A REDETERMINATION OF THE RELATIVE ENTHALPIES OF AQUEOUS PERCHLORIC ACID SOLUTIONS FROM 1 TO 24 MOLALI

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1 A REDETERMINATION OF THE RELATIVE ENTHALPIES OF AQUEOUS PERCHLORIC ACID SOLUTIONS FROM 1 TO 24 MOLALI ABSTRACT Previous tabulations of enthalpies of formation of perchloric acid solutions are not in agreement with the behavior espected for an acid as highly dissociated as perchloric acid. Measurements of the heats of neutralization of perchloric acid solutiolls with sodium hydroxide solutions were made and new values of the relative apparent partial molal enthalpies of perchloric acid solutions in the range 1 to 24 molal were con~puted from these. These new values agree with the anticipated behavior of perchloric acid. Traditionally, any deviations of strong electrolyte solutions from ideal behavior are explained in terms of interionic attraction, dissociation of structures, and solvation of the ions produced in dissociation. Considering their effects on the heat of dilutio~~, the first two will tend to make the process endothermic, while the third is an exothermic process. Lacking both theoretical and experimental information about the interionic attraction effects in concentrated electrolyte solutio~ls, one can adopt as a working hypothesis the fact that, as a first approximation, for a given electrolyte type, heat of dilution contributions from these effects will be a function only of ionic strength but not of the specific nature of the electrolyte. Specific differences in heats of dilution of strong electrolyte solutioils will then be explainable as differences in hydration and degree of dissociation. This view is confirmed by the similarity of the relative enthalpies of solutio~ls of salts of the larger alltali metal iolls with large anions (7), where we anticipate little hydration and complete dissociation. Such a simplified picture is consistent with the tabulated values of the heats of formation of nitric acid solutions (5). Nitric acid is known to be incompletely dissociated even at relatively low concentration (8, 9), so that on dilutioil an appreciable number of protons are dissociated and upon hydration liberate sufficient energy that the over-all heat of dilution of nitric acid is exothermic at all concentrations appreciably removed from infinite dilution. Perchloric acid is a much stronger acid than nitric acid. Undissociated perchloric acid can be detected at high concentrations, but below the vicinity of eight molal the concentration of undissociated molecules is too low to be detected (10). At very high concentrations it is to be expected that perchloric acid will resemble nitric acid in showing a substantially exothermic heat of dilution to infinite dilution, but in the vicinity of eight molal, as protons cease to be made available in quantity for the exothermic solvation process, there 'hfanzrscript received June bb, Contribtltion front the Departnzent of Chenzislry, University of ~Manitoba, JVtn?~ipeg, Manitoba, with financial assistance froaz the ~Vational Research Coz~ncil of Canada. =Holder of a Natio?zal Research Council Bt~rsary.

2 1592 CANADIAN JOURNAL OF CHEMISTRY. VOL. 3-1, 1930 should be a marlted tendency for the process to become less exothermic. At lower conceiltrations it would appear necessary for the heat of dilution of perchloric acid to become marltedly endothermic and, in fact, resemble such salts as sodium or potassium chloride, since, once dissociatioil is complete, thermal effects on dilution must be due almost entirely to iiiterionic attraction. The tabulated values of the heats of formation of perchloric acid solutions (2, 5) are quite different from this expectation, indicating the process to be esothermic clo\vi~ to about three molal and then to become endothermic by merely a few calories per mole. Because these older values could not be rationalized, they were suspect and a re-esaminatioi~ of the heat of dilution of perchloric acicl solutions was indicated. ESPERIMEKTAL The relative apparent partial molal enthalpies of perchloric acid solutions at various coilcentrations were determined indirectly by measuring the heat of neutralization of perchloric acid solutions by sodium hydroxide solutions of the same concentrations. The apparatus and technique have been described previously (1, 4). A stoclt solution of carbonate free sodium hydroxide was prepared, diluted, and analyzed gravimetrically as in the earlier neutralizatioil work. Reagent grade perchloric acid was used without further attempts at purification, and analyzed by weight titration against a gravimetrically standardized sodium hydroxide solution. Care was talten that the difference in concentration of a given acid-base set was as small as possible because of a lack of the data required to malte exactly the small correction for concentration difference. Similarly, all neutralizations were started at a temperature within a few hundredths of a degree of 25.00" C., because there is insufficient heat capacity data to calculate the temperature coefficient of the reaction. In the concentration region studied, the difference in composition of the vapor phase before and after the reaction leads to heat effects of negligible magnitude. The experimental results of the heat of neutralization observations are recorded in Table I. CALCULX'TION The relative apparent partial molal enthalpy of the perchloric acid solution at the molality nz, al(hc1o4, m), is then related to its heat of neutralization AHN by sodium hydroxide of molality m by the relationship: AHN = AHN" + al(nac104, ml) - al(hc104, m) m). The molality of the reactants, m, is talten as the average of the closely adjusted acid and base concentrations, and m1 is the concentration of the salt solution which results, related to m by The heat of neutralization at infinite dilution, AFINO, was talten as - 13,321 defined calories per mole (I). The values of the relative apparent partial molal

