Properties of Nonideal Binary Solutions An Integrated Physical Chemistry Experiment DavM B. Marshall, J. L. McHale, Suzanne C a m e l l , and DavM Erne University of Idaho. Moscow, ID 83843 We have recently switched t o an integrated approach in our physical chemistry lahoratory course. This is in part due to our dissatisfaction with the usual collection of physical chemistry lahoratory experiments but also because for many of the engineering students in the course this is their only exposure to both chemical instrumentation and an integrated, researchlike laboratory experience. The student response to this change has been overwhelmingly and enthusiastically favorable. The major drawback to this approach, however, is the dearth of appropriate integrated projerrs with a physical rhrmistry emphasis. The major integrated lahoratvry texts that are available (1,2) mostly rontain projects with a more synthetic or analytical emphasis. (However, they do contain some very fine physically related projects, which we have adopted; the reader is encouraged to consult these sources for further details.) The lack of appropriate thermodynamic projects in the literature is particularly noticeable. We describe here an integrated physical chemistry experiment that emphasizes a numher of thermodynamic principles in a cohesive fashion. The students are exposed to a numher of concepts concerning the hehavior of liquids and liquid mixtures and gain lahoratory experience in measuring a variety of physical properties such as viscosity, partial molar volume, density, dielectric constant, heats of solution, surface tension, and solvent-induced spectral shifts. This project can easily be extended t o cover an entire semester's work, or shortened by elimination of various measurements to a period of 3-6 weeks. Although the properties measured are rather typical parts of most physical chemistry laboratorv course exneriments. we have found that making these measurements on a common system is beneficial in terms of the level of student interest and learning. Background As is well known, the properties of ideal binary liquid mixtures in general have a linear dependence on mole or volume fraction. Nonideal binary mixture hehavior is not as easily predictable. The deviations from ideality serve a usefulnedaeoeical nurvose in alertinestudents to the scone and limitations of ideal solution descriptions, as well as providing them with insights concerning the nature of those intermolecular interactions that cause these deviations from ideal hehavior. I t is particularly useful to have the students study a mixture such as hexane-carbon tetrachloride, which behaves nearly ideally, and compare its hehavior with, for example, dimethylsulfoxide and water. Ethanol-water is also a good system. The contributions t o nonidealitv from strongdipoleldipole forces and hydrogen bonding can then he demonstrated. Data treatments can vary according to the abilities of each student. The real data can simply he plotted and compared with the curves expected for ideal solution hehavior. The student can then nostulate reasons for anv observed statistically significant beviations from ideal he"havior and reach the conclusion that nonideal macroscopic properties result from microscopic inhomogeneities. Although the technical details of modern theoretical and experimental approaches to deducing local structure in liquids are beyond the grasp of most undergraduates, the typical student can easily appreciate the variety of potential functions used to model pairwise interactions. Expressions
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for the dipole-dipole, dipole-induced dipole and dispersion incerartions, and the 1.ennard-Jones function, can be found in most standard physical chemistry textbooks. The texts hy Lesk (3) and by Laidler and Meiser ( 4 ) also contain introdurtions ro modern romputer simulation terhniqws. In order to get a more quantitative handle on the idea of local liauid structure. the review hv Chandler (5) is recommended fo; its explanation of the raiial distribution function. This reference also provides a clear discussion of the role of attractive versus repulsive forces in determining liquid structure. The Experiments We first have the students do some background reading on of binary the general theory (6)and physical properties (7,8) solutions and then choose the systems they wish t o study. We of course veto svstems that are not completely miscible or nrr unduly hazardoui to work with. Following the selertion of a binary system or systems, the student t h i n selects several nhvsical vronerties to measure. & . Properties that are suitable include: partial molar volume (from densitv measurements). .. viscositv. .. dielectric constant (from oscillometry), heats of mixing, conductance of electrolytes dissolved in the polar binary mixtures, and spectral shifts of probe molecules. General procedures for these experiments may be found in standard physical chemistry laboratory texts (9-11). Three of these possible properties are discussed in greater detail below.
