Indian Journal of ChemistryVol. 22A, November 1983, pp. 928-931

Thermodynamics of Hydrochloric Acid in Water-2-Ethoxyethanol Mixtures

A A EI-HARAKANY·, H SADEK & A S EI-LABOUDY

Chemistry Department, Faculty of Science, Alexandria University, Alexandria, Egypt

Received 7 April 1982; revised 21 March 1983; re-reoised and accepted II July 1983

The standard thermodynamic functions of transfer (.1.G~,.1.H~and .1.S~)of one mol of HCI in its standard state from waterto water-z-ethoxyethanol mixtures have been calculated from the determination of the standard electrode potentials of the cell:

Pt, Hig)/HCI (m), S(x), H20 (lOO-x),AgCl/Agat different temperatures. The variation of the standard electrode potential of Ag/AgCI electrode with dielectric constant of themedium has been also studied.

It has already been shown that in the protolytic process 1involving the transfer of one mole of HCI in itsstandard state from aqueous to another amphiproticsolvent (SH), partition equilibrium shown below is setup,

H30+ +SH~H20+SH~

As long as water content is high enough, H 30 + is thedominant species. However, in the mixed solvent therewill be a competition between water and the organicsolvent for solvating the proton. In the present study,we have used 2-ethoxyethanol as the organic solventfor studying the thermodynamics of transfer of HCIfrom water to water-organic solvent mixture.

2-Ethoxyethanol was used as the solvent, because itproved to be a good solvent in the determination of thestability constants of some metal ion complexes, theirapparent molal volumes ' and solubilities 3. However,there are no potentiometric data available in theliterature for mixtures of this solvent with water.

Values of the standard potentials of the Ag/AgCIelectrode in pure water and in water-Z-ethoxyethanolsolvent mixtures at different temperatures weredetermined and employed for the calculation of thethermodynamic functions of the transfer process. Thethermodynamic functions in turn are very useful forthe study of ion-solvent and solvent-solventinteractions.

Materials and Methods2-Ethoxyethanol (BOH) was purified as described

earlier". Ag/AgCI electrode was prepared according toCarmody's procedure". The hydrogen electrodes wereprepared by the method of Hills and Ives". A Pyepotentiometer (model 7565) accurate to 0.1 mV, anelectrometer type 602, and a cell similar to thatconstructed by Harned and Morrison 7 were used forthe measurements of the emf.

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The dielectric constants and densities of the variouscompositions of water-2-ethoxyethanol mixtures atdifferent temperatures were determined by using aWTW dipole meter type OMOI, and a pyknometer(20ml capacity) respectively. Freshly prepared doublydistilled water was used for the calibrations of both.The results are shown in Tables I and 2. Values of thecomputed Debye-Huckel constants A and B are alsoincluded in Table 2. Values of the vapour pressures ofthe mixed solvents were taken as those of thecorresponding compositions of water-z-methoxy-ethanol system", since both the organic solvents havenearly equal boiling points.

Results and DiscussionThe standard potential of Ag/AgCI electrode

Values of the measured electrode potentials atvarious temperatures and different compositions ofthe solvent mixtures are presented in Table 3. Thestandard electrode potentials of Ag/AgCI electrode on

Table I-Absolute Densities of 2-Ethollyethanol- WaterMixtures at Different Temperatures

[2-Ethoxy- Mol. Absolute density at (0C)ethanol] fro(wt %) (XI) 25 30 35 40 45

0.00 O.()()()() 0.9970 0.9956 0.9940 0.9922 0.99019.28 0.0200 0.9971 0.9952 0.9932 0.9910 0.9887

18.67 0.0439 0.9978 0.9953 0.9930 0.9905 0.987927.90 0.0718 0.9986 0.9956 0.9927 0.9894 0.986137.42 0.1068 0.9969 0.9933 0.9898 0.9861 0.982443.82 0.1349 0.9926 0.9886 0.9847 0.9805 0.976556.88 0.2087 0.9853 0.9809 0.9769 0.9724 0.968067.01 0.2888 0.9751 0.9707 0.9664 0.9618 0.957569.77 0.3157 0.971879.81 0.4414 0.958589.58 0.6322 0.9435

