GEORGE S. PARKS Stanford University, Stanford, California
INTRODUCTION
trated sulfuric acid \~:tsneutmlized by dilute potassium hydroxide solution, first directly in one step and then Thermochemistry for the purposes of the present paper shall be defined as the study of the heat effects in a two-step process in three subsequent experiments accompanying chemical reactions, with the term re- in which the given quantity of concentrated acid was actions used in the broader sense to include also proc- initially diluted to yield acid-water mixtures of t,he esses of fusion, vaporization, and transformation be- indicated strengths before the neutralization reaction. Logically, of conrse, this law of Hess may be derived tween different crystalline forms. Sometimes heat as a corollary of the first law of thermodynamics, or capacities are also given consideration in such a conprinciple of conservat,ion of energy, which was estahnection, but here, partly for reasons of spatial econlished a few years later t,hrough the studies of Mayer omy, we shall omit them except in so far as they enter into the change of heats of reaction as a consequence (21), Joule (1.6) and Helmholtz (9),and which yielded of temperature alterations. Moreover, because of the great number of organic compounds known and the TABLE 1 Comparative Data of He% f o r the Reaction: relatively better development of the systematic data 11?S04+ 2K01I = 2HtO + KISOa in this field of chemistry (and perhaps also because of the primary interests of the writer), the thermochemis-Ifcot produced b y Acid neut~alizaSum 0.I try of organic compounds will be especially emphasized ornaosilion. rlilulion tion heals in this discussion. Has04 597.2 597.2 In dealing with chemical reactions as ordinarily H1S04,H20 7718 327.1 604.9 carried out, two types of processes are important: (a) HrS04,2Hz0 116.7 483.4 600. 1 those a t constant volume and (b) those a t constant, HxSOI,5H2O 155.6 -113.4 601.0 Mean ... ... 600.8 pressure. The heat evolved in tshefirst case represents the decrease in energy of the system; that in the second case represents the decrease in enthalpy. Using the the concept that t,he chemical cnergy of a system is a notation of Lewis and Randall (2Oa), we shall desig- definite funct,ion of it,s state, as defined by such propcrnate these as - AE and - A H , respectively. At con- ties as temperi~tum,prcssnre and, in the case of solnstant pressure they are related hy the simple equation tions, concentration. Thc heat absorbed (or evolved) AH = & E + P A V (1) in the course of a chemical reaction is then a direct measure of this change in the chemical energy content THEORETICAL FOUNDATIONS of the svstem for a vonst,mt-volume urocess and is While a certain amount of theoret,ical speculation, closely related to the cnrrgy change, by equation (11, b as well as crude experinientation, had been carried out for a constant-pressure process. These two principles suffice for handling the heat previously, the icience of thcrmochemistry really derived its fundamental theoretical foundations in the effects in processes a t a given, constant temperature. decade from 1840 to 1850 with the promulgation of the However, for then calculat,ing the heat of reaction a t some other temperature, recourse must be had to a law of Hess and the first law of thermodynamics. The former of these principles, often termed the lam third principle, developed first experimentally in 1851 of constant heat summation, involves the notion that by Person (25) and a fey years later on theoretical conthe total heat effect in taking .a, chemical system from siderations by Kirchhoff (15). This Person-Kirchhoff an initial state A to a final state B is definite and law may be conveniently st,ated in the following erluaindependent of the path followed in the transformation. tion form for a ronst,nnt,-pressureprocess I t was enunciated by Hess (10) in 1840 and n-as based on a considerable number of calorimetric measurements. (2) (CH) wr P = ACP Table I summarizes one set of such measurements by Hess, in which a certain definite amount of concen- vhere AH is the increase in the enthalpy, or the heat absorbed, during t.he process and AC, represents t,be 1 Presented a t the Symposium on Thermochemistry a t the 114th meeting of the ~ ~ ~ chemioral , . ,qociety i ~at portland, ~ ~ ,difference betn-ern tthe heat capacities of the products and reactants. Oregon, September 13, 1948. 262
MAY, 1949
EXPERIMENTAL DEVELOPMENTS
Experimental developments, based on the preceding theoretical foundations, may be conveniently recorded here in terms of the following three periods: 1. The pioneer or exploratory period (185&1900), characterized by extensive stndics with rather crude results, which were often in error by 1 or 2 per cent. 2. The intermediate period (190C-30), witnessing the iqtroduction of calorimetric refinements and the consequent reduction of uncertainties, frequently to 0.1 or 0.2 per cent. 3. "Modern thermochemistry" (1930- ), with uncertainties often reduced to about 0.02 per cent. The Pioneer Period. Although a few earlier studies, such as those of Favre and Silbermann (@),are of some historical interest, the first outstanding workers in this period were Julius Thomsen (41) of Copenhagen and Marcellin Berthelot (1) of Paris. Initially they both were stimulated in their investigations by the erroneous notion that the determination of the heat effect in a reaction would yield a direct measure of chemical affinity. Thus, in 1854, Thomsen (40) stated: To measure affinity, to decompose a compound, a force is needed whose magnitude can he measured by the thermal value of the forrnxtion of the compound from the aforesaid constituents.
