HEATS O F MIXING B Y A RAPID APPROXIMATE METHOD
399
There is some question about the constancy of the probability factor n (I, 3, 4), and it is suggested that more complete data of the type presented here may indicate its dependence on time and concentration. I t is planned to collect such data on systems in which some of the complications encountered with silver chromate are not present. SUMMARY AND CONCLUSIONS
A previously implied equation for the rate of nucleation of silver chromate in its precipitation from solution (n = A kSt) is experimentally justified. For the purpose of explaining the Liesegang ring phenomenon of rhythmic precipitation this equation may be taken as valid. Methods for the study of the number of latent nuclei preexisting in solution we suggested.
+
REFERENCES AVRAMI, M.: J . Chem. Phys. 7, 1103 (1939); 8, 212 (1940); 9, 177 (1941). Boussu, R. G.: Compt. rend. 176, 93 (1923). J.: J. Chem. Phys. 7, 538 (1939). FRENKEL, LANDAU, L., AND LIFSHITZ: Statistical Physics, p . 226. Oxford (1938). STRANSKI, I. N . : Physik. Z . 36, 393 (1935). TAMMANN, G . : In Eucken-Jacob's Der Chemie Ingenieur, Vol. I, Part 3, p. 170. Leipsig (1933). (7) VANHOOK,A.: J. Phys. Chem. 44, 751 (1940). (8) VANHOOK, A.: J. Phys. Chem. 46, 1194 (1941). (9) VOLMER, M.: Kinetik der Phuserhildung. Steinkopff, Dresden (1939). (IO) VONWEIMARN, P. P.: Chem. Rev. 2, 216 (1925). (1) (2) (3) (4) (5) (6)
HEATS OF MIXIKG IT\' THE TERNARY SYSTEM ETHANOL-ACETIC ACID-ETHYL ACETATE BY A RAPID APPROXIMATE METHOD BRUCE LONGTIX
Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois Received September 96, 1941
In order to test adequately various theories of the thermodynamics of nonaqueous solutions, it is necessary to have available heat-of-mixing data on a wide variety of solutions. Such data have been accumulated only slowly by the more precise methods of calorimetry. A need was felt for a method employing simple apparatus, which could be used to give a rapid survey of the behavior of nonaqueous solutions with a precision of a few per cent. Such a method would be of value in selecting systems for further, more precise, study. At the same time the data obtained would be of sufficient accuracy for all engineering purposes and for rough quantitative tests of solution theories.
400
BRUCE LONQTI?;
A survey of the thermal effect on mixing equal volumes of two organic liquids has been made by Madgin and Briscoe (6), for some six hundred different systems. A comparison of their data with other more accurate data in the literature reveals errors sometimes as large as 50 per cent. These errors appear to be the result of neglecting the heat capacity of t,he calorimeter. Their method can be made to give an adequate precision with no extra effort, by properly including this factor. EXPERIMENTAL METHOD
The equipment used consisted of ordinary &in. Pyres test tubes and thermom-
eters graduated in tenths of a Centigrade degree. Saybolt viscometry thermometers (h.S.T.M.D.-88) having a range from 19' to 27°C. were used. They werc calibrated against each other, and all readings reduced to a common scale. Each of the pure liquids mas measured volunietrically into a separate test tube containing a thermometer. After each had reached t,hermal equilibrium, its temperatwe was recorded at 120, 60, and 30 sec. before t,he instant of mixing. Then within not more than 3 sec., the more dense liquids were poured into the test tube containing the least dense liquid. The misture was stirred manually at a fairly uniform rate, with the thermometer, and its temperature wm recorded at 15, 30, GO, 120, and 180 sec. after the instant of mising. All temperature readings were then extrapolated to the instant of mixing. The extrapolated temperature of the mixture is the temperature which would have been recorded at the instant of mixing, if the thermometer had responded instantly while the heat of mixing was evolved instantaneously. Since no time is then available for loss of heat, it represents the uniform temperature which would have been reached if all of the heat of mixing w-ere distributed through the mixture and part of the thermometer. The system consisting of the liquids plus this part of the thermometer may be considered to have undergone an adiabatic change during the instant of mixing. One may imagine a two-step process which is equivalent in its final results. First, each liquid and the portion of thermometer are heated to the final temperature. Then the mixing is carried out isothermally, removing the heat of isothermal mixing at this final temperature. Since the over-all process is equivalent to an adiabatic process, the heat supplied in the first step must be exactly the amount removed in the second step. The heat of mixing at the fcnal temperature is given as the sum of the amounts needed to heat the pure liquids and the portion of thermometer from their initial temperatures to the final temperature. Thus, one needs only the heat capacities of the thermometer and of the pure substances in order to complete the calculation. The amount of the thermometer (and perhaps of the test tube) heated by the mixing depends to some extent on the volume of liquid used. In these experiments the total volume was always 20 ml. The heat capacity of the apparatus w&s determined by the change in temperature obtained on pouring into the apparatus liquid of a different temperature. The result of twenty-six determinations gave & value of 16.1 & 0.1 joules per degree for the heat capacity of
