Experiments for Modern Introductory Chemistry Limiting Reagent, Stoichiometry, and the Mole Nicholas ~ildahl'and Ladislav H. Berka Worcester Polytechnic Institute, Worcester, MA 01 609 There is widespread and increasing recornition that many introductory (freshman) c h e m i s h laboratory programs in this country are inadequate (1-5).Twicallv, laboratory consists of cookbook experiments designed & verify principles presented a n d discussed i n lecture. Experiments often are carried out using old-fashioned techniques and rudimentary apparatus that give imprecise and inaccurate results. Many experiments mislead students as to the role of laboratory work in chemistry: mislead students as to the quality of data that madern chemists are able to obtain: convey little or none of the excitement of discovery in experimental science; and require little or no creativity from students.
problem to be solved using EAS: the determination of the value ofx-the stoichiometry-of one of three chemical reactions, given below. Reagent 1
Reagent 2
x o-phen(aq1 + FeS04(aq)
+ CuS04(aq) x C&,N03 + Ba(OHh x
H,EDTA
+ F4o-phen),S04(aq)
(1)
-1
Hk_2Cu(EDTA),+ HZSO, (2)
-1
Ba(CsH4N03)=+ 2Hz0
(3)
The TA assigns a reaction to the section and divides the section into six groups (two per spectrometer1 of four to five students each. Each group is givcn seven volumes tin the ranae 0.2-4.3 n1L1of reaeent 2 stock solution to react with - ~ - 5 mi, of reagent 1stock solution. Groups prepare their reagent llreagent 2 mixtures in 10-mL volumetric flasks. receive instkctions in instrument and computer operagon, and obtain the spectra of their seven solutions. For efficiency, the two goups sharing a n instrument run spectra alternately. As groups finish running spectra, they enter their data to a computer file (a spreadsheet file or an ASCII file) as (x, r) points, where x is the volume of reaeent 2 solution uid-and y is the ahsorbance a t the waveren& specified for the reaction studied. When all data have been entered, the TA begins a postlab discussion of the experiment while the faculty person generates a plot of student data. Arepresentative plot of data for reaction (1)is shown in Figure 1.The plot is then made available to the TA for use during the postlab discussion. During postlab discussion. the TA uses student data to guide thifitudents to an understanding of the qualitative shape of the plot in terms of the limitina reagent concept. Spekfically, students should appreciate that in the linearly-increasing region of the plot (between 0 and 2.5 mL in Fig. 11, the extent of reaction is limited by the amount of reagent 2 added; whereas, in the horizontal region of the plot b2.5 mL, Fig. 1)reagent 1is limiting. This explanation of the shape of the plot is elicited from, not presented to, the students. Through further discussion, the TA helps the students realize that a plot of absorbance versus volume of reagent 2 does not reveal the stoichiometry (i.e., the value of x) of the reaction, since there is no clear connection between solution volumes and the relative numbers of ~ -mul----ecules (or formula units) of reagents 1and 2. Introduction of the molarities of the reaeent 1 and 2 solutions bridees the gap between volumes and molecules (formula units), and the suggestion that a plot of absorbance versus (moles reagent 2lmoles reagent 1)would reveal the stoichiometry directly is ultimately forthcoming. Such a plot is then presented (Fig. 2) and the stoichiometry determined. Finally, if desired, the TAcan explore the mole concept by comparing the section data with that for a similar reaction obtained in an earlier section. Foiexample, reaction 2 can be done using a variety of copper salts. Plots of absorbance versus mass (not solution volume) of copper salt give different break points. However, when mole ratio plots are made, points for the copper salts all fall on the same curve. ~~~~~~
Consequently, the laboratory experience is often a negative one for students, including chemistry majors. A number of institutionsz have created laboratory experiences to remedy some or all of these problems. Further. several articles have appeared recentlyin this Journal addressing the laboratory issue and offering interesting solutions (6-9). At WPI, we have restructured our introductory chemistry laboratory around two major themes. First, we use the Discovery approach (9, 10) to allow induction of principles hy experiment. Second, experiments incorporate the use of modem instrumentation to give students a state-of-the-art laboratory experience that anticipates the workplace experience. In a previous article in this Journal. wc describrdan experimeni based on gas chromatograph; (11,. lIerein we describe an cxoeriment based on electronic ahsorption spectroscopy ~EAS;.Although many articles dsscrihinc!the use ofEAS have a~oearedin lhis Journal te.e.. 12-20);the experiment described here is a t a level appro: priate for beginning freshmen. I t can be performed by iarge numbers of &dents, and leads to discovery of thk concepts of limiting- reagent. - the mole. and the stoichiometry ofa chemical reaction. Execution and Results As earlier described (11).the approximately 350 students enrolled in CHlOlO are divided into about 15 sections of 20-25 students each. Two sections meet concurrently for a three-hour laboratory, in adjoining rooms, mornine and afternoon. four davs per week. The deoartment has three electronk absorption spectrometers f i r introductorv lab, sufficient for 30 students durine a threehour ~ h u sthe , title experiment can be performed by one of two concurrent sections during a period. The second section carries out a noninstrumental experiment, and the sections switch experiments in the subseauent week. The period begins with a 15-minbriefing b y k e teaching assistant (TA). in which the basic ~ r i n c i ~ l of e s EAS are k then presents the &dents with the presented. ~ h TA
..
'Author to whom correspondence should be addressed. 'For example, California Institute of Technology, the University of Michigan, and Holy Cross College.
~~~
-
Volume 70 Number 8 August 1993
671
Volume FeS0,*7H20add, mL
Moles FeIMoles o-phen
F gure 1. Plot of absorbance at 510 n m versus volme of FeSOd souton reacted wilh 5 mL ophenantnml~neso.~t on. Data represent tne poo ed enons of approximately 75 sr~dentsin tnree secuons.
