THE VAPOR PRESSURES AND THERMAL PROPERTIES OF POTASSIUM AND SOME ALKALI HALIDES BY ERNEST FRANKLIN FIOCK B.S.—University of Illinois, 1923 M.S.—University of Illinois, 1924 A DIGEST OF A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN CHEMISTRY IN THE GRADUATE SCHOOL OF THE UNIVERSITY OF ILLINOIS, 1926 THE LIBRARY OF (HE ie RF GET REE DEC 2 © 1926 Reprinted from the Journal of the American Chemical Society, 48, 2522 (1926). iO | ( 5439 7 {Reprint from the Journal of the American Chemical Society, 48, 2522 (1926). ] [CONTRIBUTION FROM THE CHEMICAL LABORATORY OF THE UNIVERSITY OF ILLINOIS] THE VAPOR PRESSURES AND THERMAL PROPERTIES OF POTASSIUM AND SOME ALKALI HALIDES By ErRnEs?T F. Frock! anp WortH H. RODEBUSH RECEIVED JUNE 10, 1926 PUBLISHED OCTOBER 6, 1926 The physicists within the last few years have made a large number of measurements of energy changes in atoms and molecules by ionization and spectroscopic methods and have postulated various electron dis- placements and dissociations as accompanying these changes. It is de- sirable to obtain direct thermal data for these postulated reactions for pur- poses of checking against the calculations of the physicists and verifying their postulates. Thus, if the reaction Na + Cl—» NaCl in the vapor state consists simply of the transfer of an electron from the sodium to the chlorine, the heat of this reaction should be calculable from the ionization potential of sodium and the electron affinity of chlorine. The comparison of this calculated heat with the more directly determined quantity should decide whether the mechanism of the reaction is the simple one postulated above. It is true that the physicists have not been able to agree upon a value for the electron affinity of chlorine but it seems probable that a satisfactory value will be established in the near future. On the other hand, the classical calculation of this sort made by Foote and Mohler? for gaseous hydrogen chloride now appears to be in fortuitous agreement only with thermal data since Barker and Duffendack? have shown that the hydrogen chloride molecule does not dissociate on ionization as they supposed. In fact, it seems likely that no chemical reaction is so simple as the elementary process that the physicist brings about in his measurement. ‘Thus the 1 This communication is an abstract of a thesis submitted by Ernest F. Fiock in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chem- istry at the University of Illinois. 2 Foote and Mohler, ‘‘ The Origin of Spectra,’’ Chemical Catalog Co., New York, 1922, p. 185. 3 Barker and Duffendack, Phys. Rev., 26, 339°(1 925). : ~~ Oct., 1926 SOME PROPERTIES OF POTASSIUM 2020 Einstein law of the photochemical equivalent applies only to the removal of an electron from an atom, a process which does not seem to correspond to any chemical reaction. Hence, the failure of any simple radiation theory of chemical action is not surprising. Nevertheless, there is still hope that thermal quantities may be calculated from the data of the physicist with high accuracy as, for example, heats of dissociation from band spectra. On the other hand, the thermal data are far from complete. In his re- cent calculations of the energy of salt vapors Latimer* was compelled to estimate heat quantities of large magnitude in the absence of satisfactory data. In the case of the alkali metals and alkali halides the heats of vaporization are the most important quantities lacking. These can be calculated from vapor-pressure data of sufficient accuracy. Kroner’s® data on potassium do not appear to be very concordant. Von Wartenberg,® Ruff,’ Maier® and Jackson and Morgan® have made meas- urements on the alkali halides but the agreement is not good. Since the method devised by Rodebush and Dixon!° for the measurement of vapor pressures at high tempera- tures promised to give accurate results on these substances, it has been used in the determinations of their vapor pres- sures, —— PUMP —— MANOMETER B Cc Experimental Part. The method of Rodebush and Dixon has been described in detail elsewhere.'!° ‘The apparatus in which the sub- stances were vaporized was made of pure nickel with welded joints by the American Nickel Corporation. ‘The modified [J form of the apparatus is shown in Fig.1. The reservoir A Fig. 1.