100.00 l.()()()() 0.9252

EL-HARAKANY et al.: THERMODYNAMICS OF HCI IN WATER-ETHOXYETHANOL MIXTURES

Table 2- Values of Dielectric Constant and Debye-Hiickel Constants A and B for 2-Ethoxyethanol-Water Mixtures atDifferent Temperatures

% Composition (2-ethoxyethanol)

0.00 9.82 18.54 27.87 37.26 47.02 56.70 66.88

25°C

D 78.32 73.91 65.22 62.55 56.09 48.02 41.10 33.41A 0.5117 0.5845 0.6734 0.7170 0.8443 1.0659 1.3461 1.8367

lO-BB 0.3292 0.3389 0.3607 0.3684 0.3890 0.4204 0.4544 0.5040

30°C

D 76.80 71.76 64.17 60.75 54.19 46.57 39.95 32.41A 0.5140 0.5691 0.6730 0.7306 0.8672 1.0885 1.3700 }.8749

lO-BB 0.3297 0.3411 0.3607 0.3707 0.3925 0.4234 0.4571 0.5075

35°C

D 75.21 69.61 63.12 58.95 52.29 45.12 38.80 31.41A 0.5175 0.5812 0.6731 0.7458 0.8927 1.1138 1.3967 1.9175

10-BB 0.3304 0.3435 0.3607 0.3732 0.3963 0.4266 0.4600 0.5113

40°C

D 73.80 67.46 62.07 57.15 50.39 43.67 37.65 30.41A 0.5197 0.5947 0.6738 0.7627 0.9212 1.1418 1.4263 1.9648

10-BB 0.3309 0.3461 0.3608 0.3760 0.4004 0.4302 0.4633 0.5155

45°C

D 71.75 65.31 61.02 55.35 48.49 42.22 36.50 29.41A 0.5294 0.6096 0.6750 0.7814 0.9529 1.1729 1.4591 2.0174

lO-BB 0.3329 0.3490 0.3610 0.3791 0.4050 0.4340 0.4668 0.5200

the molal scale (E':,J were evaluated by applying theequation",

EO' E 2 K I 2KA Jm 2 K I= + og m - - og1 - s« Jm

(1 +0.002 Mxym) =E~ - 2Kp'm ... (1)

where, E is the corrected emf, a" is the apparent ionicsize parameter which is given the values 3-7A, A and Bare the Debye-Huckel constants. The term 2K log (1+0.002 Mxym) is usually small in the studiedconcentration range of HCI (0.05-0.01 m) and isneglected'!" -13). B'm is introduced to account for thechange produced in the dielectric constant by theaddition of electrolyte 14-18. The plots of F' againstmolality of HCI at each temperature, solventcomposition, and ion size parameter always yieldstraight lines which on extrapolation to infinitedilution give" the values of E~ (standard deviation ofintercept ranged from 0.003 to 0.005). It is also foundthat E~ is independent of the chosen value of a", andthe term 2Kf3'm is too small to affect the E~ value.

The slopes of the lines obtained on plotting F'against m are then plotted against the chosen values ofthe ion size parameter, in order to estimate a' valuewhich corresponds to zero slope for each temperaturein each solvent composition. These results are shownin Table 4.

