INDUSTRIAL A N D ENGINEERING CHEMISTRY
April, 1926
41 7
The Data of Thermochemistry’ By F. Russell Bichowsky2 THE JOHNSHOPKINSUNIVERSITY, BALTIMORE, MD.
T
HE purpose of this communication is to call attention to a much neglected branch of our science, thermochemistry. The period from 1870 to 1890 was characterized by the very large amount of thermochemical data accumulated. This was the era of Thomsen and Berthelot and the work produced by their laboratories alone was enormous. Since that time thermochemical research has, with a few noteworthy exceptions, been sporadic and casual. A renewed interest in and a new systematic development of this now almost abandoned mine is greatly needed as is shown in the following discussion. Lack of Repair of Earlier Workings
This can be strikingly illustrated by comparing the heats of formation in the latest edition of a well-known and supposedly authoritative German work with the results of a systematic recalculation of largely the same data made for the thermochemical section of International Critical Tables. I. C. T. Kg. cal. HI SO^ SH3 Na2S XazQ KzSe KzSO4 KN02 SrBrs
207.8 19.4 105.2 4.9 93.8 338.5 85.9 171,2
German “tables” Kg. cal.
{
I. C. T.
Kg. cal.
Sra(As0dz
{i::: 102.0 -9.3 79.6 344.3 88.9 157.7
BaClz MgS03
Zn(CN)z FeI3 Fe(N0a)a KCN
797 205.3 237.9 -16.2 344.9 49.3 158.4 28.1
German “tables” Kg. cal.
573.3 197.0 282.0 27.9 126.4 23.9 314.3 65.4
These are merely more or less random examples. A more nearly complete list shows that over 60 per cent of the data in the German tables differs by 1000 gram-calories or more from the values computed for International Critical Tables, and more than 5 per cent differs by 5000 gram-calories. The nature of some of these discrepancies may be illustrated by two examples. Bert’helot in his original publication recorded his results in terms of the old system of combining weights with 0 = 8. In the case of AI& the compiler of the German tables failed to note that the value given in the literature should have been multiplied by 2, while in the case of Fe(N03)3 he multiplied by 2 when this should not have been done. The barium available to Thomsen was very impure and his value for its heat of solution in acids was therefore in error by about 8000 calories. A later and reliable value has been published, however, and is in fact quoted in the German table. Apparently, the German compiler made use of this new determination in calculating his values for BaBrz and BaIz, but he accepts Thornsen’s old value in the case of other barium salts, which are all in error, because of this, by more than 8000 calories. Similar remarks apply to many other compounds-for example, those of magnesium, strontium, and nitrogen. It should not be inferred that errors of these types are found only in the particular German tables to which reference has been made. I n fact, other published tables of thermochemical data are liable to be worse. What the situation teaches, if it teaches anyt’hing, is the danger of accepting published data in this field without critical review. In 1 Read a t the meeting of Section C, American Association for the Advancement of Science, n’ashington, D. C., December 31, 1924. Received December 5, 1925. 2 Assistant Editor for Thermochemistry, International Critical Tables.
particular, it teaches the danger of relying on noncritical summaries of data published in handbooks. Such collections are almost certain to be inconsistent. It is hoped that the thermochemical section of International Critical Tables will be a t least self-consistent, since it has been built up by complete recalculation f r o m the original experimental data. Difficulties Due t o Experimental Methods
This difficulty with the older data cannot be removed by recalculation. The calorimetric work by which much of the older thermochemical data was acquired was exceedingly crude in technic. Neglect of corrections now made as part of the regular routine was common. Temperatures and concentrations were often not given. but, worst of all, the reactions measured were often not complete or free from side reactions, and errors of surprising magnitude are consequently not uncomI1zon. Thus, six methods of calculating the heat of formation of nitric acid give 48.5, 49.7, 50.0, 50.9, 49.2, and 52.4 kg. ca1.-all about equally probable, yet differing in the extreme by 4000 calories. By some singular perversity this type of error seems to weigh most heavily on the most important commercial chemicals, such as nitric and sulfuric acids, the refractory oxides, lime, barites, magnesia, the salts of tin and lead, all compounds in the chromium group, the iron oxides, the sulfide ores, silica, and the silicates. The values published have probable errors of thousands of calories, and in some cases are but little better than guesses. Energy is one of the most expensive items in the cost of chemical manufacture, and this lack of accurate information of the heats of so many chemical reactions of industrial importance is shockingly uneconomic. Scientific Importance
It is always of interest to science to accumulate accurate information concerning the behavior of chemical substances, even if we do not know immediately what to do with it, for such information is likely to be of use in the most unexpected places and to the most unexpected people. Thus, notice the recent interest displayed by physicists in the heats of solution of halogen salts. Moreover, thermochemiical data have quite predictable new and important scientific applications. 1-They are of fundamental importance to thermodynamics. The most powerful and frequently the most accurate method of obtaining certainly the temperature coefficients of electromotive forces, free energies, etc., is by means of measurements of heats of reaction and of specific heats Indeed, if the socalled Third Law is true, these same measurements will furnish these quantities themselves as well as their temperature coefficients. 2-The connections of the quantum theory with atomic structure and certain other chemical problems are just beginning t o appear, but it is already evident t h a t development in this direction cannot proceed except upon a basis of accurate thermochemical values. 3--An intimate connection between the stability of crystalline arrangements and the heat of solution of the crystal IS indicated by the recent work of Born. 4-The heat of neutralization is a well-known test of the degree of ionization of weak acids and bases. Similarly, heat of dilution has a most important connection with the degree of ionization of electrolytes in general This connection can as yet only be stated in qualitative terms because of the absence of systematic data on the heats of dilution of very dilute solu-
Vol. 18, No. 4
INDUSTRIAL A N D ENGINEERING CHEMISTRY
418
tions, but the data of thermochemistry have a most important part to play in the perfection of a theory of solutions and its applications. Need of Government Research
The writer desires to take this opportunity to point out the need and appropriateness of systematic research in this field by the National Government. Indeed, a properly equipped laboratory for carrying out this more or less routine but all the more necessary systematic accumulation of accurate and consistent thermochemical data seems to be one of the most pressing needs both of pure and applied science. The development of new industrial processes may
well be left to the industries themselves, but governmental laboratories should have as one of their prime functions the accumulation of scientific data which are of fundamental importance to science and industry as a whole. The national physical laboratories of the world have demonstrated that cooperation in the establishmeht of fundamental values in such fields as thermometry and electrical units is entirely feasible. A similar program of cooperative research in the establishment of the fundamental data of thermochemistry should also be possible, and it is to be hoped that our own Bureau of Standards will take the lead in bringing about the necessary arrangements.