HEATS OF MIXING BY h RAPID APPROXIMATE METHOD
401
the apparatus as used. The values were consistent to within the experimental error for either water or carbon tetrachloride as the liquid and for all test tubes and thermometers used during the course of these experiments. Control experiments were made to determine whether there were any systematic errors in the method. In these experiments, two samples of the same liquid were mixed. The only significant sources of error indicated by these experiments were failure to have the liquids a t room temperature and variability of the room temperature. The poured liquid tends to approach atmospheric temperature during the poiiring, while a varying room temperature introduces some uncertainty in the extrapolation of the time-temperature curves. I n a majority of the experimental runs, the temperature of the liquids and of the room could be easily adjusted to be substantially equal and constant. Errors from these sources then became negligible, as evidenced both by the control experiments and by the consistency of the data themselves. Some of thc difficulty might also have been removed by jacketing the test tube which was to contain the final mixture. A study of the experimental results s h o w that thc probable error of any single value of the adiabatic temperature change is less than O.O5"C., even when no great care is taken to control the room temperature. With runs made in triplicate, the assured precision of the mean is better than 0.loC., if one takes as the basis an assured precision \+hich is three times the probable error. I n the more carefully controlled experiments, the probable error is about 0.01"C. and the assured precision of triplicate runs is about 0.02'C. T H E SYSTEM ETHAKOL-ACETIC ACID-ETHYL
ACETlTE
The system ethanol-acetic acid-ethyl acetate was chosen to test the method, since the materials are easily available in fairly pure form, while the system itself is of some industrial importance. Furthermore, i t offers an opportunity to test theories of solutions as applied to ternary systems, there being almost no data of this type yet available. Materials The low accuracy desired in these experiments permits the use of materials which might contain not more than 1 per cent total impurities. The ethanol used wag "Rossville gold shield alcohol, u.s.P.", made by the Commercial Solvents Corporation, and is considerably better than 99 per cent pure. Glacial acetic acid, guaranteed by Baker and Adamson to assay better than 99.5 per cent and having a melting point of 16.4'C., was used. The ethyl acetate (Eastman white label) gave an assay of 99.94 per cent by the method of Redeman and Lucas ( 7 ) . Physical constants In calculating the hcats of mixing from observed temperatures and the numbers of moles from measured volumes, the physical constants listed in table 1 were used.
402
BRUCE LONGTIN
The density data used were those given in the International Critical Tables. Heat capacities of ethanol were taken from the data of Kelley (4)and those for acetic acid from an equation given in the International Critical Tables (3), while those of ethyl acetate were estimated from the single value for 20°C. given by Timofeev (9).
Results Runs were made at least in triplicate for each of forty-three different proportions of the three liquids. In all, one hundred forty-six separate rum were made. The results of these runs are shown in figures 1, 2, and 3.
ETItrUiOL
ACETIC ACTD
m Y L AC%TAT%
TXUPWTOPE
OC.
kjth/es
20
1.886 1.898 1.909
23 26
FIQ.1. Data on i
,,,,, 0.01719 0.01708 0.01703
kilojoules per mi.
rnolcr pn ml,
2.138 2.143 2.149
0.01748 0.01742 0.01737
kibjoulsr
w h *ami.
1.730 1.736 1.742
0.01023 0.01019 0.01016
acid -ethyl acetate
In figure 1 the heat in kilojoules per mole of mixture is plotted against the volume fraction of ethyl acetate in the resulting mixture expressed on an acetic acid-free basis. This fraction is calculated as the ratio of the number of milliliters of ethyl acetate used, to the sum of the number of milliliters of ethanol plus ethyl acetate. Each curve represents the heats absorbed per mole of mixture in producing various mixtures containing the same volume per cent of acetic acid. Each experimental value is represented by a separate point, although some values lie too close together to be distinguishable. In figure 2, the compositions of the final mistures are represented as mole per cents on triangular coordinates. Each small circle locates a set of experi-
HEATS OE MIXING BY -4 RAPID APPROXIMATE METHOD
403
mental runs. Each curve indicates a series of mixtures for which the same amount of heat is absorbed in forming 1 mole of mixture; the curve is labelled with a number expressing the amount of heat absorbed as kilojoules per mole of mixture. Each of the three crosses locates a point of maximum or minimum heat of mixing.
FIG.2. Constant heat-of-mixing contours in the system ethanol-acetic acid-ethyl acetate. Heats of mixing in kilojoules per mole of mixture. Compositions as mole fractions.