By applying the least squares method, the variationsof both E~ and E'{. (in the mole fraction scale)19 asfunctions of temperature in all solvent mixtures areeasily computed and represented in the form ofequations (a-h):(a) 0.0%, E~ = 0.2226 -639 x 10-6(t-25)

- 2.3 x 10-6(t - 25)2E~ = 0.01624-1333 x 1O-6(t-25)

- 2.9 x 10-6(t - 25)2(b) 9.32%, E~=0.2153-641 x 1O-6(t-25)

- 4.2 x 10 -6(t - 25)2E~=O.01292-1321 x 1O-6(t-25)

-4.2 x 1O-6(t-25)2(c) 18.54%, E~=0.2061-430xlO-6(t-25)

-12.0 x 10-6(t-25)2E~ = 0.0080 - 1093 x 10 -6(t - 25)

-12.1 x 1O-6(t-25)2(d) 27.87%, E~=0.1984-656 x 1O-6(t-25)

- 5.6 x 10-6(t - 25)2E~ = 0.00501-1303 x 10 -6(t-25)

-5.8 x 1O-6(t-25)2(e) 37.26%, E~=O.l912-878xlO-6(t-25)

-3.6 x 10-6(t-25)2E~ = 0.00304 -1508 x 10-;;(t - 25)

- 3.8 x 10-6(t -- 25)2(f) 47.02%, E~=0.1817-962x 1O-6(t-25)

-7.2 x 10-6(t - 25)2E~ = - 0.00443 - 766 x 10-6(t - 25)

- 39.6 x 10 -6(t - 25)2

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INDIAN J. CHEM .• VOL. 22A. NOVEMBER 1983

Table3-The Potential of the Cell Pt.H2 (I atm)/HCI(m).ECS (x~.,)/AgCI. Ag

0.050330.040110.030110.020080.01001

0.050320.040300.030280.020010.01001

0.050410.040150.030100.020080.01010

0.050190.040390.030290.020110.01011

0.050720.040630.030360.020160.01014

0.051650.042100.029410.020940.01028

0.051930.041700.031100.020720.01047

25

E (corrected to I atm) in volts at ("q

45

0.378450.389500.403350.423250.45730

0.372700.382700.396100.415850.44910

0.365700.376400.3899(\0.408950.44180

0.359900.370000.383450.402900.43545

0.352450.362600.376600.395200.42770

0.340450.350700.366700.382600.41525

0.325750.334700.348050.364700.39590

30 35x=9.3% ECS

0.37866 0.378650.39001 0.389550.40396 0.403650.42401 0.424050.45841 0.45875

x=18.54% ECS0.37295 0.371950.38305 0.382650.39690 0.396600.41725 0.417100.45120 0.45190

x=27.87% ECS0.36540 0.365590.37620 0.376240.38950 0.390040.40935 0.409390.44240 0.44339

x=37.26% ECS.0.35845 0.357240.36885 0.367740.38265 0.381840.40180 0.401590.43515 0.43549

x=47.02% ECS0.35060 0.349330.36085 0.359130.37520 0.373530.39335 0.393230.42685 0.42603

x = 56.70% ECS0.33779 0.335320.34789 0.344520.36454 0.362120.38069 0.378370.41384 0.41197

x=66.88% ECS0.32239 0.318820.33144 0.328020.34514 0.341820.36194 0.359020.39249 0.38907

ECS = 2-Ethoxyethano1

40

0.378110.389660.403510.424060.45946

0.371750.382550.396750.417150.45220

0.363850.374800.389050.409050.44370

0.356740.367140.380890.400790.43474

0.347080.357080.371580.390730.42418

0.332570.342370.359470.375770.41007

0.314960.324360.338310.355660.38720

0.377490.388990.403290.424190.46009

0.370920.381920.396420.417120.45292

0.363050.374200.388500.408650.44335

0.354890.365140.379590.399490.43406

0.344560.355210.369260.389260.42376

0.329550.338950.356350.372900.40775

0.310880.320430.334030.351830.38293

(g) 56.70%. E~ = 0.1673 -1228 X 10 -6(t-25)- 2.4 x 10-6(t - 25)2

E~= -0.0080-1813 x 1O-6(t-25)- 2.6 X 10-6(t - 25)2

(h) 66.88%. E~ = 0.1439 - 1673 x 10-6(t - 25)+ 4.4 x 10-6(t - 25)2

E~ = -0.0231-2233 x 1O-6(t-25)+4.2 x 10-6(t - 25)2

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Table 4- Values of a" at Different 2-Ethoxyethanol (ECS)Compositions and Temperatures