3 BIDISOSTI AND BII.)KM:1SS: PEKCIILORIC ACID SOLUTIONS 15'33 EXPERIMENTAL DATA FOR THE HEAT OF NEUTRALIZATION 01; PERCHLORIC ACID AND SODIU>f HYDROXIDE SOLUTIONS AT VARIOUS &folhlities 171 (HCIOI) nz (NaOH) AHN, def. cal C. enthalpies of sodium perchlorate solutions have been determined by Colo~nina and Nicolas (G). The relative apparent partial molal enthalpies of sodium hydroxide solutions are talcen from Bertetti and McCabe (3), whose data were converted to defined calories per mole at 25' C. and corrected for an extrapolational error due to inclusion of Richards and Guclter's (11) low concentration data. Table I1 presents smoothed values of the heat of neutralization of sodium hydroxide and perchloric acid solutions at integral concentrations, obtained by applying an approximately fitting analytic fu~zction and deviation curve to the original data. Included are the relative apparent partial molal enthalpies of the perchloric acid solutions and the partial molal enthalpy of perchloric acid at integral concentrations, together with the supplementary data used in their computation.

4 1594 CANADlXN JOURNAL OF CI-IEMISTRY. VOL. 34, 1956 TABLE I1 HEAT OF NEUTRALIZATION OF NaOH WITH HClOa AT INTEGRAL CONCENTRATIONS, AND ENTHALPIES OF PERCHLORIC ACID SOLUTIONS, AT 25'C. IN DEFINED CALORIES PER MOLE?n m (reactant product --AHx rnolality) rnolality +(NaOH, rtz) m) +2(HClO4,??z) DISCUSSION The discrepancy between the Berthelot results and those of the present work is substantial, as show11 in the selected values of Table 111. We are i~lcliiled to TABLE 111 COMPARISON OF THIS WORIC WITH THE EARLIER VALUES Defined calories per mole Nolality Berthelot This work prefer the results of the present work because previous experience with the ileutralizatioil measurements, as well as an ailalysis of contributing errors, indicat-es that the maximum error in the heat of neutralization values is not apt to be greater than 20 calories per mole. The auxiliary data used in computing the relative apparent partial molal enthalpy of perchloric acid could 11ot account for a sigilificant portion of the discrepancies of Table 111.

5 BIDINOSTI AND BIERMANS: PERCHLORIC ACID SOLUTIONS 1595 The results of this work agree satisfactorily with the anticipated behavior of perchloric acid on dilution, showing a rapid curtailment of the exothermic process in the region where dissociation is know11 to be effectively complete, and below this region resembling the "strong" salts much more closely than it resembles nitric or other weaker acids in its heat of dilution properties. REFERENCES 1. BENDER, P. and BIERMANX, LV. J. J. r11n. Chem. Soc. 74: BERTIIELOT, M..4llll. chim. et phys. 27: BERTETTI, J. W. and McC~ne, W. L. Ind. Eng. Chem. 28: BIERMANN, W. J. arid WEUER, N. J. Am. Chem. Soc. 76: Bureau of Standards Circular 500, U.S. Government Printing Office, washing tor^ COLOAIINA, &I. a~ld NICOLAS, J. Xnales real soc. espati. fis. y cluim. (Madrid), B, 45: HARNED, H. S. and OWEN, B. B. 'I'lle physical chemistry of electrolytic solutions. lieinhold Publishii~g Corporation, New Yorli I~Ao, N. R. Indian J. Phys. 15: REDLICH, 0. and BIGELEISEN, J. J. Am. Chem. Soc. 65: REDLICH, O., HOLT, E. I<., and BIGELHISEN, J. J. Am. Chem. Soc. 66: ~<ICHARDS, T. W. and GUCICER, F. T., JR. J. -Am. Chem. Soc. 51: