"' Enno Wolthuir, Roger Brummel, and Paul Vanden Bout Grand
Calvin College Rapids, Michigan
Determination of Vapor Pressure A general chemistry laboratory .experiment
Laboratory manuals for the general chemistry course generally neglect a study of the properties of liquids. Most textbooks include vapor pressure-temperature curves for a few liquids. However, determination of such curves seldom is assigned to students, presumably because more than ordinary equipment is required to achieve accurate results. We are suggesting a method for obtaining good vapor pressuretemperature curves with equipment available in any general college chemistry laboratory. For some years we have demonstrated the determination of a vapor pressure-temperature curve by any of a variety of simple methods (1-5). In our experience good results are not usually obtained by these procedures. Methods involving more elaborate equipment were also investigated (6-10). However, these make use of materials not generally available to students in general college chemistry. The method of Devor looked attractive for a student experiment but requires a special type manometer not usually available. However, we did try this method hut modified it by attaching the side arm of a suction flask directly to one end of a manometer after filling the flask with the liquid vapor and recording the pressures as the liquid cooled. After several trials this method
Figure 1.
was abandoned because too much time was required for the attainment of equilibrium between the liquid and its vapor, particularly a t low pressures and temperatures. A reflux method was also tried. For this purpose a two-neck flask containing the liquid was'fitted with a thermometer and a condenser connected a t its top to a manometer and an aspirator. This method was found to have the disadvantage that the reflux condenser -
This paper reports another in a series of quantitative laboratory experiments for the general chemistry course developed originally as freshman research projects (see J. CHEM.EDUC., 34, 133 (1957); 35, 412 (1958). Work here reported was performed by Mr. Brummel and Mr. Vanden Bout during their freshman year at Calvin College.
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flooded when the liquid boiled. As a consequence, the pressure and temperature fluctuated too much. The final choice was to use a simple distillation technique. Although it may seem that a simple distillation under a variety of pressures should present no problems, it was found that a good vapor pressure-temperature curve is obtained only under certain carefully controlled conditions. Bumping was a problem in the lower pressure range, even when boiling stones were added. By far the best preventive is the old technique of using a very fine capillary to permit circulation of a stream of small air bubbles through the liquid. Establishment of true equilihrium between liquid and vapor is absolutely necessary for accurate results. At first our practice was to begin distillation a t atmospheric pressure and gradually to reduce to lower pressures. Such a procedure was found to give erratic results, particularly a t the lowest pressures. Apparently this is due to the fact that, after the liquid has been heated enough to hoil a t the higher pressure, a lowering of the pressure requires the dissipation of considerable heat energy through evaporation of the liquid before a new point of equilibrium is reached a t the lower temperature. It was found that it is far better to begin the distillation a t the lowest pressure. Thereafter the pressure is increased to a definite value and just enough heat is applied to the liquid to make it distill a t a predetermined rate, a t which time the pressure and the maximum boiling temperature are read. In this way the liquid never possesses more heat energy than is necessary to make i t boil, and better control is obtained. The apparatus required is shown in Figure 1. It consists of an ordinary distillation unit. The 500-ml. distillation flask, equipped with a thermometer and a fine capillary to prevent bumping, is attached to a bulb-type condenser and receiving flask, and the latter to the manometer and pump. The whole system is evacuated by means of an aspirator rather than a vacuum pump to avoid traps needed to protect the pump from condensed vapors. The pressure is controlled by a bleeder valve, B. A screw-type Bunsen burner base works very well. The Experiment Put 250 ml of pure liquid in the flask and evacuate the system to the lowest possible pressure with valve B shut tightly. When the liquid starts to hoil, open B a little to raise the pressure just enough to stop the boiling. Then heat the flask with a very small flame until the liquid again boils and distills a t the rate of one drop per second. As soon as the temperature and pressure are stabilized at this distillation mte, read and record them. Now raise the pressure about 30 mm, apply a little more heat until the liquid again distills a t a constant temperature and pressure and s t the same rate as before, and again record the new values. I t is important not to touch the valve, B, during this time. The only control required is the amount of heat which is applied to maintain a. constant rate of distillittion. Repeat ss many times as debired to cover the whole range of pressures.
various boiling temperatures. Plot these vapor pressure values against the temperatures to obtain the VP-T curve. Student Results
Data were determined for three liquids: water, carbon tetrachloride, and isobutanol. In order to show the accuracy obtained by the students, Figure 2 gives the curves drawn from literature data and the actual experimental values (circles) obtained by the foregoing procedure. Students working in pairs can determine the data for two liquids in a three-hour period. Literature Cited (1) (2) (3) (4)
Record the atmospheric pressure, and subtract all pressure values from it to get the actual pressures on the liquid for the
FORBES,W. R., Chem. News, 106, 88 (1912). LONG,R. H., School Sci. and Math., 49, 453-4 (1949). MARSHALL, F., Field and Lab., 3, 13-15 (1934). S m m , A,, AND MENZTES, W. C., J. Am. Chem. Soe., 32,
907-14 (1910). (5) DEVOR, A. W., J. CAEM.EUUC.,22, 1 4 6 5 (1948). (61 DUMKE,W. H., J. CHEM.EDUC.,32, 383 (1955). (7) JUDKINS, R. L., AND BREWINWON, G . P., Am. J. Phys., 19, 380-81 (1951). (8) LEONARD, J. M., AND BULTMAN, J. D., J. CAEM.EDUC., 33, 623-4 (1956). (9) OTHMER, D. F., Ind. Eng. Chem., 20, 743 (1928). (10) ROGERS,J. W., KNIGHT,J. W., AND CHOPPIN,A. R., J. CHEM.EDuC., 24, 491-3 (1947).
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