Experiments for Modern introductory Chemistry: Intermolecular Forces and Raoult's Law Ladislav H. Berka and Nicholas ~ildahl' Worcester Polytechnic Institute, Worcester, MA 01609 In response to the widely recopized prohlems with the traditional approach LO the introdurtory chemistry laboratory 11-51, we have initiated the complete restructuring 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 previously described experiments based on gas chromatography (GC) and electronic absorption spectroscopy (8,9). We now present a second GC experiment that promotes discovery of deviations from Raoult's Law in binary mixtures of simple liquids. Experiments dealing with the measurement of vapor pressure and Raoult's Law have appeared in this Journal (10-15), and an experiment involving use of GC for Raoult's Law measurements has been described (16). This experiment simply and elegantly illustrates liquid-vapor phase equilibrium of ideal and nonideal solutions. The experiment is appropriate for beginning freshmen, yet is challenging, interesting, and informative. Students discover the following principles. .Vapor in equilibrium with a binary liquid mixture can he sampled and quantitatively analyzed by GC. Deviations from the ideal vapor pressures predicted by Raoult's Law provide a "view"of the intermolecular farces between molecules in the liquid phase. These deviations are readily understood in terms of the molecular structures of the components.
Ove~iew Approximately 100 students, primarily chemical engineering, chemistry, and biology majors, are enrolled in CH1040, the last in a sequence of four introductory chemistry courses, during the spring term. They are divided into five sections of about 20 students each. Each section meets once per week for a 3-h laboram. The department has three &!as chromatographs fbr introhuctory lab, easily sufficient for 20 students during a 3-h period. The period begins with a 15-Ain briefing by the TA, in which the basic nrincinles of GC are reviewed. (Students have already usLd GCLonseveral occasions during the introductory sequence.) Raoult's Law is introduced and explained. The TA then presents the challenge to be met using GC: Study the vapor in equilibrium with mixtures of two liquids as a function of the liquid composition, to find out whether Raoult's Law is valid. The TA announces the pair of liquids (acetone/chloroform, hexandheptane, or acetnne/heptane)to be studied. The section is dlvidcd into no more than six groups (two ner chromnwmaoh, of three or fuur students each. The TA then assigns each group a specific mixture composition to prepare i d studyso that the entire range of possible liquid mixtures is covered. Composition is specified on a volume basis to simplify preparation.
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'~uthorto whom correspondence should be addressed.
Each student group then prepares three samples in septum-capped 25-mL Erlemneyer flasks. The first flask contains 1mL of the first pure liquid of the pair. The second flask contains 1mL ofthe second pure liquid, and the third flask contains 1 mL of the assiened mixture. The TA assigns groups to instruments, and reviews the sample-injection techniaue and oneration of the comnuter. This requires a n akditionali5-20 min. student's then proceed with their analvses. If two a o u ~ share s an instrument. they alternate cnjections so 'ihat'no group is inactive fo; more than 10 min (the time twicallv reauired to make a run and print the chromatogra"&). stidenis in a group permute the injection and computer operations, so everyone has an opportunity to carry out all aspects of the procedure. The TA and a chemistry faculty member are present at all times to assist the students. When students have acauired the necessary data, they return to the lab to carrv out the calculations~neces~ary to put the data in plottab& form (see Data Treatment below). Then they enter their data to a computer file as discussed below. At this point, if time permits, the TAconducts a postlab discussion of the experiment, using either the data obtained by his or her students or data obtained for the same liquid pair by an earlier section. Some auestions used to &ide this discussion are given below. If there is not enough time for a postlab discussion, it is delayed until the recitation meeting the next week. In our experience, very close to 3 h is required for the prelab briefing, gas chromatography, and data analysis. Smaller sections may complete the experiment in time to allow postlab discussion.