0
02
w
04
fmclm ol
06
c-nt
08
IO
8
FIG.3. Heats of mixing in the binary systems; comparison with previous data Figure 3 presents the data for the binary systems, together with the data of other authors. On the whole, the data may be read from these three figures with an accuracy equivalent to the experimental precision, so that no more complete presentation is warranted. All of the values were obtained in the range from 19'C. to 26"C., and much of it in the neighborhood of 23°C. Since it was found that the heats of mixing do not vary detectably over this range of tem-
404
BRUCE LONQTIN
peraturea, the data may be taken to represent approximately the heats of mixing at 23%
Discussion The crosses in figure 3 represent the data of Hirobe (2) for the system ethanolethyl acetate at 25OC., as taken from the International Critical Tables. The agreement between the two sets of data is at least as good as the consistency of Hirobe's data. The only other data available for the system ethanol-acetic acid-ethyl acetate are the heats of solution given in the International Cn'tical Tables. The limiting slopes of the binary heat-of-mixing curves in the dilute solutions, as calculated from these data, are represented by the dotted straight lines in figure 3. In all cases, except that of the dilute solution of acetic acid in ethanol, the agreement is good. Since the completion of figure 3, the original data of Berthelot for the heat of solution of acetic acid in ethanol (from which the values in the International Critical Tables were taken) have been located and found to be in good agreement with the present work. The heats of mixing are all subject to an error of f 0.3 per cent, owing to uncertainties in the heat capacity of the apparatus. This amounts at most to 3 joules per mole. The experimental errors in the mixing data, as judged from their self-consistency, amount to an assured precision of the mean of triplicate runs equal to f 10 joules per mole. The combined errors are probably less than f12 joules per mole. The more carefully controlled experiments show a better precision. The results are accurate enough to establish beyond question the existence of a saddle point in the heat content vs. composition diagram of figure 2. This point appears at the intersection of the two dotted curves which give the contour of mixtures whose heat of formation from the pure liquids is 0.180 kilojoule per mole. The existence of such a saddle point is not consistent with the simpler theories of solutions (1, 5, 8) and indicates the necessity of a more elaborate theory. The heat of mixing of ethyl acetate and acetic acid shows a change of sign as the proportion of acetic acid increases. The evidence is not conclusive, since the measured drop in temperature on mixing 18 ml. of ethyl acetate with 2 mi. of acetic acid is little more than the experimental error. However, the results are consistently in the same direction. At least the phenomenon is interesting and warrants further study. The data are more than sufficiently accurate for industrial purposes, since an error of as much as 0.1 kilojoule per mole is rarely of any industrial significance. CONCLUSIONS
1. Heats of mixing have been measured in the system ethanol-acetic acid-ethyl acetate to an accuracy of within 5 to 20 joules per mole in the neighborhood of 23°C.
DECOMPOSITIOX
OF CARBOX MOSOXIDE
B Y FERROYAGSETIC
METALS
405
2. Heats of mixing may be measured consistently to an accuracy of within 5 joules per mole using ordinary Pyres test tubes and thermometers graduated in tenths of a degree Centigrade. Care must be taken to have the liquids substantially at the steady temperature of the room at the instant before mixing, and the heat capacity of the apparatus must be considered. 3. The heat capacity of the apparatus is substantially the same for any apparatus of the same kind when used in the same way, to within these limits of accuracy. 4. The results of these measurements are adequate for survey and industrial purposes and for rough checking of solution theories. 5. Heats of mixing in the system ethanol-acetic acid-ethyl acetate cannot be represented adequately by the simpler theories of solutions. REFERENCES ( 1 ) HILDEBRAND: Solubility of .lion-electrolytes, 2nd edition. Reinhold Publishing Cor-
poration, New York (1936). (2) HIROBE:J. Fac. Sci. Imp. Cniv. Tokyo 1, 155 (1926). (3) International Critical Tables, Vol. V, p. 114. McGraw-Hill Book Company, Inc., S e w York (1929). (4) KELLEY: J. Am. Chem. SOC.61, 779 (1929). (5) LANGMUIR: Colloid Symposium Monograph 1946, 48. (6) MADGINAND BRISCOE: 3. SOC.Chem. Ind. 46T, 107 (1927). (7) RSDEYANAND LUCAS:Ind. Eng. Chem., Anal Ed. 9, 621 (1937). (8) SCATCHARD: Trans. Faraday Sot. 93, 160 (1937). (9) TIYOFEEV: Compt. rend. 112, 1261 (1891).
DECOMPOSITION OF CARBOS MOSOXIDE BY FERROMAGSETIC METALS FRAKCOIS OLMER'
Laboratory of Mineral Chemistry, $cole Nationale SupCrieure dhs Mines de Paris, France Received June 7 , lO4l INTRODUCTION
At temperatures below 1000°C. carbon monoxide is decomposed according to the following equation: 2co 4c
+ co,
Under ordinary circumstances this reaction cannot be observed, but when iron, nickel, or cobalt is present, a strong catalytic action is observed. The fact that the metals which best catalyze the reaction are the three ferromagnetic metals led the author to wonder whether their catalytic action is dependent on their 1
Present address: Department of Chemistry, University of Missouri, Columbia, Mis-
80Uri.
