DECEMBER. 1941 JOURNAL OF CHEM1CAL PHVS1CS VOLUME 9 The Entropy of Acetic Acid J. O. HALFORDl Department of Chemistry, University of Michigan, Ann Arbor, Michigan (Received August 8, 1941) From third law measurements, vapor pressures and vapor densities, the entropy of the acetic acid monomer at 25° and one atmosphere is 69.4±1.0 e.u. The value 68.7 is calculated from the vapor phase ethyl acetate equilibrium. For a model based upon acetone and approxi­ mately representing free rotation the entropy would be 72.7. If there is only a single potential minimum in the hydroxyl group rotational cycle, the large deficiency below the free rotation value is explained without assuming an exceptionally high potential barrier. A brief discussion of the effect of the number of potential minima and their relative depth is appended, and a possible source of error in third law measurements is suggested. HE entropy of the acetic acid monomer as a concluded that the entropy of dissociation of the T hypothetical perfect gas can be calculated dimer could not be evaluated from equilibrium from the third law entropy of the liquid and the data within several entropy units. This uncer­ heat of vaporization with the aid of the constants tainty would of course be carried in some measure of the reaction into the entropy of each component. The acetic acid case appears to be more favorable. Since the publication by MacDougalP of vapor This procedure or its equivalent is made neces­ densities measured at low pressures near room sary by the wide deviations of acetic acid vapor temperature, it is possible to evaluate the heat of from perfect gas behavior. The significance and reaction 1 and the composition of the mixture as accuracy of the result depend upon the complete­ a function of temperature with a high probable ness with which existing vapor density data may accuracy; From MacDougall's results and those be used to describe the vapor as a mixture of two of Fenton and Garner,4 the heat of dissociation perfect gases in equilibrium according to Eq. (1). of the dimer becomes Although it is reasonable to suppose that at iow ~H=18,853-9.76T (2) pressures this description should be valid, accu­ rate vapor densities at anyone temperature are and, for the dissociation constant, not available over a wide enough range of pres­ log k = 25.732 -4120jT -4.910 log T. (3) sures to establish the point by experiment. Conse­ quently the entropy requires support from other Constants calculated by Eq. (3) show an average sources. The calculation has therefore been made deviation of 2.0 percent from MacDougall's mean at a series of temperatures from 25 to 100° and an values. If his point at 25° is left out the deviation average value of the heat capacity between these decreases to 0.8 percent. Similarly average con­ temperatures has been derived and shown to lie stants from the Fenton and Garner data are re­ between the heat capacities of the related sub­ produced with a mean deviation of 1.0 percent, or stances ethyl alcohol and acetone. As a further and more direct confirmation an independent TABLE I. Vaporization and association of acetic acid. value of the entropy is derived from the results of TEMP. flH k (DIS.) P flH a recent study of the vapor phase ethyl acetate DEG.C DIS. MM MM YAP. equilibrium. 25 15,944 0.578 15.63 0.0957 10,874 40 15,797 2.080 34.72 .1215 11,011 In a similar study of the nitrogen dioxide­ 60 15,602 9.48 88.96 .1611 11,236 nitrogen tetroxide case Giauque and Kemp2 80 15,407 35.8 202.4 .2058 11,497 100 15,212 115.1 417.2 .2540 11,779 1 The author is indebted to Dr. Donald Brundage for making preliminary calculations. 2 W. F. Giauque and J. D. Kemp, J. Chern. Phys. 6, 41 3 MacDougall, J. Am. Chern. Soc. 58, 2585 (1936). (1938). 4 Fenton and Garner, J. Chern. Soc. p. 694 (1930). 859 860 JOSEPH O. HALFORD TABLE II. Entropy of the acetic acid monomer. reliable, however, because they are based upon low pressures measured over a range of only 20°. TEMP. S (60 R LN SI DEG.C G LIQ.) aHI aHI/T PI 760/PI GAS If gas imperfections have been adequately ac­ 2.1 38.2" 12,646 42.4' 2.731 11.23 69.3' counted for in the factor 1 +a, the heats of 40 39.67 12,445 39.7" 7.524 9.P 70.24 vaporization derived from Eq. (5) should not be 60 41.5' 12,162 36.51 24.6' 6.81 71.2' 80 43.45 11,867 33.61 69.08 4.77 72.2' in error by more than 100 calories. 