J . Chem. SOC., Faraday Trans. 1, 1982, 78, 1257-1267 Standard Potentials of the Silver, Silver Bromide Electrode in Tetrahydrofuran and Tetrahydrofuran + Water Mixtures at Different Temperatures and Related Thermodynamic Quantities BY MAHMOUD M. ELSEMONGY,*~ IBRAHIM M. KENAWY AND ABDELAZIZ S. FOUDA Chemistry Department, Faculty of Science, Mansoura University, Egypt Received 6th June, 198 1 The standard potentials of the Ag,AgBr electrode have been determined in tetrahydrofuran (THF) and in nineteen THF + water solvent mixtures from the e.m.f. measurements of the cell Pt I H,(g, 1 atm)$ I HBr (m), solvent I AgBr (s) I Ag at intervals of 5 OC from 5 to 45 OC. In solvents of high THF content, where the dielectric constant is small, it was necessary to correct for ion-pair formation.The temperature variation of the standard potential has been utilized to evaluate the standard thermodynamic functions for the cell reaction and the standard thermodynamic quantities for the transfer of HBr from water to the respective solvents. The results are discussed in the light of ion-solvent interactions as well as the structural changes of these solvents. Dipolar aprotic solvents are particularly interesting in that they solvate anions much less strongly than do protic solvents such as water.' This is usually reflected in the solvent effects on acid-base strengths, rates of reactions and conductance, for instance. A better understanding of these effects in non-aqueous, as well as partially aqueous, media can be obtained from fundamental electrochemical studies which offer useful information with regard to the nature of ion-solvent interactions.However, relatively few electrochemical studies have been made in aqueous mixtures of dipolar aprotic solvents. Recently we have reported2 the standard potentials of the Ag , AgCl electrode and related thermodynamic quantities in such dipolar aprotic solvent media as tetrahyd- rofuran (THF)+water mixtures containing up to 90% (w/w) THF at 15-55 OC. The Ag , AgBr electrode has distinct advantages over the Ag, AgCl electrode, particularly for the determination of the dissociation constants of nitrogen bases, in which AgBr is less ~oluble.~ Thus, the standard potential of the Ag,AgBr electrode has been determined by Roy et aL3 in three THF+water mixtures containing 10, 30 and 50% (w/w) THF at 5-55 OC.Thermodynamic properties of hydrobromic acid in these solvents have also been rep~rted.~ However, no work seems to have been done on the determination of the standard potentials of the Ag , AgBr electrode in pure THF or its aqueous mixtures other than for these three THF+water solvents. Thus as a part of a comprehensive study on solute-solvent interactions and the t Present address: Chemistry Department, Faculty of Science, Kuwait University, P.O. Box 5969, $ 1 atm = 101 325 Pa. Kuwait . 12571258 STANDARD POTENTIALS OF THE Ag,AgBr ELECTRODE related structure of solvents in both aqueous and non-aqueous media2*4*5 based on e.m.f. measurements of the cell PtIH, (g, 1 atm)lHBr (m), solventlAgBr (s)IAg (1) we have determined the standard potentials of the Ag,AgBr electrode in THF and in nineteen THF + water mixtures at nine different temperatures ranging from 5 to 45 OC.These have been used to evaluate the transfer functions AGP and ASP for HBr in the respective solvents. We can thus obtain information about thermodynamic properties of HBr in these solvents and about the properties and structure of the solvents. EXPERIMENTAL The preparation and analysis of hydrobromic acid solutions in the various solvent mixtures were identical to the methods described previ~usly.~ The molality of the HBr solutions ranged from 0.01 to 0.1 mol kg-l. The acid concentration was accurate to within +0.01%. THF (B.D.H. grade) was purified in the manner described previously.2 The middle fraction of the second distillate was subsequently used in the preparation of the cell solutions.The purity of the sample was verified by gas-liquid chromatography. The purified solvent was used within two days. The aqueous solvents were made by weighing, vacuum corrections being applied to all weighings. The conductivity of water used in the preparation of the solutions was < 0.7 x i2-l cm-l. The THF content of all the solutions reported was accurate to within f 0.01 %. The solutions were all stored in dark bottles shielded from the light under a nitrogen atmosphere. All solutions were swept through with a stream of purified hydrogen gas for 1-2 h before the cells, which were fitted with triple saturators, were filled, to prevent contamination of the solutions with air. All solutions were freshly prepared before taking measurements.The electrodes were prepared essentially as described el~ewhere.~ The experimental set-up and the general procedure used for the e.m.f. measurements were identical with those given previously.4* The measurements were made with three hydrogen electrodes and three Ag, AgBr electrodes for each solution, at intervals of 5 O C from 5 to 45 "C. The cells were thermostatted at each temperature with an accuracy of fO.01 O C . The Ag,AgBr electrodes were found to be stable over the entire temperature range, and the constancy of the cell e.m.f. to f0.05 mV over a period of 1 h was considered as an adequate criterion of equilibrium in the e.m.f. measurements. As a precaution, a given cell was never measured over the entire temperature range.Three series of results were made at each acid concentration. The first was from 5 to 25 O C , the second from 20 to 35 "C and the third from 30 to 45 "C. As new solutions were prepared for the measurement in each, the results serve as an excellent means of checking the reproducibility of the procedure. The e.m.f. values were generally reproducible to f 0.05 mV for different solutions. The cell measurements were in triplicate, and the mean values of these observations recorded. The triplicates generally agreed within f 0.09 mV. RESULTS AND DISCUSSION The measured values of e.m.f. were corrected in the usual way to a pressure of hydrogen of 1 atm. The properties of the THF +water mixtures over the temperature range 5 to 45 *C were derived from previous data.2$ 3$ 8 y The standard potentials of the Ag,AgBr electrode in the water-rich solvents have been determined at each temperature by the usual extrapolation technique, making use of the extended terms of the Debye-Huckel theory, and the procedure is essentially the same as that used in our previous determinatiom29 4 9 In the THF-rich solvents, where the dielectric constant is < 25, ion-pair formation occurs and hydrobromic acid behaves as a weak electrolyte.Thus corrections for ion association in these solvents were taken into account. The standard e.m.f. can no longer be obtained by the simple procedure, but a method that involves preliminary knowledge of the ionization constant of HBr isTABLE 1 .-STANDARD MOLAL POTENTIALS (Eg/V) OF THE Ag , AgBr ELECTRODE IN TETRAHYDROFURAN +WATER SOLVENT MIXTURES AT 5-45 O C ~ ~ ~~ ~~ ~ _ _ _ _ _ _ _ _ ~~ temperature/OC THF (wt %) 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 50 55 60 65 75 80 85 90 95 1 00 70.0.079 62 0.078 07 0.076 79 0.076 08 0.075 75 0.075 64 0.075 08 0.073 16 0.070 22 0.066 33 0.061 60 0.055 85 0.049 44 0.041 91 0.033 47 0.024 26 0.014 49 0.004 37 -0.006 08 -0.016 79 -0.028 02 0.077 73 0.076 21 0.075 08 0.074 19 0.073 65 0.073 18 0.072 32 0.069 84 0.066 50 0.062 11 0.057 07 0.051 07 0.044 33 0.036 35 0.027 78 0.0 18 44 0.008 62 -0.001 59 -0.012 07 -0.023 06 -0.034 13 0.075 67 0.074 39 0.073 19 0.072 24 0.071 52 0.070 71 0.069 29 0.066 36 0.062 58 0.057 82 0.052 44 0.046 03 0.038 85 0.030 78 0.021 82 0.012 23 0.002 28 -0.008 02 -0.018 56 -0.029 32 -0.040 64 0.073 42 0.072 21 0.071 12 0.070 03 0.069 01 0.067 78 0.066 02 0.062 78 0.058 46 0.053 37 0.047 55 0.040 64 0.033 31 0.024 77 0.015 59 0.005 98 -0.004 07 -0.014 50 -0.025 06 -0.035 98 -0.047 32 0.071 05 0.069 98 0.068 89 0.067 70 0.066 43 0.064 77 0.062 42 0.058 76 0.054 13 0.048 56 0.042 29 0.035 14 0.027 39 0.018 67 0.009 28 -0.000 59 -0.010 82 -0.021 31 -0.031 99 -0.042 94 - 0.054 2 1 0.068 52 0.067 49 0.066 50 0.065 16 0.063 61 0.061 54 0.058 59 0.054 62 0.049 45 0.043 58 0.036 83 0.029 32 0.021 24 0.012 26 0.002 62 -0.007 39 -0.017 68 -0.028 33 -0.039 05 - 0.050 16 -0.061 43 0.065 85 0.064 98 0.063 79 0.062 25 0.060 45 0.057 88 0.054 61 0.050 07 0.044 71 0.038 19 0.031 14 0.023 37 0.014 97 0.005 72 -0.004 18 -0.014 33 - 0.024 92 -0.035 57 -0.046 54 -0.057 53 -0.068 89 0.063 02 0.062 11 0.060 97 0.059 33 0.057 13 0.054 22 0.050 35 0.045 38 0.039 62 0.032 80 0.025 25 0.017 02 0.008 31 -0.001 13 -0.011 16 -0.021 69 -0.032 36 -0.043 22 -0.054 12 -0.065 39 - 0.076 8 1 0.060 03 0.059 26 0.057 97 0.056 02 0.053 54 0.050 17 0.045 88 0.040 56 0.034 15 0.026 94 0.019 15 0.010 63 0.001 62 -0.008 21 -0.018 54 -0.029 07 -0.039 93 -0.050 91 -0.062 05 -0.073 24 -0.084 791260 STANDARD POTENTIALS OF THE Ag,AgBr ELECTRODE TABLE VALUES OF THE CONSTANTS a, b AND c OF EQN (1) FOR EVALUATION OF Erne IN THE Ag , AgBr ELECTRODE ON THE MOLAR CONCENTRATION ( E p / V ) AND MOLE FRACTION (E$/V) SCALES CALCULATED AT 25 O C TETRAHYDROFURAN + WATER SOLVENT MIXTURES AT 5-45 OC AND THE STANDARD POTENTIALS OF 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 1 00 7.105 6.998 6.890 6.773 6.645 6.477 6.241 5.876 5.414 4.858 4.229 3.514 2.741 1.867 0.928 - 0.059 - 1.082 -2.131 -3.199 -4.296 -5.421 4.903 4.707 4.682 4.958 5.499 6.336 7.348 8.152 8.98 1 9.806 10.647 1 1.326 1 1.993 12.516 12.988 13.340 13.619 13.825 13.997 14.127 14.164 3.027 3.413 3.807 4.200 4.520 4.693 4.813 4.873 4.900 4.893 4.860 4.793 4.700 4.607 4.560 4.567 4.673 4.860 5.073 5.300 5.567 0.070 90 0.069 75 0.068 56 0.067 25 0.065 81 0.063 95 0.061 38 0.057 49 0.052 61 0.046 76 0.040 16 0.032 68 0.024 59 0.015 47 0.005 67 -0.004 62 -0.015 30 -0.026 26 -0.037 44 -0.048 93 -0.060 72 ~ 0.135 35 0.134 45 0.133 48 0.132 52 0.131 59 0.130 95 0.130 88 0.131 98 0.133 92 0.136 65 0.139 94 0.143 91 0.148 25 0.153 37 0.158 85 0.164 49 0.170 11 0.175 54 0.180 60 0.185 27 0.189 33 As in our previous following the procedure reported by Mussini et aL8 the standard e.m.f. in THF-rich solvents have been determined.STANDARD ELECTRODE POTENTIAL The least-squares values of the standard potential e (molality scale) of the Ag,AgBr electrode found in this investigation are summarized in table 1, along with the values for water as solvent.* The ion-size parameters that gave a satisfactory linear extrapolation were in the range 0.52 to 0.66 nm depending on both temperature and solvent composition. The standard deviation in is kO.05 and k0.09 mV for solvents containing 5-50 and 55-100% (w/w) THF, respectively. The values of e obtained for each solvent were fitted by the method of least-squares to = ~ - b ( t - 2 5 ) - c ( t - 2 5 ) ~ (1) where t is the temperature in O C .The parameters a, b and c are given in table 2 for each solvent, along with the values for water as the s01vent.