High and Low Stiffening Carbon Blacks’ By Ellwood B. Spear and Robert L. Moore DEVELOPMENT LABORATORIES, THERMATOMIC CARBON CO.. PITTSBURGH, PA.
NTIL recently it was generally believed that carbon blacks exhibit a pronounced stiffening effect when incorporated in a rubber mix. In this article it is shown that carbon blacks differ greatly in this respect and that they may be divided roughly into two main classes, one of which has a regnforcing effect much greater than, and the other equal to or even less than, an equal volume of ordinary zinc oxide such as is used in the rubber industry. This difference is so great a t times that, although two rubber stocks, each containing 14 volumes of a different black on 100 volumes of rubber, may have approximately the same ultimate tensile, the tensile value at 500 per cent elongation may be twice as great in the one case as in the other. It follows, of course, that the per cent stretch a t break must be much greater in the stock having the lower tensile at 500 per cent elongation. This remarkable difference in stiffening effect is exhibited to an even greater degree when the respective carbons are ground into a mineral oil vehicle, such as is suitable for making news ink. Here the rate of flow from a given orifice may be almost five times as fast in one case as in the other. The wetting equivalent of carbon black, an arbitrary term which is defined later-is a fairly reliable index of the stiffening power in a rubber mix. These relations are by no means exact, but qualitatively they are often useful. The wetting equivalent values demonstrate in general that carbons may be divided by this means into a high and low stiffening class. There is no necessary relation between the stiffening power of carbons in rubber or oils and the adsorption of (1) malachite green in aqueous solution, (2) Victoria blue in benzene, and (3) hexamethylenetetramine in benzene. Some carbons with high stiffening powers for rubber adsorb the smallest amounts of these substances named. Finally, the writers’ observations on very thin films2 of rubber stocks containing carbon black lead to the conclusion that high tensiles are always associated with the colloidal dispersion of a t least a part of the carbon black in the rubber matrix, where the particles of the carbon are too small to be resolved by the microscopic objective. There is some evidence to show, however, that such relations do not necessarily obtain in the case of stiffening effect. Stocks
U
1
Presented before the joint meeting of the Division of Rubber Chem-
istry and the Akron Section of the American Chemical Society, Akron, Ohio,
February 22 and 23,1926. I Tms JOURNAL, 17, 936 (1925).
containing the highest regnforcing carbon given in the table and others containing the lowest appear to be identical when viewed in the microscope. Myriads of tiny black particles or aggregates with intervening spaces having a brownish color are discernible in both cases. Experimental Data
The experimental data are shown in the accompanying table. The following formulas for compounding the rubber stocks were used: -FORMULA Pale crepe Carbon Zinc oxide Sulfur DPG
-1 73.5 19.06 4.33 1.84 1.27
100.00
-FORMULA Pale crepe Carbon Zinc oxide Sulfur Hexa
2-7 72.23 18.80 4.25 3.60 1.12
100.00
The different carbons used in this work are given in code form. Six of them are widely known in the rubber industry. Some of the remainder are used extensively in the production of printer’s ink and news ink. Many of them are experimental carbons that have never appeared on the market. The list includes carbons produced by widely different methods. Some are collected on a surface as in the channel or disk processes and some are made by condensation in the gaseous phase. In making the inks, 5 per cent by weight of the respective carbon black was ground for 16 hours in a ball mill into a 95 per cent (by weight) mineral oil vehicle suitable for making news inks. These figures are only relative. The figures for time necessary for a given volume of the inks to flow through a given orifice of arbitrary dimensions (Column 6) have not been reduced to such terms that they may be connected with the usual expressions for viscosity on plasticity. The wetting equivalent values (Column 7) represent the number of cubic centimeters of a neutral, pale yellow, raw linseed oil that must be added to 100 grams of a given carbon so that the mass may be pressed into a coherent ball similar to a stiff, dry putty. The oil is added from a buret in small portions a t first and finally drop by drop toward the end of the titration. The small balls of carbon and oil are squeezed out and the mass pressed as much as possible with a flexible paint knife. The end point is fairly sharp, being 2 to 3 drops for 5 grams of carbon. These values are not identical
