Donald Barton, Roy Ralph, and Kevin Kane Memorial Univers~ty of Newfoundland, st. John's, Newfoundland, Canada
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The Dissociation of Acetic Acid Dimer in the Gas Phase
Although the determination of the temperature dependence of equilibrium constants is a valuable part of the student's work, the number of suitable gas phase systems appears to be limited. The most commonly used is the dissociation of NzO,; see for example the experiment of Phipps, et al. (1) and a modification by Murray (2), or Daniels (3). We have found that the dissociation of carboxylic acid dimer in the vapor phase can he studied in the undergraduate laboratory and report here an experiment based upon the technique of Taylor (4),using acetic acid. The Experiment The acetic acid may be placed in the sample vessel A through the stopcock bore by means of a thin pipet, weighed, and distilled into trap B through joint F (see
The metallic strip regulator D, connected to an electronic relay, allows convenient and rapid temperature adjustment. In this experiment the temperature was measured on the surface of the 2-1 vessel by means of a chromel-alumel thermocouple and a student potentiometer. The air bath also contained a mercury thermometer to allow easy observation of the temperature during adjustment. The mercury levels were measured by means of a cathetometer and the pressure in torr was calculated using Hg density tables. Results
Data obtained by a student, using glacial acetic acid, are given in the table, and the calculated equilibrium constants (K,,) are plotted in Figure 2. Using the T"C
Dimer Dissociation DataP (torr)
Data courtesy J. Desmond Cousens. Weight of acetic acid; Volume: 2276 cm'.
0.1795 g.
notation of Taylor
(4) the equilibrium constant is
PM and PDare partial pressures of monomer and dimer, respectively, and P is the measured pressure. P,is the calculated pressure for no dissociation,
Figure 1. Air bath and vacuum line. The bath is 3 6 in. X 21 in. X 2 1 in. with '/*-in. .$bestor walls upon whish are coiled chrmel-A heating w i r e The walls are lined with bright aluminum foil. A, sample vessel with 3-mm stmight bore stopcwk; B, U4rop; D, metallic strip rsgulotw (American Instrument Componyl; E, 2-1 uesrel; F, ground [oint; K, door; L window; M, 20-cm diameter fan; Q, trap, R, Hg manometer, 1 5 mm od; S, outline ofair furnace; X, rtopmcks; P, to pump.
Fig. 1). It may also be introduced by expanding i t a t room temperature from the sample vessel, closing stopcock C, and condensing the extra acetic acid in the sample vessel. In the latter method, since the sample vessel is weighed before and after expansion into the apparatus, care must be taken to carefully clean the ground joint of grease before weighing. In the first method this problem does not arise. In either method it is necessary to thoroughly out-gas the sample while i t is in the sample vessel. 440 / Journal of Chemical Education
2.6
I/T Figure 2.
3.0
2.8
x
Temperature dependence of K,,
lo3
Pi = 0.1795 X 62.40 X T ~ / 1 2 0 . 1X 2.277
At 353.3"K, for example, Pi is 14.47 mm, P is 21.62 mm, and K,, is 27.94. The following treatment of errors is based upon that given by Shoemaker and Garland (5). The limit of error in weighing was neglected. The limit of error in the measurement of pressure was taken as 0.05 mm from the specifications of the cathetometer. The limit of error in the measurement of temperature was generously set at 1°C. The measurement of temperature could be improved by placing several thermocouples over the surface of the vessel and using the average temperature, by using a thermocouple well, and by calibrating the thermocouple(s). The limits of errors in 1/T and K,, were calculated from the limits of errors in T, P, and PC. The convenient method of reference (5) was used for determining the limit of error in the slope. The slope, estimated visually, giving AH = 15.1 kcal was 3,300 (log,&,)/(l/T), with a limit of error of 0.5 kcal. Discussion
The structure of the carhoxylic acid dimer appears to be well established as a ring containing two hydrogen bonds. Therefore AH/2 is equal to the average hydrogen bond dissociation energy. The equilibrium method
of determining bond dissociation energies is discussed by Benson (6). The student should refer to Coulson (7) for a discussion of the hydrogen bond. From K,, AGO and ASo are available. The student may estimate the translational entropy change accompanying dissociation (8) and observe that most of AS" is thus accounted for. This point and many others, including the range in values of AH reported in the literature, and the effect of adsorption, are discussed by Allen and Caldin in a review of work on the association of carhoxylic acids (9). Literature Cited PHIPPS,T. E., S P E . ~ M AM. N , L.,AND COOKE,T. G., J. CHEM.EDUC.,12, 318 (1935). MURRAY, J. W., J. CHEM.EDUC.,4 2 , 6 6 7 (1965). DalrrErs, F., et al., "Experimental Physioal Chemistry," (6th ed.). McGrsw-Hill Book Co.. New York. 1962.. D. . 96. TAYLOR, $ D., I.J . Am. Chem. ~ o e . , ' 7 3315 , (19k1). SHOEMAKER, D. P., AND GARLAND, C. W., "Experiments in Physical Chemistry," (Snd ed.), McGrm-Hill Book Co., New York, 1967, pp. 22, 30, and 34. BENSON, S. W., J. CHEM. EDUC., 4 2 , 5 0 2 (1965). COULSON, C. A., "Vdence," (&d ed.), Oxford University Press, London, 1961, p. 344. For example see BARROW, G. M., "Physical Chemistry," (&ded.), MeGrew-Hill Book Company, New York, 1966, p. 254. ALLEN,G., AND CALDIN, E. F.,Quart. Rev., 7 , 255 (1953).
Volume 45, Number 6, June 1968
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