~ Values of calculated by eqn (1) and the experimental values (table 1) generally agree within f0.11 V. The standard potentials @ on the concentration and AT$? on the mole fraction scales were computed at 25 OC with the help of the usual relation^,^ and are also included in table 2. Reported standard potentials of the Ag , AgBr electrode in THF + water solvent mixtures3 and our corresponding new values are collected in table 3 for comparison.TABLE COMPARISON BETWEEN THE NEW VALUES OF Erne AND PREVIOUSLY REPORTED VALUES BY ROY et aL3 temperaturePC THF W % ) 5 10 15 20 25 30 35 40 45 10 0.076 86 0.076 79 0.075 10 30 0.075 08 0.063 35 50 0.061 60 reported values3 0.057 98 0.075 11 0.073 23 0.071 13 0.068 88 0.066 44 0.063 76 0.060 94 this work 0.075 08 0.073 19 0.071 12 0.068 89 0.066 50 0.063 79 0.060 97 0.057 97 reported values3 0.072 30 0.069 34 0.066 16 0.062 41 0.058 52 0.054 50 0.050 19 0.045 53 this work 0.045 88 0.072 32 0.069 29 0.066 02 0.062 42 0.058 59 0.054 61 0.050 35 reported values3 0.058 23 0.053 08 0.047 99 0.042 30 0.036 69 0.030 98 0.025 31 0.019 34 thls work 0.019 15 0.057 07 0.052 44 0.047 55 0.042 29 0.036 83 0.031 14 0.025 251262 STANDARD POTENTIALS OF THE Ag,AgBr ELECTRODE The new values of are in good agreement with 22 out of 27 values obtained by Roy et aL3 for the 10, 30 and 50% THF+water mixtures at 5-45 OC.The differences range from 0.01 to 0.19 mV. However, Roy et aL3 expressed their values obtained in each solvent as a function of temperature, but the values calculated by their equations3 are not in complete agreement with their experimental values rep~rted.~ For example, in the 30% THF solution at 55 O C , the calculated value is 0.03507 V, whereas the experimentally reported value is 0.02930 V, and so there is a difference of 5.77 mV. Moreover, a minus sign should be added to the value of the parameter co (table 13) appearing in eqn (12) of their paper,3 and thus its value must be - 5.728 V K-2. STANDARD THERMODYNAMIC FUNCTIONS FOR THE CELL REACTION The standard thermodynamic functions for the cell reaction 4H2 (8, 1 atm)+AgBr (s) = Ag (s)+HBr (solvated) (2) were evaluated from the temperature variation of the standard molal e.m.f.in THF + water solvent mixtures. Thus, the standard changes of free energy (AG@) were evaluated using AG* = -nF@ = - F [ a - b ( t - 2 5 ) - - ~ ( ~ - 2 5 ) ~ ] . (3) The standard thermodynamic functions of the cell were computed at 5-45 OC by the usual relation~~9~ and these are recorded in table 4. The values of AGe are accurate to +9 J mol-l. It is evident from table 4 that the standard free energy changes for the cell reaction increase with an increase in either the THF content in the solvent mixture or the temperature of the solvent system. The standard enthalpy and entropy changes are all negative. At 25 O C , for example, the values of A P increase to a maximum at ca.10% THF and thereafter decrease, while the values of AH@ increase to a maximum at ca. 10 % THF, then decrease to a minimum at ca. 70 % THF, and thereafter increase again with increasing concentration of THF. STANDARD THERMODYNAMIC QUANTITIES FOR THE TRANSFER PROCESS The standard thermodynamic quantities for the transfer of 1 mole of HBr from the standard state in water to the standard states of the respective solvents HBr (in water) = HBr (in respective THF + water solvents) (4) were obtained from the temperature variation of standard e.m.f. of the cell on the mole fraction scale to eliminate the Gibbs energy change as a result of concentration changes in the transfer processlo EN8 = a’- h’ T - c’ T2. ( 5 ) The standard changes of Gibbs free energy (AGt@) can thus be represented as a function of temperature (in K) by (6) The least-squares values of the parameters of eqn (5) and (6) are given in table 5.The proper choice of a function to express the thermodynamic quantity as a function of temperature has been discussed in some detail by Ives and Marsden.ll The standard changes of enthalpy (AHt@), entropy (AS,@) and heat capacity (AC?) for the transfer process of HBr from water to the respective solvents were obtained by applying the F(wE$-sE$) = AGF = A - BT+ CT2.TABLE 4.