Actually, of course, it is - 4 F , the' free energy decrease in a reaction, that measures affinity or serves as a criterion for determining the direction of a chemical reaction and not - A H , the heat evolved at constant pressure. Whilc these tn-o quantities are closely related by the fundamental equation
they arc by no means identical, and indeed some cases exist where A P and AH even have opposite signs; i. e., a reaction may proceed ripontaneonsly and be accompanied by the absorption of appreciable amounts of heat. On the experimental side, Thomsen was a most indefatigable investigator. In the years from 1852 to 1886 he made about thirty-five hundred different calorimetric measurements, pertaining to (a) heats of neutralization of acids and bases, (b) heats of reaction of numerous nonmetallic elements, ( c ) heats of reaction involving metals and the dilution and hydration of their salts, and (d) thc thermochomistry of carbon compounds. In the latter connection he deduced a value of 157,870 calories per CIIz increment in the heat of combustion of gaseous organic compounds, which figure compares favorably with the presently accepted value of 157,443 ealorics derived by Prosen and Rossini (28b). For combustion studies with volatile materials Thomsen devised his "aniversal burner," which may be regarded as a c.rude precursor of the modern flame calorimeter of Rossini (31). Berthelot started his extensive thermochemical studies in 1864 and continned them for more than thirty years. In many cascv he p:mlleled the work of
263
Thomsen, but their results frequently differed by more than one per cent. An especially noteworthy contribution by Berthelot was the invention of the constant-volume "bomb calorimeter," in which the stout steel bomb was platinum-lined to avoid corrosion errors and was fdled with oxygen a t an initial pressure of twenty-four atmospheres (Z). Such abomb, surrounded with water containing a mercury-in-glass thermometer and suitable stirring equipment, thus constituted a calorimeter for carrying on combustions in the neighborhood of room temperature and with temperature rises of about two degrees. The bomb calorimeter provided a convenient method for measuring the heats of combustion of organic substances, and consequently it was quickly adopted by investigators in this field all over the world. Noteworthy among these were Stohmann (36) and collaborators at Leipzig, and Louguinine and Zubov in Moscow. By 1900 a very considerable body of thermochemical data, subject to uncertainties of the order of one per cent, had thus been developed (16). These uncertainties arose jointly from (a) impurities in the substances available for study and (b) deficiencies in the thermochemical apparatus and procedure. In thosedaya the heat equivatent of the calorimeter was frequently computed from the heat capacities of the constituent materials, thermometers at their best were mercury-in-glass of the Beckrnann t y i e which often limited precision measurements of temperature to about 5 0 . 1 per cent, and the common provisions for taking carc of heat exchanges with the surroundings as well as evaporation losses left much to be desired. The Intermediate Period. This period, extending to about 1930, witnessed great improvements in calorimetric methods, although i t provided only moderate additions to the existing body of thermochemical data. Among these improvements may be noted the introduction of electrical thermometers of the resistance and thermocouple types which permitted temperature measnrements to better than O.OOOlO,improved jacket control both for the adiabatic and the "ordinary" methods of calorimetry, superior stirring of the propeller type, the elimination of evaporation losses, and the use of corrosion-resistant metals and alloys, such as tantalum and the illium (84) of the Parr bomb. Some of the more outstanding of this era, and their contributions, should be briefly noted. T. W. Richards (SO) brought to a high state of development the adiabatic calorimetric procedure, which had first been tried by Person (Z5) in 1849. Jaeger and Steinwehr (1Z) introduced electrical calibration for calorimetric apparatus, and later Dickinson (4) at the United States Bureau of Standards and Smietoslawski (38) in Europe were instrumental in obtaining the adoption of highly purified benzoic acid as the standardizing substance for bomb calorimeters. W. P. White (43) made a very thorough theoretical analysis of the relative merits of various calorimeters and procedures in' his American Chemical Society monograph on "The Modern Calorimeter." As other important contribu-
