J. PhyS. Chem. 1081, 85, 3713-3714
3713
Thermodynamics of Transfer of Hydrochloric and Perchloric Acids from Pure Water to Aqueous Glycerol at 25 O C J. H. Stern’ and S. L. Hansen Department of Chemistry, California State Unlversity, Long Beach, California 90840 (Received: May 4, 198 1; In Flnal Form: July 17, 198 1)
Enthalpies of transfer of HC1 and HC104at very low concentrations (ca. 0.002 rn) from pure water to aqueous glycerol were determined calorimetrically by the enthalpy of dilution method over the mixed-solvent span from glycerol mole fraction X3 = 0.0477 to 0.831 and are contrasted with analogous S-shaped enthalpies of transfer (AI?,) of the two acids from water to aqueous ethylene glycol. Free energies of transfer 4Gz of HC1 based on electromotive-force measurements (up to a maximum X 3 = 0.16) and limiting slopes lim (X3 -,0, X2 -,0) (6AO2/8X3)Xzshow that the order of increasing aqueous solvent preference of HC1 is glycerol, ethylene glycol, and water. Combination of AG2 with M 2yields positive values of T A&, and thus, in the limiting interaction region, 4G2will decrease with increasing temperature. In the second region, where M2is negative, the difference between HC1 and HC104 curves is correctly predicted by the Born equation. The steep rise of AZ?z in the glycerol-rich region is partially attributed to specific changes in the inner solvation spheres of the protons and the two different anions.
In roduc ion The thermodynamic properties of dilute aqueous strong acids (HX) have been shown to change remarkably when transferred from pure water to various aqueous nonelectrolyte mixed s~lvents.l-~These changes also reflect the effect of the added nonelectrolyte on the-solve_nt propertigs of water and may be characterized by AG2,AH2,and T AS2 for the transfer HX(rn2, H20) = HX[rn2,H20(1- X,),ne(X,)] where X 3 is the mole fraction of the nonelectrolyte (ne) and m2 is the molality of HX. This contribution reports on the enthalpies of transfer of very dilute (ca. 0.002 rn) HC1 and HC104from water to aqueous glycerol, over wide ranges of mixed-solvent compositions (from X3 = 0.0477 to 0.831). Free energies of transfer based on electromotive-force_data5 (to a maxjmum X3 = 0.16) were combined with AH2 to yield T AS2. The effect of the large difference in size of the chloride and perchlorate ions and their solution properties should become much more evident in the mixed solvent than in pure water, where leveling effects obscure their specificity. Experimental Section Calorimetric Procedure. The determination of AR2via the enthalpy of dilution method and the calorimeter have been described in detail elsewhere! The reported data are based on 167 runs, including those for the correction of dilution enthalpies for the appropriate quantity of heat evolved when water from the acid-filled ampule is mixed with the aqueous glycerol in the Dewar.4 Materials. All materials were analytical-reagent grade. The water content of stock glycerol was determined by Karl Fischer titration. Results and Discussion Enthalpies of transfer of HC1 and HC104 are shown in Tables I and 11and are plotted in Figure 1 as a function of the mole fraction of glycerol. Figure 1 also shows AG2. The S-shaped AR2 curves are similar to those for the (1) J. H. Stern and J. M. Nobilione, J. Phys. Chem., 72, 1064 (1968). (2) J. H. Stern and J. M. Nobilione, J. Phys. Chem., 72,3937 (1968). (3) J. H. Stern and J. M. Nobilione, J. Phys. Chem., 73, 928 (1969). (4) J. H. Stern and S. L. Hansen, J. Chem. Eng. Data, 16,360 (1971). (5) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions”,2nd ed. (rev.), Butterworths, London, 1965. 0022-3654/81/2085-3713$01.25/0
TABLE I: Enthalpies of Transfer of Hydrochloric Acid from Water to Aqueous Glycerol X, AB2,kcal/mol X, AZ?,, kcal/mol 0.0477 0.0947 0.157 0.231 0.369 0.362 0.527
*
0.31 0.08= 0.45 i: 0.15 0.42 f 0.11 0.05 f 0.0@ 0.04 i: 0.12 0.18 t 0.14 -0.29 0.08
*
0.607 0.683 0.758 0.813 0.822 0.831
-0.51 -0.72 -0.07 1.42 1.61 1.37
* 0.06 0.08 0.19 t 0.14 i 0.06 i 0.06
i: k
Uncertainty intervals are overall estimated experimental errors. See ref 4. For unknown reasons this point is inconsistent with the remainder of the data and was given zero weight. a
TABLE 11: Enthalpies of Transfer of Perchloric Acid from Water to Aqueous Glycerol X,
A P 2 , kcal/mol
X,
A R 2 , kcal/mol
0.0477 0.0947 0.157 0.231 0.276 0.362 0.446
0.21 c 0.02 0.09 f 0.04 -0.39 t 0.07 -0.76 t 0.03 -0.73 i 0.14 -1.31 t 0.19 -2.04 i: 0.09
0.527 0.607 0.683 0.758 0.813 0.831
-2.22 i: 0.09 -2.76 i: 0.11 -2.63 0.05 -2.27 i 0.12 -1.50 t 0.20 -1.63 i: 0.20
*
transfer of both acids from water to aqueous ethylene glycol2and may be divided into three regions. The first consists of endothermic peaks with maxima for HC1 and HC104 a t X3 = 0.1 and 0.05, respectively. The second region is characterized by minima a t X 3 = 0.6-0.7, with the third glycerol-rich region ending with steeply rising enthalpies. In the first region, AG, increases linearly with XB,up to the limit of measurement. If it is assumed that AGz behavior in aqueous glycerol is similar to that in aqueous ethylene glycol, it may be expected that this linearity wijl continue well into the glycerol-rich region. The simple AG2 behavior is thus the result of enthalpy-entropy compensation. The unfavorable free energy of transfer is a measure of the relative preference of HC1 for the two solvents, and thus pure water is the solvent of fir@ choice. The limiting slope lim ( X 3 0 , X 2 0) (6AG2/8X3)xz for HC1 in aqueous glycerol is (6 kcal/mol)/X3. Comparison with the
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0 1981 American Chemical Society
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The Journal of Physlcal Chemlstry, Vol. 85, No. 24, 1981
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Stern and Hansen
In the second region of negative enthalpies, the large negative differences between HC104 and HC1 may be attributed to electrostatic and structural contribution effects. Since the chloride ion is relatively small, the orienting electrostatic effects of high fields may be much more important than the structural effects arising from changing the hydrogen-bond network in aqueous glycerol, relative to pure water. With the much larger perchlorate ion, on the other hand, the relative electrostatic orientation effects would be smaller than with chloride ion because the field is weaker; however, the structural effects on the hydrogen-bond network are increased. There is no satisfactory theory that would describe nonelectrostatic structural interactions, and qualitative interpretations based on structure-making and structure-breaking arguments are often inconclusive and misleading.6 A rough idea of the relative electrostatic contribution difference between HC104 and HC1 may be obtained from considering the Born enthalpy of transfer equation.’ It predicts that the ratio of electrostatic enthalpies is inversely roportional to the ionic radii. Thus, on this basis alone for HC104 should be lower than for HC1, in agreement with experimental results. In the third region beyond the minima ARzvalues rise sharply, particularly for HC1. This region may be interpreted as one in which glycerol begins to penetrate the inner hydration structure of the ions, with preferential solvation becoming important above the observed minima. No dependence of on final concentrations of the acids in the mixed solvent was observed, and thus the rise in enthalpies of transfer cannot be attributed to ion pairing in the glycerol-rich solvent region. Thus, both HC1 and HC104 appear to behave as strong acids across the entire measured composition range.
d2
(6) A. Ben-Naim, “Hydrophobic Interactions”, Plenum Press, New York, 1980. (7) For a detailed treatment of the thermodynamics of transfer using the Born equation, see D. Feakins, B. C. Smith, and L. Thakur, J. Chem. SOC.A , 714 (1966).