-sTANDARD MOLAL THERMODYNAMIC FUNCTIONS OF THE CELL REACTION IN VARIOUS TETRAHYDROFURAN -k WATER SOLVENT MIXTURES AT 5-45 "C tempera- THF (wt %) ture 1°C 0 10 20 30 40 50 60 70 80 90 100 -AG*/J rnol-I 6768 5947 6043 506 1 5224 4080 4310 3006 3302 1838 -AHe/J rnol-I 25 610 29 304 28 287 31 959 31 059 34 708 33 926 37 551 36887 40488 -ASe/J K-l mol-l 67.7 84.0 77.2 93.3 86.7 102.7 96.1 112.1 105.6 121.5 5 15 25 35 45 7685 7299 6855 6353 5792 7404 7063 6648 61 59 5597 7298 6898 641 1 5837 5176 7254 6684 6022 5266 441 8 4778 3756 2645 1442 149 3226 2105 895 - 402 - 1787 1 404 225 - 1044 - 2403 - 3852 - 581 - 1785 - 3087 - 4486 - 5983 -2712 -3918 - 5230 - 665 1 -8178 5 15 25 35 45 17 593 19 247 20 960 22 730 24 560 15 883 17 963 20 116 22 343 24 644 17 204 19 673 22 230 24 874 27 606 21 807 24 437 27 159 29 975 32 883 31 918 34 486 37 145 39 894 42 734 33 187 35 678 38 258 40 925 43 681 32 937 35 490 38 134 40 867 43 691 31 537 34 309 37 178 40 146 43 212 29 324 32 366 35 515 38 772 42 136 5 15 25 35 45 35.6 41.5 47.3 53.1 59.0 30.5 37.8 45.2 52.5 59.9 35.6 44.3 53.1 61.8 70.5 52.3 61.6 70.9 80.2 89.5 97.6 106.6 11 5.7 124.8 133.9 107.7 116.5 125.3 134.1 142.9 113.4 122.4 131.4 140.4 149.4 115.5 125.3 135.0 144.8 154.6 115.2 125.9 136.7 147.4 158.11264 STANDARD POTENTIALS OF THE Ag,AgBr ELECTRODE TABLE 5.-vALUES OF THE CONSTANTS a’, b’ AND C’ OF EQN (5) FOR THE EVALUATION OF @ IN TETRAHYDROFURAN + WATER SOLVENT MIXTURES AT 5-45 O C AND THE VALUES OF THE CONSTANTS HBr FROM WATER TO TETRAHYDROFURAN + WATER MEDIA A, B AND C OF EQN (6) FOR THE EVALUATION OF THERMODYNAMIC QUANTITIES FOR TRANSFER OF THF - b’/ 10-4 c’/ 10-6 A/102 B/J K-’ C/10-2 (wt %) -d/10-’ V V K-’ V K-2 J mol-l mol-l J K-2 mol-l 0 10 20 30 40 50 60 70 80 90 100 5.1847 12.9923 17.1396 14.6353 1 1.3669 7.2292 3.2818 0.8837 2.0168 6.5626 12.6780 6.2245 11.2311 14.81 14 14.8691 13.9303 12.22 10 10.1416 8.5643 8.9034 1 1.2687 14.5001 3.027 3.807 4.520 4.813 4.900 4.860 4.700 4.560 4.673 5.073 5.567 - 75.3313 115.3455 91.1837 59.6485 19.7256 - 18.3607 - 4 1.4986 - 30.5653 13.2940 72.2984 - 48.3066 82.8506 83.4072 74.3491 57.8573 37.7940 22.5761 25.8476 48.6695 79.8468 - 7.5258 14.405 1 17.2321 18.0716 17.6856 16.1419 14.791 1 15.8814 19.7407 24.507 1 usual thermodynamic relations4 to eqn (6).The standard transfer thermodynamic quantities calculated at 5-45 O C are collected in table 6. The values of AGt* are accurate to f 17 J mol-l. The standard Gibbs free energy of transfer is an index of the differences in interactions of the ions (for example, H+ and Br-) and the solvent molecules in the two different media.The values of AGF decrease negatively, pass through minima (at ca. 30 and 20% THF at 5-25 and 35-45 OC, respectively) and thereafter increase to positive values with increasing THF content in the solvent. The observed negative decrease in AGte values suggests that the transfer of HBr from water to the THF + water solvents is increasingly favourable. The negative values of AGP support the view that water is less basic than the mixed solvent, whereas the positive AGte values indicate that HBr is in a higher free-energy state in pure THF than in water, and therefore the transfer process is not spontaneous. Hydrobromic acid thus is more strongly stabilized in water-rich solvents (with maxima at ca. 30 and 20% THF at 5-25 and 35-45 O C , respectively) than in water, whereas for THF-rich solvents the solute is more strongly stabilized by solvation with water molecules.The standard transfer enthalpy and entropy show similar trends. At 25 O C , for example, the values of AHte and ASP decrease, pass through minima at 70 and 90% THF, respectively, and thereafter increase with increasing THF content in the solvent. The values of AHP and ASte could provide an insight into the solvent structure. The transfer