J. Chem. SOC., Faraday Trans. 1, 1984, 80, 1409-1414 Enthalpy of Solution of Calcium Chloride in Aqueous Mixtures of Methanol, Ethanol and Propan- 1-01 at 298.15 K BY STEFANIA TANEWSKA-OSINSKA* AND JOLANTA BARCZYNSKA Department of Physical Chemistry, University of Lodz, 91-416 Lodz, Poland Received 27th May, 1983 Enthalpies of solution of CaC1, in aqueous mixtures of methanol, ethanol and propan-1-01 have been measured over the entire range of mixed-solvent compositions. Plots of the standard enthalpy of solution against composition exhibit a maximum in the water-rich region and a minimum in the alcohol-rich region. Enthalpic pair-interaction coefficients have also been calculated: these were found to be positive for electrolyte-alcohol pairs in water and negative for electrolyte-water pairs in alcohols.Solutions of electrolytes in water + organic-solvent mixtures have been investigated by many A number of studies conducted in our laboratory have been devoted to the physico-chemical properties of NaI in aliphatic alcohols and their mixtures with ~ a t e r . ~ - ~ We are currently concerned with 1 : 2-type electrolytes, and the subject of the study reported here was CaC1,. We present enthalpies of solution of CaCI, in aqueous mixtures of methanol, ethanol and propan-1-01 obtained calori- metrically at 298.15 K. EXPERIMENTAL Puriss-grade anhydrous CaCl, produced by POCh Gliwice (Poland) was dried in a glass-tube furnace under a continuously flowing atmosphere of gaseous HC1 at 530 K. The HC1 gas stream was then replaced by argon and heating was continued for an additional 0.5 h, after which the product was furnace-cooled under argon.The CaC1, was then transferred into a specially designed container from which glass ampoules were filled with the desired doses of the salt. The degree of reagent purity was established by analytic methods. The content of C1- ions in the anhydrous CaC1, was determined potentiometrically and found to be 99.9%. All the alcohols used in the study were puriss grade produced by POCh Gliwice (Poland) and were purified by standard methods.* Measurements of the enthalpies of solution of anhydrous CaCl, in the water + alcohol mixtures were performed with the aid of an isoperibol calorimeter. The calorimetric vessel had a capacity of ca. 180 cm3. A balanced Wheatstone bridge with an NTC-type thermistor temperature indicator with a resistance of 40 k n at T = 293.15 K was used as the measuring instrument.The voltage of the unbalanced bridge was determined using a MERA V-534 (Poland) digital voltmeter. The overall temperature sensitivity of the calorimeter was found to be ca. 1 x lop4 K, and heat effects could be measured with a precision of ca. 0.5%. The heat effects resulting from breaking ampoules in the reaction vessel were found to be negligible. 14091410 SOLUTION ENTHALPY OF CaCl, IN AQUEOUS ALCOHOLS RESULTS AND DISCUSSION The measurements of the heat of solution of CaCl, in aqueous mixtures of methanol, ethanol and propan-1-01 were performed at 298.15 K over the whole composition range. The isotherms AHm =f(rn), where rn is the molality of CaCl, obtained exhibit decreasing exothermicity of the thermal effect of solution with increasing CaCl, concentration.As an example table 1 shows the integral enthalpies of solution of CaC1, in water+methanol mixtures containing 5 and 15 mol% methanol. Table 1. Integral enthalpies of solution (AH,/kJ mol-l) or CaC1, in water + methanol mixtures at 298.15 K 0.0014 - 80.0 0.0019 0.0038 - 79.0 0.0020 0.0047 - 79.3 0.0023 0.0048 - 78.8 0.0024 0.0085 - 78.4 0.0027 - - 0.0029 - - 0.0040 AH0 = - 80.4 & 0.3 0.0040 - AH0 - - 79.6 - 79.1 - 79.3 - 79.2 - 79.0 - 78.8 - 78.3 - 78.9 z= - 80.6 & 0.5 a Mol% of alcohol. Concentration of CaC1, in mol kg-l of solvent. AH* is the standard enthalpy of solution of CaCI, & the standard deviation of the mean.The measured enthalpies of solution of CaCl, were extrapolated to infinite dilution by the method proposed by Criss and C ~ b b l e . ~ The dielectric constants and densities were taken from the literature.lOJ1 The standard solution enthalpies of CaCl, in mixtures of the three alcohols with water are presented in table 2 and fig. 1. In both systems analysed in the present study, i.e. CaCl, + H,O + EtOH and CaC1, + H,O + n-PrOH, there are maxima in AH0 (fig. 1) corresponding to ca. 12 and 8 mol% alcohol, respectively. Maxima in the standard solution enthalpies of electrolytes are characteristic of all water + alcohol systems investigated thus 57 