Standard partial molal heat capacities of sodium perchlorate in water from 0-90.deg. and in anhydrous methanol from -5-55.deg.

Abstract

Heats of solution of NaC104 have been measured calorimetrically in water at several temperatures ranging from near zero to 90°C and in anhydrous methanol from -10-55°C at concentrations ranging from about to 1.2 X m. The data have been extrapolated to infinite dilution by means of an extended Debye-Huckel equation to obtain standard heats of solution at the various temperatures. These were treated by the integral heat method to evaluate standard partial molal heat capacities for this salt in the two solvents as a function of temperature. The heat capacity curve for NaClO4 (aq) shows the characteristic maximum exhibited by other aqueous electrolytes while the heat capacity curve for the methanolic NaC104 appears to be at a maximum at the lower end of the temperature range and decreases as the temperature is increased. This suggests that the behavior of the standard partial molal heat capacities in the two solvents is similar, but that the maximum occurs in methanolic solutions at much lower temperatures. P a r t i a l molal heat capacities a t infinite dilution, e,:, have been extremely valuable for elucidating the structure of aqueous solutions of electrolytes (1, 7 , 18). Within the past few years, C,," data have been reported (1, 3, 6, 9, 18) for aqueous electrolytes over rathzr wide temperature ranges. The temperature dependence of C,," is even more interesting, because it is very sensitive to structural changes in the solution with a change in temperature. Heat capacities of electrolytes in nonaqueous solutions should be equally useful in understanding the structure of these solutions. However, up until now only three papers (6, 13, 16) have reported partial molal heat capacities of solutes in nonaqueous solutions. Only in the last of these (16) were the data obtained a t sufficiently low concentrations t o give reliable values a t infinite dilution.The traditional method of obtaining c,," through direct specific heat measurements is not suitable for nonaqueous solutions, because measurements must be made a t high concentrations where ion-pair formation is significant and because of uncertainties in extrapolating t o infinite dilution from high concentrations.Fortunately, the integral heat method (3) of evaluating c,," avoids the most objectionable aspects of the direct specific heat method. The method involves measuring the heat of solution of a solute a t different temperatures, and since accurate calorimetric measurements can be made in the range 0.001-0.02m, the question of nonideality and ion-pair formation is not nearly so serious. Furthermore, application of the Debye-Huckel equation in extrapolation of heat data requires only the first derivative of the dielectric constant, which is generally known with a fair degree of accuracy. A t any given temperature the heat of solution can be expressed bywhere 8, and 8 2 are the partial molal enthalpies of the solvent and solute, respectively, R1" and H2" the molal enthalpies of the pure solvent and pure solid solute, respectively, and n1 and n2 are the respec...

Apparent molar heat capacities and volumes of LaCl3(aq), La(ClO4)3(aq), and Gd(ClO4)3(aq) between the temperatures 283 K and 338 K

“…While all four sets agree with each other at T=298.15 K, the disagreement at other temperatures is significant. Values calculated from C o p,2 (NaCl,aq) (13) and C o p,2 (NaClO 4 ,aq) (32) support our results at the lowest and highest temperatures studied, but new experimental measurements are clearly needed. The major uncertainties appear to lie with results for the perchlorates.…”

“…Pogue et al, (30) and Mastroianni and Criss (32) are plotted in figure 12. While all four sets agree with each other at T=298.15 K, the disagreement at other temperatures is significant.…”

“…w, calculated from V°2 (HCl,aq) (30) and V°2 (HClO4,aq) (30) ; r, calculated from V°2 (HCl,aq) (30) and V°2 (HClO4,aq) (31) ; q, calculated from V°2 (LaCl3,aq) and V°2 (La(ClO4)3,aq), this work. (13) and C°p,2(NaClO4,aq); (32) r, calculated from C°p,2(HClO4,aq) (18) and C°p,2(HCl,aq) (21) ; q, calculated from C°p,2(HClO4,aq) (30) and C°p,2(HCl,aq) (30) .…”

An electrolyte CPA equation of state for mixed solvent electrolytes

“… Apparent molar heat capacities of NaClO4 at infinite dilution in water, methanol, and N , N ‐dimethylformamide from Mastroianni and Criss and Chang and Criss …”

Standard state heat capacities of aqueous electrolytes and some related undissociated species

“…For example, Mastroianni and Criss (30) have applied the integral heat method to obtain values of C @ for NaClO,(aq) over the range 0-95"C, including CpO= 1 7 . 8~ On the basis of the eight value for A,CpO of ionization of water cited (7,3 1,35,36) in the two preceding paragraphs, we choose A C = -215 J K-I mol-I as the best available value ,.…”