—Appa- is 4.5 cm. in internal diameter and 2.5 cm. deep. The ‘tus central tubes B and C are 0.9 cm. in inside diameter. Instead of Sees constrictions in the tubes, loose plugs were introduced. ‘These plugs have a central hole 3 mm. in diameter for the ascent of the vapor and a slot E for the return of the condensed liquid. This proved to be a great improve- ment as it prevents clogging of the apparatus with drops of liquid. The apparatus was heated in an electric furnace with an insulating cover and the regulation was accomplished by a hand rheostat. ‘The tempera- ture did not vary by more than 0.2-0.8°. ‘Temperatures were measured 4 Latimer, TH1s JouRNAL, 45, 2803 (1923); 48, 1234 (1926). 5 Kroner, Ann. Physik, 40, 438 (1913). 6 Von Wartenberg and Albrecht, Z. Elektrochem., 27, 162 (1921). Von Warten- berg and Schulz, ibid., 27, 568 (1921). 7 Ruff and Mugdan, Z. anorg. allgem. Chem., 117, 147 (1921). 8 Maier, Bur. Mines Tech. Papers, No. 360. 9 Jackson and Morgan, J. Ind. Eng. Chem., 13, 110 (1921). 10 Rodebush and Dixon, Phys. Rev., 26, 851 (1925). 2524 E. F. FIOCK AND W. H. RODEBUSH Vol. 48 by a platinum—platinum-rhodium thermocouple which was repeatedly calibrated against the boiling point of sulfur and the melting points of potassium chloride and potassium sulfate.1! ‘The apparatus was sealed to the glass pump and manometer system with de Khotinsky cement. Argon was used to furnish an inert atmosphere. Sulfuric acid was used in the manometer. The levels in the manometer were read to 0.02 mm. with a cathetometer and the density of the sulfuric acid was determined aftereachrun. About 25 g. of material was introduced into the apparatus for a determination. The boiling point of potassium in the neighborhood of one atmosphere was determined by boiling the metal in a nickel tube and measuring the temperature of the vapor with a thermocouple protected by a nickel shield. ‘The lower end of the tube was heated in an electric furnace and a heating jacket was placed around the tube above the liquid line to pre- vent excessive cooling of the vapor. Argon was used as an inert at- mosphere and the total pressure was obtained as the sum of the readings of a mercury manometer and the barometer. A constant temperature could be obtained in the vapor even while the rate of heating of the liquid and vapor jackets was varied considerably. Purity of Materials Mallinckrodt’s metallic potassium was used. It was freed from oil and oxide and introduced into the apparatus in a clean state by repeated filtering and distillation in a vacuum. Spectroscopic tests indicated less than 0.1% of sodium in the initial material. The salts used were recrystallized from products purchased as being of high purity and with one or two exceptions probably contained negligible amounts of impurity. ‘The sample of potassium bromide was furnished by Professor Braley of this Department. It had been recrystallized twice but was not known to be free from chlorides. ‘The cesium chloride was purchased as pure from Eimer and Amend, but it was not recrystallized. Tests showed it to be free from any considerable amounts of sodium and potassium. Tests on the material after a run showed that no appreciable amounts of nickel were dissolved. Results The results are given in the tables. In addition, an empirical equation has been fitted to each set of data. This equation is linear in log p and 1/T in every case. ‘This is not surprising in the case of the salts since the meas- urements cover a comparatively short range of temperatures. In the case of potassium, however, where the range is considerable, the absence of curvature is surprising since the difference in the heat capacities of the liquid and vapor must be two or three calories. ‘The authors have noticed, in fitting equations to vapor-pressure data for other metals, that a linear equation often seems to fit as well as one that contains a term for AC, and they are at a loss to explain this. It may be due to deviations of the saturated vapor from the perfect gas law at higher pressures. 4 Roberts, Phys. Rev., 23, 386 (1924). Oct., 1926 SOME PROPERTIES OF POTASSIUM 2525 Previous tests of the experimental method on substances of known vapor pressure, such as mercury, have indicated that the accuracy of the method is limited only by the uncertainty of the temperature control. The plot shows the data to be highly consistent and the absolute error of the meas- urements is believed to be less than 1% on the average. TABLE I VAPOR PRESSURES OF METALLIC POTASSIUM Pressure, mm. of Hg Pressure, mm. of Hg Temp., °C. Obs. Calcd. Temp., °C. Obs. Calcd. 