JOURNAL OF CHEMICAL EDUCATION
264
tors of new thermochemical data (15)-we may also mention Roth in Germany, Verkade in Holland, Swarts in Belgium, Berner in Norway and Keffler in England. Thanks to more or less use of the calorimetric improvements just noted, most of these data involved uncertainties of the order of only one or two tenths of one per cent. However, the amount of such available data was by no means adequate to meet the growing needs of those who were making calculations in the field of chemical thermodynamics. Thus we find that Lewis and Fhndall (20b) in their free energy calculations were frequently forced to resort to the indirect, noncalorimetric evaluation of a heat of reaction, -AH, by means of the van't Hoff relationship for the change of equilibrium constant with the absolute temperature
where K is the equilibrium constant for the particular reaction and R is the gas equation constant. And again, for testing the so-called third law of thermodynamics in the case of certain chemical reactions, Gerke (7, 19) provided Lewis, Gibson, and Latimer with extremely reliable e. m. f. data at several temperatures, whereby values of iW,accurate to within 30 calories, were computed by the Gibbs-Helmholtz equation
long story brief, it subsequently developed that Richards and Davis must have had approximately one per cent of unrecognized water i n their methanol sample, and the extremely accurate value of Rossini in 1931 finally eliminated the discrepancies between the thermodynamically calculated equilibrium constants and those found experimentally (5,22). TABLE 2 Heat of Combustion of Methanol (for Liouid in Int. Kioules a t 250' C.) Investioator
Thomsen Stohmann Richards Roth, Muller Roth, B a s e I. G. Farbenfabrik Rossini
:
Year
- A H ver mol
1880 1889 1920 1927 1931 1931 1932
725.2 714 714.2 719 726.0(727.8)" 726.7 726.25(+0.20)
@Initiallyreported as 727.8 but later revised to 726.0 kilojoules.
This methanol problem exemplifies one characteristic of the work in the intermediate period: the improvements in thermochemical methods were frequently adopted on a piecemeal or incomplete basis, which often vitiated the final result to a serious extent. Thus, the combustion result of Richards and Davis for methanol should have been reliable to within 0.2 per cent on the basis of the physical measurements NFE (5) involved but the uresence of one ver cent of impurity . . in the material b&ned was entireiy overlooked. in which F is Faraday's constant, and E and N are, Modern Themnochemistry. The period dating from respectively, the reversible e. m. f. of the galvanic cell about 1930 may be considered to belong to "modern and the number of equivalents of chemical change inthermochemistry." It bas been characterized by a volved in the reaction. consistent utilization of the various improvements in Of course, such indirect methods for the evaluation calorimetric technique which were developed in the of heats of reaction cannot be utilized in a particular ease where the equilibrium constant has not been previous three decades and which should reduce errors in the physical measurements to 0.01 per cent or less in measured accurately for a t least two temperatures or many cases (57). I t has also seen vast improvements where the reaction cannot be effectuated in a suitable galvanic cell. This is the situation encountered with in the preparation and purification of the substances under study, and in the determination of the exact many organic reactions which may have been merely character and completeness of the chemical processes postulated or which, if feasible, have not been investiinvolved. Its problems in detail have been discussed gated under equilibrium conditions. very effectively in Rossini's excellent review (85). For example, during the 192k30 decade, when a On the present occasion we shall take space to menthermodynamic analysis of the methanol synthesis became important, the derivation of AH for the tion only some of the more noteworthy researches of this era. Among these should be classed Rossini's develreaction opment of a flame calorimeter (51) and his application 2Hs CO = CHIOH of it to the accurate determination of the heats of comappeared most practical through use of the law of Hess bustion of hydrogen, carbon monoxide, methanol, and and the available combustion data for the three sub- other gaseous or readily volatile substances. Some stances involved. However, a comparison (58) of the very important improvements have also been made in values available prior to 1930 for the heat of combustion bomb calorimetry a t the National Bureau of Standof liquid methanol is given in the upper portion of ards, including Washburn's theoretical work (@) on Table 2 and shows a variation over a 1.5 per cent range. the conversion of combustion results a t high pressures At first, the preference among these was usually to the standard-state process a t one atmosphere, Jesawarded to thevalue reportedby Richards andDavis (ZD) cup's redetermination ( I S ) of the combustion value of but the equilibrium constants calculated thereby were benzoic acid, and the development of a procedure of found to be a t least ten to twenty times the experi- relative bomb calorimetry by Prosen and Rossini (28a) mental results which were later obtained. To make a for a comparative study of the thermochemistry of a
+
'
group of isomers, such as the nine heptanes. Among LITERATURE CITED M., "Thennoehimie." Gauthier-Villam et Fils. other recent workers in bomb calorimetry may be cited (1) BERTHELOT, Paris, 1897. Huffman (ii), Gilbert (S),and Parks (23). BERTHELOT, M.,AND VI:II.I.Z, Ann. chim. phys., 4 546 A truly classical series of researches is to be found in (1885). the hydrogenation studies of Kistiakowsky (17) and DAVIES,G. F., AND E. C. GILBERT,J. Am. C h a . Sac., 63, coworkers a t Harvard. Other outstanding develop1585, 2730 (1941). DICKINBON, H. C., Bu& BUT.Standards (u. S.), 11, 189 ments in thermochemistry are represented by the (1914); Sci. Paper No. 230. measurements of heats of solution by Lange (18) in DODGE,B. F., Ind. Eng. C h a . , 22,89 (1930). Germany and Gucker (8) in this country, the microFAVRE,P. A., AND J. T. SILBERMANN, Ann. chim. phys.. 34, calorimetric determinations of Swietoslawski (39) in 357 (1852). GERKE,R. H., J. dm. C h m . Sac., 44, 1684 (1922). Poland prior to World War 11, and measurements (after GUCKER, F. T., H. B. PICUARD, AND R. W. PLANCK, ibzd., Sturtevant) of heat effects in slow reactions, as carried 61, 450 (1939). out by Sturtevant a t Yale University. HELMHOLTZ. H., "Uher die Erhaltung der Kraft," 1847; The extent to which "modern thermochemistry" reprinted in OSTWALD'S "Klassiker der Erakten U'issen'has been improving the quality of the available thermoschnften," No. 1. HESS, G. H., Pogg. Ann., 50, 385 (1840); 52, 97 (1841); chemical data is well illustrated by the results presented reprinted in OSTWALD'S "Klassiker der Exskten \Viesenin Table 3 for the molal heat of combustion of cycloschaften," No. 9. hexane. This striking comparison of the experimental HUFFMAN, H. M.,AND E. L. ELLIS,J. Am. C h a . Sac., 57, results of three early investigations with those of three AND H. M. HUFFMAN, ibid., 69, 41 (1935); R. SPITZER, 711 modern studies, where the over-all uncertainties have --- (19471 JAEGER,W., AND H. VON STEINWEHE,Verhandl. deut. now been reduced to the order of 0.02 or 0.03 per cent, phpik. Ges., 5, 50 (1903); Z. physik. Chem., 135, 308 appeared recently in a paper by Prosen, Johnson, and (1928). Rossini (87). JESSUP. R. 5.. J. Research Natl. BUT.Standwds, 29, 247 \----,-
TABLE 3 Heat of Combustion of Cyclohexane (for Liquid in Kcal./Mol a t 25" C.) IwestigatDr
Year
- AH
Zuhov Richards Roth Parks Huffman Rossini
1898 1915 1915 1940 1943 1946
936.81 (+0.94) 940.64(*1.89) 938.65(*1.89) 936.72 (+0.35) 936.62(+0.32) 936.88(+0.17)
Equally important, moreover, to the practical user of such data in chemical thermodynamics and other fields is the fact that this modern era has so far been witnessing a very marked increase in the quantity of data published. Thus, there is already available a very considerable body of highly accurate thermochemical values. CONCLUSION
Within a period of about one hundred years thermochemistry, starting with errors of several per cent, has been brought to a stage where the uncertainties are frequently of the order of a very few hundredths of one per cent. However, modern thermochemical data are now being used extensively for free energy calculations by equation (3), and even such relatively small errors correspond to changes of about 200 calories in the free energy of formation of cyclohexane and to uncertainties of perhaps 40 per cent in resulting equilibrium constants. On the other hand, the corresponding entropies in many cases can be evaluated with uncertainties equivalent to only 30 calories of free energy (54). Accordingly, our goal should be yet a tenfold reduction in the present thermochemical errors.