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Reagents
Reagent-grade acetone, chloroform, hexane, and heptane were obtained from reliable commercial sources and used as received. Equipment At WPI we use Perkin-Elmer AutoSvstem 9000 Gas Chromatographs equipped with wide-gore fused-silica capillarv columns (PE no. N9312743. methvl 50% ~ h e n v l sificone-liquid phase) and flame ioni~ationdetectokF& this experiment we use injector and detector temperatures of 150 and 200 "C and a carrier gas (He) flow rate of 10 mWmin. Column temperature is 35 "C forhexaneheptane, and 45 OC for acetone/chloroformand acetoneheptane. Each GC is interfaced with a DTK 386 computerMP Lasej e t 111graphics printer. Data collection, analysis, and display are accomplished using Labcalc 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, making them ideal for student use. Data plots and fits were carried out using ProPLOT Scientific Graphics, Version 1.0, from Cogent Software. Volume 71 Number 7 July 1994
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RESULTS Intermolecular Forces and Raoult's Law
Name Section Coworkers Date Liquid pair: Liquid A =ZSX@!E
Density = MW
=
Mixture 1
VoiA 0.2 rnL
Voi B 0.8 rnL
Liquid B
Density
= h&m@ =-
MW
=
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lo3x MO! A
lo3 x MoI B
Volumes Fraction A 0.20
Mole Fraction A
Relative Area 13
Relative Area, Total
SignalArea (Vapor Pressure) Data
Flask no.
Vol Fract A
Inj
Signal Area A
Signal Area B
Relative Area A
1
1
1
3 u
-
2 3
3.842 3L5
4 5
3L8 362
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32.6
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Mean Ann 2
Aa
4
4
L
0.72
0.92
1.64
L
1
1
1 6 1 2 3 4 5
Mean Attn 3
1
222
1&Z
ax 223
E.5 u.5
ELZ
lac
229
36
BJL 1 6
1 2 3 4
a2.Q
5
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Mean Attn Conclusions based on section results:
rn Im 19;L
1 6
Figure 1. Reproduction of student Results sheet. Sample-Injection Technique Careful: Failure to dilute the vapor sample with air results in poor signal reproducibility due to condensation of vapor in
the syringe during compression.
To obtain reliable and reproducible signal areas from vapor samples, we found it necessary to dilute the vapor sample with air before injection. Dilution of a 0.2-mL vapor sample with 0.6 mL air gives excellent reproducibility. 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 be accomplished. 614
Journal of Chemical Education
They are then told to eliminate the highest and lowest signal areas for each GC signal, leaving them with about five areas for each signal. From these they compute the average area for each signal (one for a pure liquid; two for a liquid mixture) in each of their three runs. Reproducibility in signal area is better than f3%. Data Treatment
Figure 1is a reproduction of the student Results sheet for the experiment, with representative data entered for the two pure liquids and the one assigned mixture. For each pure liquid and for each component of the mixture,
students calculate a mean signal area, as indicated in the figure. Signal areas are then converted to relative vapor pressures using relative P,
of Ain mixture =
signal area of A over mixture . sgnal area of A over pure A
where the two liquid components are designated Aand B. Conversion to relative vapor pressures (relative signal areas) in this fashion eliminates systematic errors due to the use of different gas chromatographs or differences in injection technique from group to group. Finally, each student group enters its mixture data to a computer data file as three points in the following formats.
..
volume fraction of liquid A, relative P,.,(A) volume fraction of liquid A, relative Pqsp(B) volume fraction of liquid A, total relatlve P,
The total relative vapor pressure is the sum of the relative vapor pressures of A and B over the mixture. The TA then generates a plot of student data that can be distributed later to the students and used as the basis for a postlab discussion. Results
Representative plots of student data for the three liquid pairs investigated are shown in Figures 24. In all cases, the total relative vapor pressure &d the relative vapor pressure for each component are plotted against volume fraction of one component in the liquid mixture. Negative deviations from Raoult's Law for acetone/chloroform,near ideality for hexaneheptane, and extreme positive deviations for acetoneheptane are clearly seen. Data giving plots with essentially these forms were obtained by all sections. In no case was an inwrrectly shaped plot obtained due to error in student injection technique. Overall, the experiment was highly successful in tern of student enthusiasm and learning. Students were particularly pleased to use modern scientific instrumentation to obtain data that cannot be readilv obtained anv other way The experiment provides an exc&ent introdukon to modern laboratom methods for students and oresages the career experiences that some will have. (cop{es of The experimental procedure, prelab briefing, and instructions to teaching assistants are available on request.)
Vol Frad hexane
Figure 3. Relative P,dvOlUme composition plot for hexanelheptane total Pmp;o PVap hexane; A P- heptane).
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Postlab Discussion
We believe that the nature of this discussion should be largely left to the faculty person or TAin charge of the laboratory. However, we present an outline of the discussion we use at WPI to serve as a model. The outline is most effective when used with an overhead transparency showing the vapor-pressure/composition plot obtained collectively by students.
