Striking a Balance: Experiment and Concept in Undergraduate Inorganic Chemistry John E. Frey Northern Michigan University, Marquette. MI 49855
In response t o a perceived need and in accordance with the euidelines of the Committee on Professional Traininr! of the Lmerican Chemical Society, we have offered, since 1980,a four-credit course for undermaduate chemistrv. majors - and minors entitled: CH215 Chemlstry of the Elements
Descriptive chemistry of the elements and their compounds in relation to the periodic table. Emphasis is placed on chemical behavior, bonding and structure, and thermodynamic, electrochemical, and spectral properties. The laboratory involves the synthesis and characterization of compounds of common elements, including salts, complexes, and covalent compounds. The course is based on the premise that inorganic chemist~ nromesses hv the mutual interolav of the activities of u experimental, tGeoretical, and tecdnoiogical chemists and that a balanced understanding of inorganic chemistry requires knowledge and experience in each of these aspects of the subiect. Therefore the lecture and the laboratory portions of the course are given equal weight in both c&tact time (3 h each per week) and in emphasis and are integrated as closely as possible. Technological aspects of inorganic chemistry are presented in the lectures in order to demonstrate how general principles and knowledge are applied to produce useful materials and how the products of technolbgy inspire in turn developments in the science. Anecdotes are used frequently t o present the historical and human a s ~ e c t of s chemistrv. 'The lectures ar; organized into three main segments: chemical bonding and structure (3 weeks), main group elements (7 weeks), and transition elements (5 weeks). A brief review of chemical bonding and crystal and molecular structure is followed by a systematic tour through the periodic tahle with emphasis on the descriptive chemistry of selected elements and groups. Aspects of thermodynamics, equilibrium, electrochemical potentials, ligand-field theory, spectroscopy, etc., are developed as necessary, andstructural and graphic models are used frequently to clarify electronic in&r&tions and chemical structure. Each of the laboratory exercises and syntheses in the specially prepared lab manual' involves the synthesis and characterization of compounds of at least one element from each of the chemical families, IA through VIIA, and several elements from the first transition series (Table 1). The order of laboratow topics ~arallelsthat of the lecture topics so that the majo; segments of the course are chronolo~icallyinte-
grated. The syntheses are adapted from procedures found in standard references2tomeet the s~ecificneedsof the course. Generally the reagents are inexpensive and are used in amounts that yield 5 to 15 n of product per student. The 12 . lab exercises iilustrate a variety of synthetic principles and techniques, as well as diverse reaction forms and types (Table 2). bond and structural types (Tahle 3), thermodynamic and kinetic behavior in aqueous solution, and separation and purification techniques. Table 1. CH215 Laboratory Exerclws and Preparations (References In parenthews) 1. Transition Metal Reactions 2. NH4BF4. KBF. (20. pp 29.30) 3. Snl, (Zd, p 10) 4. (NHhPbCls. (NH&SnCIe(Zf, p 83)
".
5. N01/N20, (a, p 124) 6. TiOSO, solution (21,p 128) 7.
Table 2. CH215 Reactlon Forms and.Typ.38 Combination: Sn -~~~
Supplement, Notthern Michigan University: Marquette. MI. 1989. (a) Jolly, W. L. The Synthesis and Characterlzatlon of horganlc Compounds: Prentlce-Hall: Englewood Cliffs, NJ, 1960. (b) Moeller, T., Inorg. Synth. 1957,5, 114. (c)Palmer, W. G. Experimentallnorgm Ic Chemlstry: Cambridge University: Cambridge, 1954. (d)Pass, G.: Sutcliffe. H. Practical Inorganic Chemishy; Chapman and Hall: London. 1968. (a) Schlessinger, G. G. InwganicLaboratoryPreparations: Chemical Publishing: New York, 1962. (1) Walton, H. F. horganlc Preparations;Prentice-Hall: Englewood Cliffs, NJ. 1948. 684
Journal of Chemical Education
+ 21.
-
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-- - + - + + + ~.