6 v 12* l3 Their appearance is attributed to the stabilizing effect of a small addition of alcohol on the three-dimensional structure of water.3* 14* l5 Although within experimental error the positions of the maxima in A H 0 (CaCl,) (fig.1) for aqueous ethanol and propan-1-01 mixtures are the same as those for other electrolyte^,^ the shapes and heights of the peaks are different. These differences may be due to the higher valence of the Caz+ cation or to the greater number of ions arising from one ‘molecule’ of the electrolyte. An especially distinct difference between the behaviour of the electrolytes studied earlier3* 4 9 l6 and CaCl, is observed in methanol + water mixtures. As can be seen in fig. 1, the CaC1, + H,O + MeOH system exhibits no maximum in A H e . The flat shape of the function AH* =f(mol% methanol) in the range 0-17 mol% methanol (fig. 1) suggests the existence of two thermal effects which compensate each other.First, theres. TANIEWSKA-OSINSKA AND J. B A R C Z Y ~ K A 141 1 Table 2. Standard enthalpies of solution (AH*/kJ mo1-l) of CaC1, in water + methanol, water +ethanol and water + propan-1-01 mixtures at 298.15 K water + methanol water + ethanol water + propan-1 -01 &a A H 0 A H 0 &a A H 0 - 0 2 3 5 7 15 17 20 30 40 50 70 80 85 90 93 95 96 100 - 80.8 - 80.7 - 80.4 - 80.5 - 80.3 - 80.6 - 80.3 -81.3 -85.5 - 92.0 - 94.1 - 103.5 - 107.4 - 110.7 - 112.5 - 113.0 - 1 13.9 -113.6 - 107.9 0 5 8 10 12 15 20 30 40 50 60 70 75 80 85 90 93 1 00 - 80.8 - 78.0 - 76.4 - 74.6 - 74.0 - 75.6 - 77.2 - 82.1 - 87.6 -91.4 -97.5 - 102.7 - 104.8 - 105.1 - 106.3 - 104.2 - 101.0 -91.7 - 0 5 6 8 9 10 15 20 30 40 50 60 70 80 85 90 100 - 80.8 - 76.6 - 75.9 - 74.6 - 74.8 - 75.2 - 77.7 -81.4 -87.1 -93.5 - 98.9 - 103.7 - 107.7 - 112.4 - 105.6 -98.8 - 86.3 a Mol% of alcohol.is the structure-making effect that addition of methanol exerts on water, giving rise to an increased solution enthalpy observed in other water + alcohol +electrolyte systems. We can assume that the other effect, accompanied by a decreased enthalpy of solution, represents the partial incorporation of methanol molecules in the ion hydration envelopes. From 17 mol% methanol in water upwards, the solvent structure in CaCl, solutions becomes disturbed in a similar way to the case of other electrolytes.** l6 In each case this is due to an increased exothermic thermal effect. We have calculated the interaction coefficients17 of ion-alcohol-molecule pairs in water (table 3) on the basis of the enthalpies of electrolyte transfer from water to water + alcohol mixtures determined by us.For the sake of comparison, table 3 also contains pair-interaction coefficients for aqueous alcohol solutions of NaI and NaCl. The positive values of the coefficients h,, for all three electrolytes suggest that ions interact with alcohol molecules only weakly, which may mean that they interact more strongly with water molecules. A comparison of the pair-interaction coefficients for NaCl-alcohol and NaI-alcohol pairs in water shows them to be similar. Thus in the three alcohols studied changing the anion has no effect on the values of these coefficients. On the other hand, their positive values increase on increasing the length of the non-electrolyte chain. The pair-interaction coefficients for CaC1,-alcohol pairs in water increase more rapidly with increasing length of the alcohol molecule than do those for NaCl or NaI.The values of the coefficients h,, for all three electrolytes with propan-1-01 in water1412 SOLUTION ENTHALPY OF CaCl, IN AQUEOUS ALCOHOLS 0 mol % alcohol 5 0 I Fig. 1. Standard solution enthalpy of CaCl, in alcohol + water mixtures as a function of solvent composition: 0, water +methanol; 0, water +ethanol; A, water +propan-1-01. Table 3. Enthalpic electrolyte-non-electrolyte pair-interaction coefficients (hxy/J kg molP) in aqueous solution electrolyte non-electrolyte solvent hXY ref. NaCl NaI CaC1, NaCl NaI CaCl, NaCl NaI CaC1, MeOH MeOH MeOH EtOH EtOH EtOH n-PrOH n-PrOH n-PrOH I50 157 49 290 298 I62 370 395 340 18 16 this work 18 16 this work 18 16 this works.TANIEWSKA-OSINSKA AND J. B A R C Z ~ S K A 1413 are identical within experimental error. One can thus suppose that the structures of the solvation shells of the three ions studied here are identical or almost identical. It has also been found that the pair-interaction coefficients h,, for CaC1,-MeOH and CaC1,-EtOH pairs in water have considerably smaller positive values than those for NaI and NaCl solutions in the same solvents, suggesting that interactions between CaCl, and alcohol molecules (especially methanol) in water are stranger than is the case with NaI and NaCl. Quite possibly this effect may hinder the ordering of water structure by methanol molecules.Further analysis of the plots presented in fig. 1 shows that A H e exhibits minima in the range of high alcohol contents in the mixed solvent. These minima in the standard solution enthalpy of the electrolyte in mixtures of the first three alcohols with water have been observed for the first time in this work. The composition of the mixtures corresponding to the minima are shifted toward lower alcohol contents in the following order : methanol > ethanol > propan- 1-01. However, similar minima were observed for mixtures of butyl alcohols and water in the presence of NaI,5 although not for NaI solutions in mixtures of the first three alcohols with water. The appearance of these minima may reflect changes either in the structhre or in the interactions of water + alcohol mixtures studied in the range of high alcohol contents.The effect of NaI on the structures of aqueous methanol, ethanol and propan-1-01 mixtures is probably too small to reveal the properties of the mixed solvent. On the other hand addition of CaCl,, which contains a greater number of ions and a bivalent cation, makes the change distinct because of the dominant ion-solvent interaction. The appearance of minima in the standard enthalpies of solution of CaCl, in mixtures the first three alcohols with water may be attributed to the formation of a certain kind of alcohol-water associate, which could be similar to those proposed by Franks and Ives for the t-butyl alcohol+water mixtures.19 If this supposition is justified, then we can compare the position of the minima observed in the presence of CaCl, (fig. 1) with those of NaI in t-butyl alcohol + water mixture^.^ The minimum in the standard enthalpy shifts from ca.95 mol% alcohol in methanol + water mixtures to ca. 75 mol% alcohol in t-butyl alcohol + water mixtures. The pair-interaction coefficients calculated for CaC1,-water pairs in alcohols (table 4) have negative values; this has not been observed to date for other salts in Table 4. Enthalpic electrolyte-water pair-interaction coefficients (h,,/J kg molP) in non-aqueous solvents electrolyte non-electrolyte solvent hX, ref. NaI H2O MeOH 583 20 NaI H2O EtOH 532 20 CaCl, H2O -1083 this work EtOH - 1167 this work NaI H2O n-PrOH CaC1, H2O n-PrOH - 1225 this work CaC1, H2O MeOH 174 20 the first three alcohol + water mixtures.With increasing length of the alcohol chain, the values of the pair-interaction coefficients become increasingly more negative. The affinity of this ion for water molecules probably irlcreases too. Further work on 1:2 electrolytes in alcohol solutions is in progress.1414 SOLUTION ENTHALPY OF CaC1, IN AQUEOUS ALCOHOLS C. M. Slansky, J. Am. Chem. Soc., 1940, 62, 2340. K. P. Mischenko and G. M. Poltoratskii, in Problems of Thermodynamics and Structure of Aqueous and Non-aqueous Electrolyte Solutions (Plenum, New York, 1972). G. A. Krestov, in Thermodynamics of Ionic Processes in Solution (Khimiya Press, Leningrad, 1973). A. Dadgar and M. R. Taherian, J. Chem. Thermodyn., 1977,9, 71 1. S. Taniewska-Osinska and H. Piekarski, J. Solution Chem., 1978, 7, 891. S. Taniewska-Osinska, H. Piekarski and A. Kacperska, in Thermodynamics and Structure of Solutions (Ivanovo, USSR, 1976), vol. 4, p. 123. S. Taniewska-Osinska and H. Piekarski, in Thermodynamics and Structure of Solutions (Ivanovo, USSR, 1979), vol. 6, p. 15. * A. Weissberger, E. S. Proskauer, J. A. Riddick and E. E. Toops Jr, Organic Solvents (Interscience, New York, 1955). C. M. Criss and J. W. Cobble, J. Am. Chem. Soc., 1961,83, 3223. lo Y. Y. Akhadov, Dielectric Properties of Binary Solutions (Pergamon Press, Oxford, 1981). l1 Y. Tashima and Y. Arai, Mem. Fac. Eng., Kuushu Univ., 1981,41, 215. l2 N. Dollet and J. Juillard, J. Solution Chem., 1976, 5, 77. l3 Y. Pointud, J-P. Morel and J. Juillard, J. Phys. Chem., 1976, 80, 2381. l4 0. Ya. Samoilov, Zh. Strukt. Khim., 1966, 7, 15; 175. l5 G. Nemethy and H. A. Scheraga, J. Phys. Chem., 1962,66, 1773. l8 H. Piekarski, Can. J. Chem., 1983,61,2203. W. G. MacMillan and J. E. Meyer, J. Chem. Phys., 1945, 13, 276. l8 G. Perron, D. Jolly and J. E. Desnoyers, Can. J. Chem., 1978,56, 552. l9 F. Franks and D. J. G. Ives, Q. Rev., 1966, 20, 1 . 2o H. Piekarski, A. Piekarska and S. Taniewska-Osinska, Can. J. Chem., in press. (PAPER 3/863)