Experiments for Modern Introductory Chemistry The Temperature Dependence of Vapor Pressure Nicholas ~ i l d a h l and ' Ladislav H. Berka Worcester Polytechnic Institute, Worcester, MA 01609 In reiponsr to the widdy recognized problems w ~ t hhe traditional approach to the ~ntroductonchemistry lnboratorv (1-5). ., wkhave initiated the comnlete restructurine of our program around two major themes. First, our new program incorporates the Discovery approach (6, 7). Second, experiments incorporate the use of modern instrumentation to give students a state-of-the-art laboratory experience that anticipates the workplace experience. We have ~reviouslvdescribed exneriments based on eas chromatoe) electrokc absorption spect~oscopy(8-16. Eaphy ( ~ kand We here nresent a h e a d s ~ a c eGC experiment that enables discovery of the tempe;ature dependence of the vapor pressure of a pure liquid. Articles dealing with the measurement of vapor pressure have appeared in the Journal (11-22). The experiment that we discuss here simply and elegantly illustrates liquid-vapor phase equilibrium of pure liquids. The experiment is a t a level appropriate for beginning freshmen, yet is challenging, interesting, and informative a t the same time. Students discover that
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vapor in equilibrium with a pure liquid can he sampled and quantitatively analyzed by gas chromatography; the vapor pressure of a pure liquid increases more rapidly than linearly with temperature; the variation of vapor pressure with T can be used to measure the magnitude of intermolecular farces in the liquid phase. Overview Approximately 350 students, primarily engineering and science majors, are enrolled in CH1020, the second in a seauence of four Introductorv Chemistrv courses. durine the winter term. They are divided into 15 sections of no more than 30 students each. Each section meets once per week for a three-hour laboratory. The department has three gas chromatographs for introductory lab, easily sufficient for 30 students during a three-hour period. The neriod beeins with a 15-min briefine bv the teachine assistant ( T ~ ) , - i nwhich the basic principies of GC a; brieflv reviewed (students have alreadv used GC on one occasion during the introductory sequence) and the concept of equilibrium vapor pressure i s introduced and explained. The teaching assistant then presents the students with the problem to be solved using GC:
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To study the vapar in equilibrium with s pure liquid as a function of temperature, to find out the form of the temperature dependence.
The teaching assistant tells the section which three liquids will be studied (usually acetone, hexane, and ethanol). divides the section into no more than six a o u n.s (two per chromatograph) of four to five students each, and as. signs each group one of the three liquids to study. The TA assigns groups to instruments, demonstrates the technique of headspace sampling, and reviews the sample injection technique and operation of the computer. This rew i r e s an additional 15-20 min. Students are then shown 'Author to whom correspondence should be addressed. 258
Journal of Chemical Education
the constant temperature bath and the septum-capped, 25-mL volumetric flask containing their liquid, that has been submerged in the bath for sufficient time to allow temperature equilibration. We have found i t most convenient to begin a t the lowest T to be studied (usually -10 "C), and to work up to 20 "C in increments of 10 "C. Students then proceed with their analyses. If two groups share a n instrument, they alternate injections, so that no group is inactive for more than 10 min (the time typically required to make a run and print the chromatogram). Students in a group permute the injection and computer operations, so that everyone has a n opportunity to perform all aspects of the procedure. When all groups have completed sampling a t the lowest temperature, the TA resets the constant T bath for the next higher temperature. We are able to accomplish t h e temperature change i n 7-8 min, during which time students are expected to work on Questions that accompany the experimental writeup. The TA and a Typical Student data Sheet for Acetone T(K)
Injection #
280.4
1 2 3 4 5
Mean: 285
1
2 3 4 5 bdean:
287.6
1 2 3 4 5
Mean: 290.3
1 2 3
4 5
Mean: 293
1 2 3 4 5
Mean:
Signal Area Relative PvapAttenuation 13.4 13.2 13.6 13.1 13.3 13.3 16.3 16.8 16.4 16.3 16.4 1R4 18.9 19 18.6 18.6 18.4 18.7 21.3 20.9 20.8 20.9 20.7 20.9 22.3 22.1 21.9 22.4 22.7 22.3
0.597
32
n 737
32
0.839
32
0.938
32
1 .0
32
acetone -- AH = 31.4kJ AH(CRC) = 32.3 k -1.0 -1.5
ethanol
0.4 0.2 : a0260 ,/,
270
280
290
,I
ethanol -- AH = 41.2 kJ AH(CRC) = 43.3
-0.5
300
T(K) Figure 1. Relative Pvapfremperatureplots for acetone, hexane, and ethanol. chemistry faculty member are present a t all times to assist the students. When students have acquired data a t all temperatures, they return to the lab to perform the calculations necessary to put the data in plottable form (wide infra),then enter their data to a computer file a s discussed below. At this point, if time permits, the TAconducts a post-lab discussion of the experiment, using either the data obtained by hisher students or data obtained by a n earlier section. If there is insufficient time for a post-lab discussion, it is deferred until the recitation meeting in the subsequent week. I n our experience, very close to three hours is required for the pre-lab briefing, gas chromatography, and data analysis. Smaller sections may complete the experiment in time to allow post-lab discussion. Experimental ~ea~ents
Reagent grade acetone, hexane, and ethanol were obtained from reliable commercial sources and used a s received.