We begin at the board with the Raoult's Law equations for a mixture ofAand B.
In tenns of relative vapor pressure
t " " I " " I ' " ' I " ' ' ~ " " 3
0
0.2
0.4
0.6
0.0
1O .
Vol Fract acetone
Figure 2. Relative Pv&olurne composition plot for acetoneichloroPWpaetone; A Pvwchloroform). form (-total P,;
VOI Fran acetone
Figure 4. Relative P,dvolume composition plot for acetoneheptane total P,; 0 P, acetone; APVap heptane).
(0
Volume 71 Number 7 July 1994
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We discuss with the students how each of the following plots should appear.
W h a t behavior should be shown by t h e following liquid pairs? acetone:CSz (positive) ~entane:hexane(ideal) kthano1:ethanol (ideal) methano1:hexane (positive) CSz:CC1, (ideal) acetone:HzO (negative)
1. relative vapor pressure of A v c m u ~XA 2 IEINBVC vapor pressure of 1%\.enus X A 3. total relative vapor pressure versus XA
Hopefully, they will suggest three linear plots
Discussion of these pairs is based on the amount of similaritv between the molecules of the two substances. Some simple guidelines are
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We then project a transparency of the actual plot of relative P,, versus mole fraction of liquid A obtained by the students and discuss the shape of the plot in terms of ideality or nonideality of the mixture. The following questions can be used to dired the discussion. For liquid pair A:B, which indicates any of the three pairs, we begin with the following. Do the vapor pressures conform to Raoult's Law? How do the" deviate from Raoultk Law? (Are thev ereater or less than expected? In the mlxture, are the rnolecule~more content or less mnten, tu be ~n the I>qud phnw than in the puw Ilqulds9 Enplain. What are intermolecular forces? Does t h e data tell us anvthina - about the magnitudes of A-A intermolecular forces? E-B intermolecular forces? A-B intermolecular forces?
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We take the discussion a t least to this point and further if possible. If the students do well with the discussion, we progress to the following. Should heat be produced or absorbed when A and B are mixed? Why? S h o u l d the total liouid volume be less than. eoual to. or greater than the sum of volumes of pure liquids that were mixed? Why?
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Journal of Chemical Education
bath nonpolar-ideal both polar-negative one polar, one nonpolar-positive
Acknowledgment
We thank the National Science Foundation (Grant no. 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. Report onfhaNotio~l&kn~F~NiofiiiDiscipIinin'y Workhopsan Undeqmduat4 Education; Directorate for kience and Engineering Eduestion, National kienee Foundatim, April, 1989. 2. AD Explomtim of the Natat and Quality of Undergaduate Education in Sciena, Mathematics, and EngineeMg;Repat of the National Advlmli Gmup of Sigma Xi, January, 1989. 3. Morean. D. Tha &&"tist 1991.Februarv. 1. 4. Lagow~agki.J. J. J . Cham. Edue. 19W,67,1. 5. Lagowski J. J. J Cham.Edvc. 1991,68,271. 6. &ci. R. W.: Ditzler. M. A. J Cham. Educ. 1991.68.226. 7. Paris,M R.:Avmes,D. J. J. Chem. Educ. lW. 67,510. h Rurna. D S R ~ k nL. H . Kldald.S K .I Chrm N u 1983.70. AIUll 9 fildahl. Y K. B c r b . L H J C h r m Mur lR83.70.671 10 Van Hmkc C K J C k m Ed", 1891. 2nd rrfmnc~afhcrem .m.Wl . 11. Cheng,R.J.:Hsu,C.-H.:Taai,Y-F. J Chom.Edu. 19=, 69,581 12. Wilaon,A S.' Cham. Edue. 1990,67,598. 13. Kovee. J. J.Chem E d u . 19&5,62.1090. 14. StWe1.M. J. J. Chem. Educ. 1998.60.5W. . . 15. Koubek, E. J. C h . E d u . 198% 60,1069. 16. Rieti,R. W.;Ditzler.M.A.;Janett,R.M.ALobomto~.C~nt4mdI1ddtiiAppmh to f klkzhing of Chamist'y; Abstracts ofthe National Meeting of the Amarican Chemical Satiety: San Francisco, CA;Apd, 1992.
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