406.2 4.60 4.57 509.5 32.80 3352 427.9 eee €.26 528.5 44.83 45.9 448.6 11.18 11.0 754.3 744.0 741.7 469.1 16.23 aul 63 757 0 763.1 q6L-2 489.4 20.00 23.5 759.8 783.3 782.0 login p = —4433/T + 7.1830 AH = 20,260 cal. per gram atom TABLE II VAPOR PRESSURES OF SODIUM CHLORIDE Pressure, mm. of Hg Pressure, mm. of Hg enpr. G: Obs. Caled. ‘Lemp: sc Obs. Caled. 976.5 6.12 6.19 1079.5 Zora 23.23 1002.5 8.71 8.83 TOS Ae alae 31625 1028.4 12:/37 12.38 113012 41.27 41.46 1054.1 LieG7 17.09 1155.4 54.16 54.45 logio p = —9419/T + 8.3297 AH = 48,050 cal. per gram molecule . TaBLe III VAPOR PRESSURES OF POTASSIUM CHLORIDE Pressure, mm. of Hg Pressure, mm. of Hg O bs. 24.85 34.00 45.39 54.54 Calcd. 24.81 33.69 45.27 54.75 Pressure, mm. of Hg Obs. 20.82 25.69 31.69 38.68 46.74 Temp., °C. Obs. Calcd. Wemp., °C. 906.0 4.30 4.19 1036.9 932.0 6.26 6.15 1062.6 959.0 9.03 9.01 1088.0 985.2 12.84 12.85 1105.0 iid es 18.09 17.98 logio Pp = —9115/T + 8.3526 © AH = 41,660 cal. per gram molecule TABLE IV VAPOR PRESSURES OF POTASSIUM BROMIDE Pressure, mm. of Hg Temp: °C: Obs. Caled. ‘Temp:, -CG: 906.0 6.32 6.32 993.9 923.8 8.15 8.16 1011.1 941.4 10.44 10.42 1028.4 959.0 13.23 13.21 1045.6 976.5 16.60 16.62 1062.6 logio p = —8780/T + 8.2470 AH = 40,130 cal. per gram molecule Caled. 20.76 25.70 31.70 38.81 46.10 E. F. FIOCK AND W. H. RODEBUSH Vol. 48 TABLE V VAPOR PRESSURES OF POTASSIUM IODIDE Pressure, mm. of Hg Pressure, mm. of Hg Temp., °C. Obs. Caled. Temp., °C. Obs. Caled. 842.9 5.29 5.27 950.2 23.31 23.38 852.0 6.07 6.05 976.5 32.25 32.39 879.0 9.00 8.98 1002.5 43.88 43.13 897.0 11.538 11.56 1028.4 60.08 59.30 923.8 16.62 16.62 logio P = —8229/T + 8.0957 AH = 37,610 cal. per gram molecule TABLE VI VAPOR PRESSURES OF CESIUM CHLORIDE Pressure, mm. of Hg Pressure, mm. of Hg aL ein Dec Obs. Calcd. Temp oc: Obs. Calcd. 824.7 4.15- 4.30 959.0 28.60 28.54 852.0 6.49 6.55 985.2 39.42 39.39 879.0 9.77 9.74 1011.1 53.13 53.46 906.0 14.18 14.238 1019.9 58.48 59.14 932.0 20.32 20.18 logio p = —8282/T + 8.1772 AH = 37,854 cal. per gram molecule TaBLE VII DEVIATIONS OF CALCULATED VALUES FROM OBSERVED VALUES Total Max. dev., Av. dev., Max. dev., Av. dev., Substance observations mm. mm. 0 % K 17 met 0.71 3.3 ri NaCl 23 0.70 re | 137 0.5 KCl 24 .40 .10 4.6 8 KBr 10 64 .09 1.4 3 KI 22 .78 .14 1.5 4 CsCl 23 .34 .09 3.5 Tt Thermal Properties In Table VIII are shown the principal thermal data for the substances investigated. ‘The entropies of the solid at 298° K. have been calculated from specific-heat data except in the case of potassium iodide and cesium chloride, for which the values were estimated. ‘The heat of fusion of po- TABLE VIII HEATS OF SUBLIMATION K NaCl KCl KBr KI CsCl Ssolid 298°K- 16.5 17.6 19.8 22.6 »- S433 Ty Sa6.6: > 10%o 1043 1001 950 918 ASy <6 Gry 6.1 6.2 6:2 6.2 : ASyaporization to 1 atm. at Ty 19 . 65 24 . 9 25 . 0 24 . 5) 23 . 8 24 . 2 298 frcdmT 0.24 “Str a3 6.1. 6.80 5.6 Ty Svapor 298°K- 1 atm- 38.0 55.6 Dic 59.4 60.2 59.3 Oct., 1926 SOME PROPERTIES OF POTASSIUM at tassium is given by Bernini,!* and for sodium and potassium chlorides by Plato.'* ‘The other heats of fusion have been estimated. ‘The heats of vaporization are calculated from our data by the Clapeyron relation. AC, is assumed to be —5 cal. TABLE IX HEATS OF SUBLIMATION Lattice Heat of soln. AH gublimation at 298°K- energy of gaseous ions NaCl 54 182 248 KCl 52 163 235 KBr 50 156 216 KI 47 144 195 CsCl 47 Berl 225 culate its lattice energy. In Table IX are given the values in kilogram calories of the heats of sublimation of the crystals at 298°K. calculated by us, the values of the lattice energies as calculated by Born’ and the heats of solution of the gaseous ions calculated by Born’s’ formula AE = (Ne?/2r)[1 — (1/D)] (1) The figures for the lattice energies are the original calculations of Born and would be changed somewhat by new values of the constants involved. The calculation of the heat of solution involves the arbitrary choice of the “‘atomic radii.’” Born himself does not claim a high accuracy for his lattice energy value but apparently the only test of his theory that has been made with dependable data is the comparison made by Richards and Saerens!® of experimental compressibilities with those calculated by Born and here the agreement seems as good as one could expect. All other tests of his theory involve uncertain ionization potentials or ‘“‘electron affinities.” Latimer* has recently attempted to verify the formula for the heat of solu- tion of gaseous ions by the use of very uncertain data. It seems rather more plausible to assume that the formula is correct and draw what in- ferences we may. When we examine Table VIII we notice first a satis- factory parallelism between the values for the heat effects in the three columns. By far the most striking features, however, are the extremely small values of the heats of solution of the solid alkali halides in spite of the large values of the heats of vaporization and the lattice energies. ‘This means that the lattice energy is nearly equal to the heat of solution of the 22 Bernini, Physik. Z., 7, 168 (1906). 