100 45.28 11,564 30.9' 169.0 2.9' 73.2' The entropies of vaporization of the monomer and dimer are most readily obtained with the aid of individual heats of vaporization calculated 0.3 percent at three of their four temperatures. from the equations Results obtained by Drucker and Ullmann" at three temperatures from 80 to 110° are on the MIl (vap.) =t:.H (vap.)/2 average 5 percent above the constants given by +(1-a)t:.H(dis.)/2 (6) Eq. (3). If the weight at higher temperatures and were given to the Drucker and Ullmann results it t:.Hz (vap.) =t:.H (vap.) -at:.H (dis.). (7) would be impossible to bring Eqs. (2) and (3) into line with the remaining data without an unrea­ For 60 grams of liquid the entropy at 25° is 38.2 8 sonably large t:.Cp term expressed in greater according to Parks, Kelley and Huffman. The detail. The heat of dissociation according to Eq. added liquid entropy for higher temperatures is (2) is 15,944 cal. at 25° as compared with calculated from the heat capacity given in the 16,400±800 obtained by MacDougall from his International Critical Tables. 9 After conversion to own data alone. Since it seems apparent that his calories this becomes Cp = 12.9+0.056T in the equilibrium constant at 25° is low, and his mean appropriate temperature range. The results of the heat of reaction therefore high, the correct value entropy calculation are shown in Table II for the should lie within the lower half of his range of monomer and in Table III for the dimer. uncertainty. Equation (2) gives heats of dissoci­ The mean heat capacity of the monomer and ation within these limits up to 60°. dimer vapors is now derived from the entropy by The heat of vaporization is evaluated from the means of the equation dS/dT= Cp/T. For the vapor pressures in the International Critical monomer, Cp is 17.4 and for the dimer it is 44.1. Tables,6 which are reproduced with an average Because of structural similarity it is reasonable to deviation of 0.1 percent from 20 to 100° by the assume that the monomer value should be near equation the mean of the heat capacities of ethyl alcohol and acetone. From Table IV these are, respec­ logp=18.8302-2630/T-3.562 log T. (4) tively, 16.8 and 19.9 at the mean temperature of For the vaporization of 120 grams of the acid 62°. For reaction 1, t:.Cp is - 9.3 as compared with -9.76 shown in Eq. (2). This discrepancy t:.H=(1+a)(12035-7.08T). (5) is probably due to an oversimplification of Eq. Table I contains the heat of dissociation, the (5), for which the true heat capacity, there repre­ degree of dissociation a, the dissociation constant, sented as -7.08(I+a), should have the more the vapor pressure and the heat of vaporization calculated from the above equations at five TABLE III. Entropy of the acetic acid dimer. temperatures. The heats of vaporization listed in TEMP. R LN S, the International Critical Tables 7 and marked as DEG.C aH, aH2/T p, 760/p, GAS questionable are as much as 320 cal. lower than 25 9349 31.3" 12.90 8.1 " 99.66 those appearing in Table I. MacDougall's values, 40 9092 29.04 27.20 6.6' 101.76 60 8722 26.1' 64.27 4.91 104.4" 11,000 at 25° and 11,270 at 40° are corre­ 80 8326 23.58 133.3 3.46 107.03 spondingly high. They cannot be taken as more 100 7916 21.2' 248.2 2.2' 109.5" "Drucker and Ullmann, Zeits. f. physik. Chemie 74, 567 (1910). 8 Parks, Kelley and Huffman. J. Am. Chern. Soc. 51, 6 Vol. 3, p. 218. 1969 (1929). 7 Vol. 5, p. 138. 9 Vol. 5, p. 114. THE ENTROPY OF ACETIC ACID 861 detailed form TABLE V. Entropy of ethyl alcohol, ethyl acetate and acetone at 25°. ~Cp= 2aCp1 +(1- a)Cp2 +~H (dis.) da/dT - 2Cp(1). (8) S t;,H p S LIQ. VAP. :\IM GAS Since the discrepancy involves only a fraction of Ethyl Alcohol 38.4 10,125 58.6 67.3 Ethyl Acetate 62.0 8,596 93.3 86.7 an entropy unit, the second approximation re­ Acetone 47.8 7,488 229.2 70.6 quired to produce consistency would not be justified by the data used. The above calculation using individual heats of mean temperature dependence of ethyl alcohol vaporization is different in form from the method and acetone and the constant term is set to give outlined by Giauque and Kemp.2 Their approach the value 17.4 at 62° consistent with the entropy leads to two equations, the one expressing the calculation.