process of ions from water to a mixed solvent includes a number of changes connected with building up and breaking down the structure.12 Further, the structure- forming processes are exothermic and accompanied by a decrease in entropy, and the structure-breaking processes are endothermic and lead to an increase in entropy.The negative and decreasing values of AHte and ASte assume that ions are breaking the water structure more effectively than in the mixed solvent. Water alone is therefore a more structured solvent than the THF + water mixtures. On the other hand, the positive entropy and enthalpy of transfer of HBr from water to water-rich solvents can be attributed to a greater degree of structure breaking by HBr in these solvents than in water. The values of the heat capacity (ACF) are all negative and decrease with increasingTABLE 6.-sTANDARD THERMODYNAMIC QUANTITIES (MOLE-FRACTION SCALE) FOR THE TRANSFER OF HBr FROM WATER TO TETRAHYDROFURAN +WATER SOLVENT MIXTURES AT 5-45 O C tempera- THF (wt %) ture /"c 10 20 30 40 50 60 70 80 90 100 F F m r m m AG,e/J.rnol-l - 438 140 - 15 676 444 1245 937 1845 1467 2479 11 710 14 325 12 712 15 239 13 749 16 185 14 821 17 164 15 929 18 175 40.5 52.0 44.1 55.2 47.6 58.5 51.1 61.7 54.7 64.9 98.4 89.8 101.9 93.0 105.5 96.3 109.0 99.5 112.5 102.7 - AHte/J mol-l -AS,*/J K-l mol-l -ACF/J K-l mo1-l 5 15 25 35 45 -81 - 138 - 180 - 206 -218 - 365 - 378 - 362 -317 -244 - 749 - 608 -431 -221 25 - 734 - 454 - 138 214 603 1014 1626 2267 2938 3639 204 1 2682 3355 4059 4795 3065 3696 4367 5077 5827 398 1 4570 5209 5896 6633 5 15 25 35 45 - 171 1 - 1284 - 843 - 387 84 - 390 426 1271 2144 3046 4214 5190 6200 7245 8324 8017 9040 10 100 11 195 12 327 15 593 16 431 17 298 18 195 19 121 15 344 16 243 17 174 18 137 19 132 13 943 15 061 16 219 17 416 18 652 11 731 13 119 14 555 16 041 17 576 5 15 25 35 45 - 6.4 - 4.9 - 3.4 - 1.9 - 0.4 - 2.7 0.2 3.0 5.9 8.8 12.5 15.9 19.3 22.8 26.2 26.2 29.8 33.4 37.0 40.6 59.7 62.7 65.6 68.6 71.5 62.5 65.7 68.9 72.0 75.2 61.1 65.1 69.0 73.0 76.9 56.5 61.4 66.3 71.2 76.1 5 15 25 35 45 41.9 43.4 44.9 46.4 47.9 80.1 83.0 85.9 88.8 91.7 95.9 99.3 102.8 106.2 109.6 100.5 104.1 107.8 111.4 115.0 82.3 85.2 88.2 91.2 94.1 88.3 91.5 94.7 97.9 101.1 109.8 113.8 117.7 121.7 125.6 136.3 141.2 146.1 151.0 155.91266 STANDARD POTENTIALS OF THE Ag,AgBr ELECTRODE TABLE 7.-ELECTRICAL AND CHEMICAL PARTS OF THE THERMODYNAMIC QUANTITIES ACCOMPANYING DIFFERENCE BETWEEN THE FREE ENERGIES OF TRANSFER OF THE CHLORIDE AND BROMIDE IONS THE TRANSFER OF HBr FROM WATER TO TETRAHYDROFURAN + WATER SOLVENT MIXTURES AND THE (AGte’/J mol-l), ALL CALCULATED AT 25 OC 10 20 30 40 50 60 70 80 90 100 485 969 1404 1632 1915 2746 4773 8397 14 367 - 73 I66 300 486 746 1126 1694 2555 4052 7449 - 253 - 528 - 731 - 624 - 302 119 573 800 315 - 2240 47 169 339 599 843 1149 1290 1423 1294 804 - 890 1102 586 1 950 1 12 906 15 036 16 008 15 751 14 925 13 751 0.4 1.1 2.1 3.6 5.3 7.6 10.0 13.3 17.9 27.7 - 3.8 1.9 17.2 29.8 42.3 50.9 55.6 55.6 51.1 38.6 temperature.The values of AC? decrease, pass through minima at ca. 40% THF, then increase to maxima at ca. 70% THF, and thereafter decrease again with increasing THF content in the solvent. To study the ion-solvent interaction, the method adopted by Khoo and Chan13 was followed. In this method,14 consider a function AGte’ on the mole-fraction scale given (7) The difference between the free energies of transfer of hydrochloric2 and hydrobromic acids gives the difference between the free energies of transfer of the chloride and bromide ions.The values of Act*’ so calculated at 25 OC are given in table 7. The values of AGP’ are positive for all the solvents and increase with increasing THF content in the solvent. This is qualitatively in agreement with the Born theory, which predicts