Figure 2. Plot of absorbance at 510 nm versus (moles Felmole o-phen). These are the same data as plotted in Figure 1. Open circles are points that were omitted from linear regression analyses.
This type of discussion can be constructed so that the students "discover" the mole concept. The discussion takes about 20 minutes and reinforces students' understanding of the concepts of limiting reagent, stoichiometry, and the mole. In our experience, the total time required for the pre-lab briefing, solution preparation, EAS, data analysis, and poselab discussion is close to three hours.
2. A solution containing 2.500 x lo3 M Ba(OH)2 was prepared by dissolving 0.7926 g Ba(OH)2.8H20in 900 mL of previously hiled and cooled distilled water, then diluting to 1L with boiled and cooled distilled water. This solution must be kept capped tightly between uses to avoid formation of BaC03.
Experimental
At WPI we use Hitachi U-2000 Eledronic Absorption Spectrometers. Each spectrometer is interfaced with a DTK 386 computer/HP Lasej e t I11 graphics printer. Data collection and display are accomplished using Labcalc software from Galactic Industries. Spectra were obtained using disposable plastic cells with a transmission window between 280 and 750 nm. With some sacrifice in precision of data, the experiment could be oerformed adeauatelv using~pertronlr20 Colorimeters. ~ a t plots a and fiis were carried out usina Prol'l.OT Scientific Graohics. Version 1.0, from ~ o ~ e n t ~ o f t w a r e .
Reagents
All chemicals were reagent grade and were obtained from reliable commercial sources. Solutions for reactions 1 3were prepared as follows: Reaction I
1. A solution containing 5.00 x lo-' M o-phenanthroline and 0.072 M NH20H.HCI (hydroxylamine hydrochloride, was prepared by dissolving 99.1 mg o-phenanthroline monohydrate in 500 mL pH 4 acetate buffer, adding 50 mL of 1.44 M aqueous NH20H.HC1, and diluting to 1 L with acetate buffer. 2. A solution containing 3.33 x lo-' M FeSOa.7Hz0was prepared by dissolving 92.7 mg FeSO4.7H20in 100 mL water, adding 0.6 mL concentrated H2S04, and diluting to 1L with water. This solution should be prepared no sooner than one day prior to the start of the experiment. Reaction 2
1. A solution containing 5.00 x lo3 M KEDTAwas prepared by dissolving 1.4612 g KEDTA in -900 mL of pH 4 acetate buffer, then diluting to 1L with acetate buffer. M CuC12was pre2. A solution containing 1.000 x pared by dissolving 1.7049 g CuC12.H20in sufficient water to give 1L of solution. 3. A solution containing 1.000 x lo-' M CuSOI was prepared by dissolving 2.4968 g CuSO4.5Hz0 in sufficient water to give 1L of solution. Reaction 3 1. A solution containing 2.500 x loA M p-nitrophenol was prepared by dissolving 0.3478 gp-nitrophenol in =900 mL of water, then diluting to 1L with water.
672
Journal of Chemical Education
Equipment
Conclusions Overall, the experiment was highly successful in terms of student enthuiiasm and learning. students were particularly pleased to use modem scientific instrumentation to examine a simple chemical reaction. The experiment provides an excellent introduction to modem lnhoraton, methods for students, and presages the career experienEes that some of them will have. Copies of the experimental procedure, prelab briefing, and instructions to teaching assistants are available on request. Acknowledgment We thank the National Science Foundation (Grant #USE-9151507, under the ILI Program) and Worcester Polytechnic Institute for funding the purchase of instrumentation and the development of the laboratory experiment described in this paper. We thank the Educational Development Council of WPI for summer salary support for NKK. Literature Cited 1. Report on the National Science Foundation Diseiplma?. Wmkahops onundergradmate Education, oiretorate for and E n g n ~ r i n gEducation, National 5"enee Faundation.April1989.
science
2. An Exploplaratla of the Natvre and Q d t y o f Unde~graduateEducation in Science, Mathematics, and En&,eeting, Report afthe National Advieory cmnp of sigma Xi, January 1989. 3. Morgan, D. The Scientist 1991, Februluy,p 1. 4. Lagowski, J . J . J. Chem Educ 1990,67,1. 5. Lagowski, J. J. J Cham Educ. 1991,68,271. 6 . Erwin, D. K J. Chem Educ. 1991,68,362. Middendorf, D. V. J. Cham. 7. Tabbutt. F D.;Kelly, J.J.; Cale. R. S.; Barlow, C. H.: Ed- I W S 66 9 4 0 ~ 3. Amey R. L. J Chem Edue. 1982,69,A148. 9. Rieci, R. W.; DiWer,M.A. J. Chem Educ. 1991.68.228. 10. P a r ? s , M . R . : h e s , D. J. J. Ckem. Educ 1990,67,510.
Bums, D. 5.: Berka. L H.; Kildahl, N. K J. Cham E d m . 1998.70,AlOO. Kildahl,N. K J Cham.Edur. 1883, 69,591. Waltem. D.; Birk, J. P. J C h . Edue. 1990,67,A252. Esde. C T : Walmelev, F.J. Cham. Educ. 1991.68.336. . . pa& R. A II C l w n ~ d u r1991,68,549. . Riee,G. W J C h . Edue. 1990,67,430. Parody-Momale, Camara-Artigss, A; Sanchez-Ruiz, J. M. J. Chem Edvc 67.988. 18. Malerich, C. J. J. Cham. Educ 1982,69,1W. 19. Thom6en.M. W J. C h . Educ. 1992.69.828, 20. Walmsley, F.J Chem. Educ. 1992,69,533. 11. 12. 13. 14. 15. 16. 17.
Volume 70 Number 8 August 1993
673