(1942); 36, 421 (1946). JOULE,.J. P.. Phil. Mag., 23,435 (1843). KHARAYCH. M. S.. BUT.Standards J. Research. 2.359 (1929). K~CHHOFP,G., P O ~ Q . Ann., 103, 203 (1858); 104, 6i4 ''Khssiker der Exakten (1858); reprinted in OSTWALD'S Wissenschaften," No. 101. KISTIAKOWSKY, G. B., AND COWORKERS, J. Am. Chem. Soc., 57, 65, 876 (1936); 59, 831 (1937); 61,1868 (1939). LANGE, E., AND A. L. ROBINSON. Z. physik. Chem., 148A. 97 (1930); Chem. Rev.. 9, 93 (1931). LEWIS,G. N., G. E. GIBSON,AND W. M. LATIMER, J. Am. C h m . Soe., 44, 1013 (1922). LEWIS. G. N., AND M. RANDALL,"Thermodynamics," McGraw-Hill Book Co., New York, 1923; (a) pp. 49-57; (b) pp. 298-300. MAYER, J. It., Ann., 42, 233 (1842). PARKS,G. S., AND A. M. HUFFMAN, "The Free Energies of Some Organic Compounds," Chemical Catalog Co., New York. 1932.. . D. 112. PARKS,G. S., AND COWORKERS, J. Am. Chem. Sac., 61,3543 (1939); 68, 2524 (1946). PARR.8. W.. ibid.. 37, 2515 (1915). ~EnsbN,C. c., A&. ;him. p h w , 27, 270 (1849). PERSON, C. C., ibid., 33,448 (1851). PROSEN, E. J., W. 1%. JOHNSON, AND F. D. ROSSINI,J. R e search Neil. Bur. Standards, 37, 53 (1946). PROSEN,E. J., AND F. D. ROSSINI,(a) ibid., 27, 289 (1941); ih) . . ihid.. 34.. 266 (1945). RICHARDS, T. W., AND H. S. DAVIS,J. Am. Chem. Soc., 42, 1614 (1920). RrcnARDs, T. W., L. J. HENDERSON, AND G. S. FORBES. Proc. Am. A d . . 41.. 1 (1905). . RossrN~,F. D., Bur. Standards J. Research, 6, 1 (1931); 12, 735 (1934). ROSSINI.F. D.. ibid.. 8. 130 (1932). ~ossslrr:F. D.: ~ h e hkev., . i s , 233 (1936). RUEHRWEIN, R. A,, AND H. M. HUPFMAN, J. Am. Chem. Soe.. 65. 1624 (1943). . . STOHMANN, F., C. KLEBERAND H. LANGBEIN, J. prakt. Chem., 39, 503 (1889). STURTEVANT. J. M.. J. Am. C h m . Sac.., 59.. 1528 (1937): J. Phys. ~ k e m .45, , 127 (1941). STURTEVANT, J. >I., Chapter on "Calorimetry" in Weiss-
.
.
.
266 bcrger's "Physical Methods of Organic Chemistry," Interscience Publishers, New York, 1945, p. 311. W., J. Am. Chem. Sac., 39, 2595 (1917). (38) SWIETOSLAWSKI, (39) S w m ~ o s u w s ~ rW., , "Microcalorimetry," Reinhold Publishing Corp., New York, 1946. J., Pogg. Ann., 92, 34 (1854). (40) THOMEN,
JOURNAL OF CHEMICAL EDUCATION (41) THOMSEN, J., "Thermochemi~cheUntersuehungen" Barth, Leipsig, 1882-86. (42) WASHBURN, E. W., Bur. Standards J. Research, 10, 525 (1933). (43) WHITE,W. P., Modern Calorimeter," Chemical Catalog Co., New,York, 1928.
he