13 Plato, Z. phystk. Chem., 55, 737 (1906); 58, 369 (1907). 14 Born, Verh. deut. physik. Ges., 21, 13 (1919). The lattice energy is the energy increase involved in the separation of the ions of the crystal to an infinite distance from one another. % Born, Z. Physik, 1, 45 (1920). 16 Richards and Saerens, THIS JOURNAL, 46, 934 (1924). 2528 E. F. FIOCK AND W. H. RODEBUSH Vol. 48 gaseous ions or, in other words, that the electrical forces of the ion are neu- tralized to about the same extent in solution as in the crystal lattice. This would seem to justify Born’s tacit assumption that the energy relations of an ion depend only upon its charge and its ‘‘effective atomic radius.” If we assume the dielectric constant of water infinite in Equation 1 for the heat of solution of gaseous ions, the energy value given is not altered ap- preciably, but the expression becomes identical with that for the neutral- ization of the ions in question by the closest approach of an ion of opposite sign and equal radius. ‘This approximates the energy change in the con- densation of a gaseous ion in the lattice!’ and hence it appears plausible that the two heat effects should be so nearly equal. In the process of the hydration of an ion, water is not to be pictured as a homogeneous medium of high dielectric constant. Rather the process consists of the neutraliza- tion of the ionic charge by the more or less polar molecules. In Equation 1 no correction is made for the radii of the water molecules nor for repul- sive forces, and hence it is not surprising that the values in Col. 3 are larger than in Col. 2. Latimer has pointed out the apparent lack of specific action between the ion and the solvent. It seems certain, however, that the tendency to co- ordinate the solvent molecules as auxiliary valence groups is a function of the charge and effective radius of the ion and hence a specific property. Likewise, the tendency of a solvent to become coérdinated is a function of the effective radius and the potential polarity of its molecules. A final point for comment on the data in Table VIII is the small value of the heat of sublimation of a salt compared to its lattice energy. ‘This must mean that when a molecule vaporizes from the lattice, the two ions approach more closely and the molecule becomes less polar. If we subtract the heat of sublimation from the lattice energy, we have the heat of ionization of the salt vapors. ‘The values in Col. 3 must cer- tainly represent a maximum value for the lattice energy, while we suspect the figures in Col. 2 to be near the right value. This would indicate that the heat of dissociation of sodium chloride vapor into sodium and chloride ions is in the neighborhood of 128 calories. Summary The vapor pressures of potassium and five alkali halides have been meas- ured. The thermal data have been calculated for these substances. Some inferences favorable to Born’s theory of lattice energy have been drawn. URBANA, ILLINOIS 17 Neglecting repulsive forces the potential energy of the lattice is (0.145 e?)/ (r+ + r-) (where r+ and r- are the respective radii) per bond, per ion, and each ion has six bonds. ) VE TEL UN The writer was born at Olney, [linois, October 17, 1902. At the age of four his primary in- struction was begun in Phoenix, Arizona. In 1910 he entered the fourth grade of the public schools at Olney, [linois, and remained in those schools until his graduation from high school in 1919. In the fall of that year he entered the University of Illinois, and received from that in- stitution the degrees of Bachelor of Science in Chemical Engineering in 1923 and Master of Sci- ence in Chemistry in 1924. For the term 1924- 1925 he held a graduate scholarship in Chemistry, for 1925-1926 a quarter time assistantship in the department of Physical Chemistry, and for 1925- 1926 a University fellowship in Chemistry. The summer months of 1924 and 1925 were spent at the United States Bureau of Standards. PUBLICATION W. H. RopesusH AND EH. F. Fiock. The Measurement of the Absolute Charge on the Earth’s Surface. Proc. Natl. Acad. Sci. II, No. 7, 402 (1925). 112 07 = 30 2887372 Z