ECS(wt %)

0.009.32

18.5427.8737.2647.0256.7066.88

253.26.63.74.55.56.06.45.6

453.74.84.84.14.96.76.14.8

303.26.04.44.55.66.66.35.0

353.94.85.13.66.36.45.94.8

403.64.64.34.74.86.56.55.0

·Limit of error in a" values is within the first decimal place

Standard electrode potential of Ag] AgClelectrode and dielectric constant

Fig. 1 shows the plot of E~ against the reciprocal ofthe dielectric constant of 2-ethoxyethanol-watermixtures at 25°C, where departure from the theoreticalBorn line20 is observed. This behaviour was also foundin the case of water-methanol+' and water-methylcellosolve/? systems, and it was attributed inpart to the uncertainty in the radii of the solvated ions,which are expected to vary in the various solvents andthus invalidate the basis of a linear relationship.

Thermodynamic functions of transferUsing the values of the standard electrode potentials

in pure water and water-Z-ethoxyethanol mixtures atdifferent temperatures, the change in the standardthermodynamic functions of the transfer of one moleof HCl from water to the mixed solvent can beobtained from the relations 19

AG~ = -F('E~ - wE~AS~ = -(dAG~/dT)

... (2)

... (3)

and,AH~ = AG~ + TAS~ ... (4)Table 5 illustrates the values of these thermodynamicfunctions at 25°C and 35°C. It should be noted that thevalues of AS~ in Table 5 are derived by applying theleast squares method to relation 3. The temperaturesused in the calculation were 25°, 30°, 35°,40° and 45°Cfor each of the studied solvent mixture. Accordingly,

0·22

':.,E0·18

O·,~O~-----,!20~--""'-)O

1000/0

Fig. I-Variation of E',., with 1000jD at 25°C

EL-HARAKANY et 01.:THERMODYNAMICS OF HCI IN WATER-ETHOXYETHANOL MIXTURES

Table5-Values of Standard Free Energy, Enthalpy andEntropy Changes of Transfer in Water-z-ethoxyethanol

System at 25° and 35°C

[ECS] &G~ &H~ &S~(wt %) (cal mol :") (cal mol '") (cal mol -I deg -I)

At 25°C

9.32 77 77 018.54 190 190 027.87 259 259 037.26 305 -1007 -447.02 477 -1758 -856.70 559 -2689 -1166.88 907 -4427 -18

At 35°C9.32 80 80 0

18.54 156 156 027.87 256 256 037.26 332 -1023 -447.02 446 -1864 -856.70 676 -2682 -1166.88 1094 -4420 -18

ECS = 2-Ethoxyethanol

I1H~ values were computed from Eq. (4) at eachtemperature.

It can be seen that I1G~ values are positivelyincreased by increasing percentage of the organicsolvent indicating that the transfer of Hel from waterto water-2-ethoxyethanol is a non-spontaneousprocess. This suggests that the affinity of the organicsolvent for Hel is less than that of water, i.e., water ismore basic component of the mixed solvent. This viewis also consistent with the fact that the "escaping"tendency of H'Cl is greater in 2-ethoxyethanol-watermixtures.

Besides the less discriminating free energy changes,which are not affected by structural factors, the

changes in entropy and enthalpy might offer somelimited interpretation of the structural changes. Since,the structure-breaking process is endothermic whereasthe structure-forming process is exothermic, it mightbe possible to predict that at low concentrations of theorganic solvent, both H + and Cl - ions are stillsolvated by the more polar water molecules. At higherconcentrations of 2-ethox..yethanol, I1St and I1Ho arefound to have negative values and decrease withincreasing concentration of the organic solvent.

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