored in hwds.) 8) A colorless, highly toxic gas dissolves with enormous ease in water yielding electrically conducting stomach acid! (1)--(as) -(as) -(9) toxic nonconducting cation anion (The product, often sold as muriatic acid, gives a white precipitate immediately on addition of aqueous solutions of silver nitrate.) +
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9) Powdered aluminum and rouge yield, on ignition, a brilliant display of visible light, corundum, and a molten metal! (1) -(3) +- (8) = (3) + silvery red corundum molten! (The thermite reaction)
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10) A rock-like substance plus liquid water fizzes with evolution of a soot-producingcombustiblegas! (8) (1) (9) rock-like noncombustible combustible
,
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1s)
slaked lime (The reaction in carbide lamps) 11) Steel wool dipped into a blue solution instantly turns copper-colored! (Mistaken by alchemists for transmutation of an element) (3) +-(ad (as) (3) base metal blue greenish copper-colored (A reactive metal displaces a more noble metal from solution.) +
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Journal of Chemical Education
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Answers 1) C~ZHZZOII = 11Hz0 + 12C 2) 2Mg 0 2 = 2Mg0 3) 2Mg COz = 2MgO + C 4) 2Na + 2Hz0 = ZNa+ + 20H- Hz 5) 2Na + Cb = 2NaCI 6) 2NaCI + HZSO~ = 2HC1+ NazSO4 7) NHa HCl = NH&I 8 ) HCl + H20 = H30+ + C19) 2A1+ Fez03 = A1203 2Fe 10) CaCz + 2H20 = C2H2+ Ca(OHI2 11) Fe Cu2+= Fez++ Cu 12) nCOz + nHzO = (C.HIO). nOa
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Notes Five of the reactions mav be viewed a s nroton-transfer reacthe act;& tions (4,6,7,8,lO). FOUI bf the reactiondi~~ustrate series (3.4.9.11). Three of the reactions oroduce a rock-salt . . structure (2, 5, 7). If the carbon produeed in reaction 1 is burned with oxygen (a spectacular transformation), the net chemical reaction is the reverse of reaction 12. Summary and Related Remarks Descriptive chemistry is deceptively difficult. It looks like merely a matter of words. A bright orange crystalline solid heated produces in a brilliant volcanic display copious quantities of a green solid, a colorless, odorless, tasteless liquid, density about 1glml, and a water-insoluble, colorless, odorless, tasteless gas slightly less dense than air. A chemist might conjecture that the liquid product is hydrogen oxide (H20), a frequent product of combustion; that t h e noncondensable gaseous product is dinitrogen (Nz), a frequent product of explosions; and that the solid meen mass is prohahly an oxide of a transition metal, for compounds of such elemrnts are frequently highly colored and the elements themselves exist in different states of oxidation. A elowinesplint test on the gas shows immediately that i t is both no;combustible (with the oxveen of air) and noncombustionsupporting (fir a carbohidiate substrate). It is a relatively inert gas. beine neither a eood reducine - kineticallv. sneaking. . agent noragood oxidizingagent. Determination01 initial and final masses01 thesolidsand the volume of theaas would establish, with a table of relative atomic masses,'that the balanced chemical equation is (NH4)2CrzOl = Crz03 4 H z 0 Nz. T h a t is descriptive chemistry. It's not easy. I t takes years of work with basic chemical techniques or, in less time, thousands of dollars worth of modern chemical instrumentation toestablish the chemical formula of a single pure substance. And even after all of that work has been done, its application to particular cases cannot be reduced to routine computational procedures. Creating a chemical description of a chemical transformation is not a job for a chemical technician. It requires the skills, insights, knowledge, and imagination of a chemist. Prohlems on descriptive chemistry are much more difficult than number-crunchine nroblems. Number crunchine. however sophisticated the &kground theory, is, for beginning students, usually a matter of a few preliminary arithmetical or algebraic manipulations followed by substitution into a given formula. Descriptive chemistry, however, is a matter of choosing the right formulas-from among perhaps hundreds or thousands of possibilities. Descriptive chemistry is the hardest p a r t of chemistry to teach well. And, curiously, as with mathematics-orw with any language--the hardest part comes first. Once arithmetic and
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algebra have heen taught, it is relatively easy to teach calculus. Once basic descriptive chemistry has been taught, it is relatively easy t o teach the rest of chemistry. Given the primacy of descriptive chemistry in the hierarchy of chemical thought, one might suppose that descriptive chemistry wouldconstitute the major part of begidning chemistry courses. But that's not the case. Bright students crank out correct numerical answers using Nemst's equation on AP exams, then write for the formula of lead, Ld, for DOtassium carbonate, PC02.A simple extrapolation suggests how little they know of descriptive chemistry. The low level of hasic chemical literacy among beginning students of chemistry has led increasing numbers of chemists to suggest that there should he more descriptive chemistry in beginning chemistry courses. And since teachers, responding to Dressures from ~rincioalsand oarents. often must teach to exams, and since the exams may not examine what teachers should teach, the route t o curriculum improvement, it has been suggested, is to change the exams. Yet that's not easy to do. For when auestions on descriptive chemistrv are submitted for use o n AP exams, they are usually rejkcted by the exam's reviewers, as being mere memorization and unsophisticated. It's easy to be unsophisticated about what's sophisticated. It's easy t o think that sophisticated teaching requires advanced content: that asophisticated introduction ro modern chemistry should get as soon as possible to orbital approximations to eigenfunctions of partial differential equations and Cihbsian thermodvnamics. Exactlv the oooosire is the rase. We learn t o thing critically-theUessen&of sophisticated t h o u g h t h y thinking about things we understand thoroughly. So~histicatedteachinzreouires so~histicatedtreatment of simple content, not simpli'stic treatment of sophisticated content. The latter stvle of teaching is almost as far from the central spirit of scienEe as one can pet. For science is nothing
if not something done well, with absolute confidence. Much of the new introductory chemistry is, unfortunately, the old descriptive chemistry a t its worst: mere memorization of things that seem important but that are taken entirely on faith. Modern descriptive chemistry is not a swing of the pendulum backward. I t is a swing forward. I t is old friends in new contexts. As a student I saw sodium react with water, sodium chloride fizz with sulfuric acid, hydrogen chloride and ammonia dissolve in the fountain experiments, conc. ammonia and conc. hvdroehloric acid form white smoke. and moistened calcium carbide form a combustible gas. ~ n I dwas amazed. But I had no idea that those startling different transformations of matter could all be viewed as occurring by the same mechanism: a simple transfer of a proton from one electron pair to another. That mental picture is almost as amazing as the optical images themselves. Descriptive chemistry in 1984 is much more exciting than it was in 1944. It is not only the key to chemistrv-the central science. I t is the key to the core of the entire curriculum: reading, writing, and arithmetic. For there is no better olace to learn to read critically, to write clearly, a n d to use arithmetic creatively than in the word-laboratorv . of. a bezinninz chemistry course. LetS elevate our guns higher. Let our goal be: Better Sensory Experiences and Better Mental Images for Better Curriculum Content and Better Education Through Modern Descriptiue Chemistry. Acknowledgment -
The author acknowledges the inspiration behind this paper and the energizing effect of the startled looks on the fares of nonscience students in his foundations course in chemistry during work on daily quizzes on the descriotive chemistrv of lecture demonstrati&* dependably set up f& the class hy Marie Benevides.
Volume 61 Number 11 November 1984
987