T(K)plotsforacetone, hexane, Figure 2. In (Relative Pvap)/Reciprocal and ethanol. Equipment
At WPI we use Perkin-Elmer AutoSvstem 9000 Gas Chromatogrnphs cqulppcd wlth widr-tm.c fuicd silica wpillarv columns PE Sumber N9312713,rnethvl 31Jr; vhrnsl silicone liquid phase) and flame ionization detectok. F& the experiment described here, we use column, injector and detector temperatures of 80, 150, and 200 "C, respectively, and carrier gas (He) flow rate of 10 m u m i n . Each GC is interfaced with a DTK 386 computer/HP Lasej e t 111 graphics printer. Data collection, analysis, and display are accomplished using Lahcalc software from Galactic Industries. Injections of vapor samples (typically 0.2 mL) are made using Dynatech 2.0-mL gas-tight syringes. These syringes are of excellent quality and durable, so are ideal for student use. Variable temperature was achieved and maintained using a Forma Masterline Constant Temperature bath and circulator. Data plots and fits were carried out using ProPLOT Scientific Graphics, Version 1.0, from Cogent Software. Sample injection Technique !lb obtain reliable and reproducible signal areas from vapor samples, we have found i t necessary to dilute the vapor
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March 1995
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sample with air prior to injection. Dilution of a 0.2-mL vapor sample with 0.6 mL air gives excellent reproducibility. Failure to dilute the vapor sample with air results in poor signal reproducibility, due to condensation of vapor in the syringe during compression. Students are told to make as many successive injections of vapor as they can during an 8-min GC run. Typically, as many as seven or eight injections can he accomplished. They are then told to eliminate the highest and lowest signal areas a t each temperature, leaving them with about five areas for each T. From these they compute the average signal area due to vapor a t each temperature. Reproducibility in signal area is better than 3%. Data Treatment The table contains representative student data for acetone. For each temperature, students calculate a mean signal area, a s indicated in the table. Signal areas are then converted to relative vapor pressures using eq 1. Signalareaat T Relative P,,., a t temperature T = Signalareaat 20 'C ( 1 ) Conversion to relative vapor pressures (relative signal areas) in this fashion eliminates systematic errors due to the use of different gas chromatographs or to differences in injection technique from group to group. Finally, each student group enters its vapor pressure-temperature data to a computer data file a s a series of points in the format, (T(K),P,,,).The TA then generates a plot of student data that can be distributed subsequently to the students and used a s the basis for a post-lab discussion. Results Representative plots of student data for the three liquids investigated are shown in Figure 1. In all cases, the total relative vapor is plotted against Kelvin tempera. pressure ture. An exponential incriase in P i , with temperature is seen in all cases. Data giving plots having essentially these forms were obtained by all sections. I n no case was an incorrectly-shaped plot obtained due to error in student injection technique. Clausius-Clapeyron plots of the data
260
Journal of Chemical Education
are shown in Figure 2. Good linearity and AH,., values are obtained for all three liauids. Overall, the experiment was highly successful in terms of student enthusiasm and learning. Students were particularly pleased to use modern scientific instrumentation to obtain data that cannot be obtained readily in any other way. The experiment provides an excellent introduction to modern laboratory methods for students, and presages the career experiences that some of them will have. Conies of the ex~erimentalprocedure.. .rel lab briefing. -. and instructions to teaching assistants are available onrequest. 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. Literature Cited 1. Reporton the National S6enceFoundationDisdplinaryWorkshapr onUndergrsdu. ate Education, Diredorate far Seience and Engineering Education, National Science Foundation. A p d 1989. 2. An Expiomlion o i t h Nature ~ end Quoiify of Uindergmduczl Edumlion in Science, Mothamalics, ond Endmerirw, R w n d t h e Nolioaol Aduisov Group ofSigmo 3. Morgan, D.The Sci~nfisl1991, February, p 1. 4. Lagowski, J. J.J. Chem. Educ. 1590.67,1. 5. Lseowski. J. J.J. Chem Educ. 1991.68.271.
I 2 Scan .I n 11 J ( ' h w F l u , i9M 5-..ltiL I? Dr.' 011 .I .\ d l'n.x, F o . r 1980.5- bh' 14 l ~ ~ c I : ~ ~ \l?n,:,r .c: r . C J Clev F d . r 1981 i B . 2 ;
15. Leuinson, G.S. J. Chrm. Edue. 1982.59.337 16. Lone. . . ". J. W. J.Chsm. Edw. 1982.59.933. 17. Schabec P M.J. Chsm Edur. 1385.62.345. 18. Richardson, W. S.:Jones, R. F. J Chem Educ. 1981.64.968. 19. Hall, P.K: Wollaston. G.J Chem. Educ. 1987,M. 969. 20. Hsmhly. G. F. J. Chem. Educ. 1990,67,278. 21. Soars. J. A.:Grieve. C. J. J. Cham. Educ 1990.67.427.