~- . ~ -
Displacement: NHIHF* + H3B03 NH,BFl Disproportionation: 2H202 2HZ0+ 0* Metathesis: NH'BF. + KC1 KBF, + NH&I Dlmerization: 2 N 0 2 N2O4 Neunalizatlon: Hacac + COS1- acacCa02.8H.0 Precipitation: CaCI. + H P . Gas Formation: Pb(NO& NO2 Oxidation: PbC12 + CI. PbCi, ~
Reduction: 2C?+
Zn
2Cr"
Zn2+
Complexation: Cu2+ 4NHs CU(NH~),~+ Chelatlon: YO2+ 2acac- VO(acac)~ Table 3. Structures and Aqueous Behavlor of Chemlcal Comoonentr Component BF,-
' Frey, J. E. Chernishy of the Elements. Lab Manual and Lecture
CaOr8H10(2c,p 177). NaBOp4H2O(n,p 128)
Snl, PbCla2-
SnCls2NO,/N,O, OZ2CU(NHJ),~+ CuCl
VO(acac), Cr,(OAc), FeG0.P-
Structure
Aqua Equll
Kinetics
covalent covalent covalent covalent covalent malent
tetrahedral tetrahedral octahedral octahedral bent
mrd-cw
sq planar
stable unstable unstable unstable unstable unstable unstable unstable stable unstable stable
inert lablle labile labile labile inert labile
Bond T y p
ionic mrd-mu metal-metal c00r6mv
8q pyramid
Octahedral octahehal
inert inert lablle
Hg Particular attention is given t o the principles and methods 0 \ of optimizing preparative yields by controlling reaction conC ditions, digestion, cooling filtrates, adding common ions, / '-0-v-0-c using nonaqueous wash solvents, etc. Students are asked to balance equations, t o determine limiting reagents and per\c -0 ............................ o cent yields, and to calculate the costs of reagents and solvents used. / Each exercise is organized in a standard format that inH& \ cludes an introduction, preparative procedure, tests, and lab CH3 report. Emphasis is placed on understanding the basis for Mo'ecU'a' srmcture Of V0(acac)2. each step and safety precaution in the procedure. Chemical tests are used to highlight the chemical characteristics of the . product. Numerical data and results are entered in the tables of the formal lab report and narrative answers toquestions are entered in the blank spaces provided. Percent yields of product. Part B consists of questions reThe students are confronted with the contrast between garding the colors and the oxidation states of elements in the concrete experiences and observations in the laboratory some of the substances that occur in the reactions, incomexposition of on the one hand, and the verbal and plete equations to be balanced, and calculations of electrothe concepts, theories, and models presented in the lecture chemical potentials for some pertinent redox reactions. Part the text on the other hand, ~h~ apparentincongruity C consists of queries regarding the molecular structure of the the empirical results between the twois resolved by product and the chemical function and costs of the reagents in a framework of relevant generalizations, principles, theeusedinthe synthesis. ries, and models that include the periodic law, the Aufbau Subsequent to the completion of the laboratory exercise a principle, the molecular orbital theory, thermodynamics, lecture is devoted t o the chemistry of vanadium with emphachemical equilibrium, acid-base and complexation consis on aspects of its chemistry that are relevant to the synthecepts, electrode potentials, etc. sis of VO(acac)~and reactivity of vanadium and vanadyl The manner in which lecture and laboratory segments are ions. Some of the topics covered include: the reduction pointegrated for Exercise 9. Oxidation states of vanadium: tential diagrams for the vanadium, zinc, hydrogen, and sulSynthesis of VO(acac)2 is based in part on the exemplary foxy systems; the colors of compounds with vanadium in approach of ophardt and stupgia,3 part A of ~~~~~i~~9 consists ofthe procedure for preparing V O ( ~ ~~h~ ~ ~first ) ~ . different oxidation states; and the structures and colors of vanadate polyanions as a function pH. stage of the synthesis involves the reduction of VzOb to V02f Students are given copies of the IR and UV-vis spectra by ethanol. and the molecular structure of VOfacac)~(see figure). Their 4H+ 2VOz++ CHsCHO + 3H20 VzOs+ C&OH attention is drawn to the IR band a t 480 em-', which is shown to arise from a stretching mode of the V=O bond. The sweetish-pungent odor of acetaldehyde and the gradual The role of orbitals in the formation of d?r-pa bonds change of color from brown to green indicate the progress of between transition metal and nonmetal atoms is introduced the reaction during the hour-long reflux. The second stage at this time, Further attention is drawn to the three bands in involves the addition of acetylacetone (Hacac) and Na2C03 the UV-vis spectrum of VO(acac)n a t 14900, 16700, and to the vanadyl solution to yield the insoluble dark green 25000 em-' as well as the deep minimum a t 20000 cm-1, ,product. whichcauses the green color ofthe product. Tlie three bands are shown to arise from the threefold splitting of the d V02++ 2Hacac + C0z2- VO(acac)2 + H20 + C02 orbital energies3 in the square-pyramidal ligand field of NH4V03 and mL each Of O.l In P*B VO(acac)z. This section provides a dramatic opportunity to HC1 to two test and The Of show students various aspects of the relation between the A NH4V03 turns brigbt yellowon the addition of molecular and electronic structure and the spectra and color amount of SO2 is bubbled into test tube 1; the solution turns of a transition metal complex. from yellow to blue. Students who have taken a course of organic chemistrv appreciate the use of organic reagents, suchas ethanol (as 2VOzt (yellow) SO2 2V02+(blue) + S O P reductant for V,O:,) and acetylacetone (as a chelating agent Several granules of zinc are added to test tube 2; this solution for vanadyl) in inorganicsynthesis, and the role of kewenol turns from yellow to aqua to violet with vigorous bubbling. tautomerism and electron delocalization in the stabilization of the chelating group acac-. These observations point up 2VOzf + Zn + 4H+ V3+ (aqua) + Zn2++ ZHZO the artifirial distinction between inorganic and organic chemistrv and hivhlieht the underlvinv 2V3++ Zn 2VZ+(violet) + Zn2+ " - unitv of all chemistry inheknt in tG p&odic law. Zn 2Ht Zn2+ + Hz Although chemistry is ultimately an experimental science, the manipulations, observations, and measurements of Students are asked to identify the various oxidation states chemistry become more comprehensible and compelling by their colors and t o rationalize their production by means when placed in a conceptual context and when visualized by of appropriate Latimer-type4 reduction potential diagrams. structural models. It is not necessary that students be preThe Laboratory Report consists of three parts. Part A is a sented with empirical facts before they receive theoretical table with blank spaces to record data needed to determine interpretations of them or vice versa. I t is i m ~ o r t a n thowev, the limiting reagent and t o calculate the theoretical and er, that we, as chemistry teachers, strike a balance between the presentation of the tangible laboratory experiences that yield the raw facts of chemistry and the interpretation of Ophardt, C. E.; Stupgia. S. J. Chem. Educ. 1984,61.1102-1103. these facts with the concepts, principles, and models Latimer. W. M. Oxidation Potentials, 2nd ed.;Prentice-Hall: Enthrough which chemists give them meaning and significance. glewood Cliffs. NJ, 1952.
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Volume 67 Number 6
August 1990
885
