The Vapor Pressure of Liquid Binary Solutions An Experiment for the Physical Chemistry Laboratory
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Luis H. ~lanco,' Carmen M. Romero, and Ricardo Munar Facultad de Ciencias, Universidad Nacional de Colombia, Bogota, Colombia
The ideal solution model and the deviations from i t are usually dealt with during the first physical chemistry course. Sometimes even in the general chemistry program, a few hours are spent introducing this topic. Raoult's law i s stated and plots of vapor pressure a s a function of composition for binary systems are used to illustrate ideality and deviations from it. Z Textbooks of general chemistry, physical chemistry, and chemical thermodynamics often use a few typical examples: benzene-toluene a t 60 "C or ethylene bromide-propylenebromide a t g &@ an undefined temperature for ideal behavior; acetone-chloroform and acetone-carbon disul.g d re fide a t 35.17 "C are by far the favorites for sys- . tems showing deviations. The graphs are repro@ duced in Figure 1( I , 2). Two questions could be asked: What i s the 0 0.2 0.4 06 08 1.0 reason for the rather awkward choice of temperatures? Who is the author of the original data? These questions are not easily answered 600 with the information available in most textbooks. The first one could have a variety of an- 2 sOo swers, but we know of none entirely satisfac- E 400 tory. T h e second question, a t l e a s t for t h e deviant systems, can be answered a s follows: J. 300 van Zawidsky in 1900 (3). Only a few authors 200 have cited this work i n their references. The usual practice is to present the plots, even with- a 100 out mention of temperature. oa 0.2 0.4 0.6 0.0 1.0 Getting a n experiment to show this behavior i n the laboratory i s quite difficult. So far we X CS2 have not found a descri~tionof a ~roceduredesigned for this purpose in the more commonly Figure 1. Textbook examples of vapor pressure of binary solutions. used laboratom manuals. Taking this into account along with the importanceof the subject other altitudes the necessary adjustments of the heights and the characteristics of the examples used i n the books, should be made. we have developed and have used for several semesters the Water is circulated a t the selected temperature through experiment described in this paper. jacket E by means of ports G. The apparatus is tested for air leaks, measuring the barometric pressure. The vacuum Experimental Procedure pump is connected through stopcock A. The mercury colThe apparatus is shown in Figure 2. I t i s a modification umn should rise until the value of the local atmospheric of the one described by Driscoll(4). The barometer principressure i s reached. When this happens the stopcock is ~ l ise used. The t e m ~ e r a t u r eis controlled bv circulating closed. If there is no leak present, the height of the merwater from a n external thermostated bath a t convenieni cury column should remain constant. rate. The stopcock and the mound joint must be of vacuum To get vapor pressure data one should proceed as follows: quality to air leaks. air is allowed to enter slowly into glass tubing C. Once the The glass tube C is 72 cm long. Its internal diameter i s 7 pressure inside is atmospheric, ground joint B i s opened. A mm. The total height, including the ground joint and the small quantity of the liquid (pure or solution) is placed in stopcock is 86 em. The side connection has 6 cm of length. C by means of a pipet or a syringe. The ground joint is put The glass jacket E is 60 cm long and has a n internal diameback in place, and the vacuum line is connected very careter of 5 cm. The chosen heights take into account the fact fully, then the vapor space is re-evacuated. The solution (or that atmospheric pressure in BogotA is around 560 mm Hg pure liquid) evaporates sufficiently to fill the gas volume because the city is located a t 2600 m above sea level. For below stopcock A, driving the mercury to whatever height i t needs so that the pressure in the vapor space plus the 'Corresponding author. pressure due to the mercury column equals the pressure of
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the room. The height of the column is recorded. The correction due to the pressure of the liquid solution column, can be made, if is considered necessary. The evaporation of some of the liauid mixture to establish the vapor pressure, changes the liquid composition. If t h e vapor space i s small with respect to the liqu i d volume, t h e effect becomes trivial. The total height of the glass tubing C should be calculated, according to the local atmospheric pressure, to have a small vapor space. During the experiment the liquid sample is riding on top of the mercury column. To get a reading for a new sample, it is necessary to clean tube C completely and to clean and outgas the mercury again. For this purpose, tube C is taken back to atmospheric pressure, and the liquid sample is removed using filter paper or a syringe. The liquid t h a t remains in the tubing is elimi-
This experiment has been programmed for the first
.ohvsical . chemistw course for several semesters. The tem-
perature usually is chosen to be the traditional 25 "C. However, from time to time, the values from literature; i.e., 30 "C, 35.17 "C, and 60 "C are used. For the ideal behavior we have programmed the systems: benzene-toluene and n-hexane-n-heptane. For negative deviations, acetone-chloroform. For positive deviations dioxane-water (5).Some of these systems are difficult to handle due to the high values of the vapor pressures. We use them because they are the most commonly found in literature. Once in a while, when students ask specifically about it, the system carbon disulfide-acetone is used. However, we do not program i t regularly, due to the very high vapor pressure of carbon disulfide and some of its other properties that make necessary special precautions. Some representative results are shown in Figure 3. The temperatures were kept constant to t0.1 "C. The experimental data can be compared with literature. For the pure compounds the values can be found in the standard handbooks. In this case, the comparison shows that they are quite acceptable, given the method used and the level of the students (third semester. chemistw maiors). The results depend a great deal on the" ski~iandcare of the students. As it mav be seen in Fieure 3. the behavior of the u systems is illustrated clearly, so that the purpose of the experiment is fulfilled. This applies to the average student we
Figure 2. Experimental setup. Vacuum quality stopcock; Vacuum quality ground joint; (C) Glass tubing; (D) Thermometer lilO C div: (E) Glass jacket; (F) Mercury; (G) Water inlet and outlet: and (H) Meter stick marked with l-mm (A) (B)
:34/ nated by vacuum evaporation, and the mercury is outgassed. The usual precautions should be taken to avoid damage of the vacuum pump. Care must be taken to avoid having mercury in the space above stopcock A. A cold trap should be placed between the apparatus and the vacuum line. The next liquid sample is added, and the experiment is repeated a s explained. The solutions are prepared, weighing the p u r e components i n q u a n t i t i e s t h a t should be sufficient to obtain five or six milliliters of solution. The concentration values should cover the complete mole fraction range. In a regular four-hour laboratory session, 6 to 10 readings with duplicate determinations can be obtained. Usually this number of data readings is enough to draw the vapor pressure versus composition curve. Different groups of students work different systems. Results are pooled, and the class as a whole has access to re~resentativeexamples of the three kinds of behavior through Figure 3. Examples of vapor pressure versus solution composition curves obtained in the data taken by themselves. laboratory. Volume 72
Number 12 December 1995
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have in our classes. If one feels that some student is specially careful, a more demanding system could be assigned. F i m e 4 is an example of this. The system butanol-water was used ab 25 "C. Even if the agreement with literature values is not complete, the behavior of a system with small vapor pressures and partial miscibility can be illustrated. Acknowledgment We acknowledge the contributions of our students in the development of these experiments. We would like to express our appreciation to the reviewer for his comments and his suggestions for improving the manuscript. Literature Cited 1. laxls,G. N.;Randall, M. Thermodynam~cs,2nd ed.: McGmw-Hill: New York. 1961: no. 215-216.
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chsng, R. physieoi chemistry ~ i t ~h ~ ~ to i ~ i ~ i~o g i~svderns: c ~ ti i M ~~ C M~ ~ I I~~ " : New Yark,1977: p. 260. 3. "on 2axldskv. J. 2.Phvsik. C h r m 1 9 W . 3 5 . 1 2 9 2.