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Handling .Precautions
Carbon tetrachloride is a suspected cancer agent, and dimethyl sulfoxide is an irritant and a contact hazard, since i t can carry other, more dangerous, substances dissolved in i t is a flamthrough the skin. N,N-dimethyl-4-nitrosoaniline mable, toxic solid and should he handled with due care. All solvent mixtures should he prepared in a fume hood and contact with skin avoided. Densitv and Partial Molar Volumes Measurement of binary liquid mixture densities allows discussion of c o m ~ l e xformation in nonideal mixtures. Partial molar volum& may also be obtained via the intercept method (9-11). Dimethyl sulfoxide-water is a particularly convenient system to study, as i t shows a large density maximum near 0.5 mote fraction in DMSO, corresponding to a partial molar volume minimum near 0.8 for DMSO and a minimum near 0.45 for water. Comparison with the linear hehavior exoected for an ideal solution clearlv demonstrates the deviation from ideality and suggests to the student possihle reasons for the deviation. includine. the vossihle sroichiometry of the complex formed. Care should he taken to use Dure DMSO and douhlv distilled water, as impurities will affect the measurements. Proper temperature-control (constant-temperature baths) is of course also necessary. The 1)MSO w~tersolut~onsshould he stored tightly rovered since DMSO 1s stn)nalv hvgroscopic and the mole frart~onof water will increase oier time in poorly covered containers. Viscoslfy Viscosity is also a conveniently measured property using Volume 64 Number 4
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as a Figure 2. Energy of me lowest transition of N,Ndimethyl4n~ros~niline functionof reciprocal dielectric constant of DMSO-water mixtures. Figure 1. Viscosity of
DMSO-water
mixtures, ploned against mole fraction
DMSO.
standard Ostwald viscometers. DMSO-water is again an intersting system, as mixtures of these two solvents exhibit viscosities higher than either of the two pure components, with a large viscosity maximum near 0.35 mole fraction in DMSO, as shown in Figure 1. This allows discussion of the effects of strone c o m ~ l e xformation on the kinematic D. ~ O.D erties of liquids, through increases in the effective molecular size uf the flowinr liquid. Again, cleanliness of the viscometers, proper ternp&a&e co&rol and purity of the solvents is crucial to obtaining good results. Carbon tetrachloride, with a viscosity of 9210 glcm s, is a convenient standard. Dielectric Constant, Spectroscopic Shifts, and Solvation
A particularly interesting experiment is the measurement of s ~ e c t r a shifts l of robe molecules. We have found that N,N-dimethyl-4-nitrosoaniline (available from Aldrich) is a usctul nrohr. The maximum wa\deneth of its viaihle ahsorption hand varies from 398 nm i n e t h e r to 440 nm in water. I t is thus a very sensitive probe of changes in solution properties. We have the students plot the energy of the transition versus a variety of functions of the mixture properties. A representative plot is shown in Figure 2. For instance, the transition energy may be plotted against the reciprocal of the mixture dielectric constant (obtained via oscillometry), the Onsager reaction field function of the dielectric constant (12).and the Block and Walker dielectric constant f u n c t i o n ' ( ~ j j . ~ h efunctionsdeal, se with increasing lewl of sonhisticntion. with modifications in the local dielectric experienced by a probe molecule in solutions of increasine ~ o l a r i t vand level of directed interactions. The Onsaeer f u k o n , eb 1below, is suitable for the description of solvint interactions in "moderatelv ~ o l a r "environments. However, 11 does not account complft;ly for effects of local dielectric saturation and rherefore does not describe highly polar mix370
Journal of Chemical Education
tures very well. The Block and Walker function (eq 2) represents the effects of polar solvent molecule clustering around polar solutes more effectivelv. Abboud and Taft have described the application of th;~lock and Walker function to avariety of phenomena that depend on solvent polarity (14).
Com~arisonof these functions eives the student insieht into the Garinus types of interacti&s operating in liqzds and binary liauid mixtures. I t is ~articularlvuseful to comDare resul& fo; low-polarity versui high-pola>ity, non-hydrogenbonding versus high-polarity, hydrogen-bonding mixtures. The students gain a very clear picture of the contributions of van der Waals, dipole-induced dipole, and dipole-dipole forces in accounting for the overall mixing and solvating behavior of liquid mixtures. Literature Cited
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4. Laidlor, K. J.: Melaer. J. H. Phyairnl Chemlalry: Benjamin-Cumminga: Menlo Park,
CA, 1982.
5.
Chsndier,D.Acc.Chem.Ros. 1574.7.247. H.:omiiie-~homaa. W. J.
~ ~ t ~wiiey: ~ N ~~W Y~O . ~ ,1981; t i ~ 7. Riddick, J. A.: Burger, W. B. O w n i c Soluenla: Physical Properfie8 and Methods of Punlficnlion, 3rd ed.; Wiiey: New Yark. 1970. ~ system h ~ in ~ concpnmpd i ~ ~ ~ 8. ~ i m m e m s n J.~ ,~ h ~ ~ icone~ on^^~ 01-Binary Solutions; Intcrscienee: New York. 1959-1'360.4voia. 9. Shoemaker. D. P. et el. Expdrnants in Physical Chemistry. 4th d.:McCrsw-Hill: NOWYnrk, 1978. 10. Dsnieis,F.et at. Experimon