that the bromide ion should be in a lower free-energy state than the chloride ion in mixed solvents of lower dielectric constant than water.14 Therefore, the Born equation may be expected to show an increasingly better fit as the THF content of the solvent is increased. The Gibbs energy of transfer may be divided into an electrostatic part AGZ, caused by the change in the dielectric constant of the medium, and a chemical part AGg, due to the difference in solvation and other ion-solvent interactions:’.by AG,,’ = AGp(HC1) - AGP(HBr) = AGP(Cl-) - AG,e(Br-). AGF = AG: + AGg Similar equations exist for the other thermodynamic quantities AHP and ASt,. Both the electrostatic and chemical parts of the standard thermodynamic quantities for the transfer process were calculated by using the usual relations,lV4 and the values so computed at 25 OC are given in table 7. The values of AGg are all positive and increase with increasing THF content in the solvent. The chemical part of the free energy (which appears to be negative for water-rich solvents and pure THF) decreases, passes through a minimum at ca.30% THF, then increases to a maximum at around 70% THF and thereafter decreases with increasing THF concentration in the solvent, finally becoming negative in anhydrous THF. As AGg is an indicator of the acid-base properties of mixed solvents,lV4 theM. M. ELSEMONGY, I. M. KENAWY A N D A. S. FOUDA 1267 negative AG: values indicate that the chemical reaction in the transfer process is spontaneous, and the spontaneity (and so the basicity) of the water-rich solvents increases, reaches a maximum at cu 30% THF and thereafter decreases with increasing THF concentration in the solvent. The more negative value of AG: obtained for anhydrous THF indicates that the transfer process is more favourable. On the other hand, the positive values of AG: obtained for THF-rich solvents indicate that the transfer process is unfavourable.Thus, as far as chemical reaction or solvation is concerned, hydrobromic acid is in a lower Gibbs-energy state in water than in these TH F-ric h solvents. The electrostatic parts of the enthalpy and entropy of transfer are all negative, whereas their chemical contributions, which appear to be positive only for 10% THF solvent, decrease, pass through minima at ca. 70% THF and thereafter increase with increasing THF content in the solvent. The large negative AHg values for THF-rich solvents reflect the smaller enthalpy changes involved in creating a correct configura- tional change of the solvent for the transfer process. This view is further supported by the negative values of ASg, which are associated with structural changes as far as the chemical interaction or solvation @r the transfer process is concerned. This phenomenon produces overall order and hence AS2 values are negative. We thank Mrs Laila Abu Elela for assistance with the-computations and helpful suggestions. K. H. Khoo, J. Chem. Soc. A, 1971, 2932. M. M. Elsemongy, Electrochim. Acta, 1978, 23, 881. R. N. Roy, E. E. Swensson and G. LaCross, J. Chem. Thermodyn. 1975, 7 , 1015. M. M. Elsemongy, A. Fouda and M. F. Amira, Electrochim. Acta, 1981, 26, 255. M. M. Elsemongy and A. S. Fouda, J. Chem. Thermodyn., 1981, 13, in press. C. Carvajal, K. J. Tolle, J. Smid and M. Szwarc, J. Am. Chem. Soc., 1965, 87, 5548. F. E. Critchfield, J. A. Gibson and J. L. Hall, J. Am. Chem. Soc., 1953, 75, 6044. T. Mussini, C. M. Formaro and P. Andrigo, J. Electroanal. Chem., 1971, 33, 177. H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolytic Solutions (Reinhold, New York, 3rd edn, 1958), p. 459. lo R. A. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworths, London, 2nd edn, 1965), l1 D. J. G. Ives and P. D. Marsden, J. Chem. 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