2633
SELF-IONIZATION OF ETHYLENE AND PROPYLENE GLYCOLS
Thermodynamics of Self-Ionization of Ethylene and Propylene Glycols by K. K. Kundu, P. K. Chattopadhyay, Debabrata Jana, and M. N. Das Physical Chemistry Laboratories, Jadavpur Uniuersity, Calcutta-32, I n d i a
(Received October 13, 1969)
Autoprotolysis constants (K,) of ethylene and propylene glycols have been determined at nine temperatures (5-45"), from emf measurements of cells of the type Pt, Hz(g, 1 atm) IMS, MBr, glycol/AgBr-Ag where S denotes the lyate ion and M stands for Na or K. The experimental data can be expressed in the form of Harned-Robinson type equations, using least-squares calculations, as given below for ethylene glycol and propylene glycol. 3487'25
+ 0.927 + 0.01079T
pK, = 3372*00 ___ T
+ 2.571 + 0.01120T
pK,
=
~
T
Thermodynamic quantities, A H o , AGO, AS", and AC,", for the processes have been evaluated at nine temperatures by using the well-known relations comprising the constants of the above equations. The evaluated thermodynamic quantities associated with the self-ionization of these solvents and of water have been utilized to throw some light on the structural aspects as well as on the relative acid-base strengths of the three solvents, using Born's model to compute the electrostatic parts of the thermodynamic functions.
The autoprotolysis or self-ionization constant of an amphiprotic solvent is quite a significant quantity in respect to its importance in giving a quantitative measure of the extreme limits of acidity and basicity in the solvent. Hence, the autoprotolysis constants of many amphiprotic solvents and their aqueous mixtures have been worked out. As expected, it has been found that since this self-ionization process accompanies charge separation, it is highly dependent on the dielectric constant as well as the intrinsic acidity and basicity of the solvent. This contention has also been amply demonstrated, especially in the cases of mixed solvents like dioxane-water,' methanol-water,2 ethanol-water3 and glycol-w ater. Besides the free energy changes, heats and entropies of self-ionization of amphiprotic solvents should be no less important in offering a clearer insight with regard to the changes in structural aspects of the solvents and thus in forming a clearer understanding of the energetics involved in the overall process. The present paper describes our findings on these aspects for two amphiprotic solvents namely, ethylene glycol and propylene glycol.
Methods Since the thermodynamic parameters such as AGO, AH", AS", and AC," for self-ionization of an amphiprotic solvent can be easily calculated from the constants of the equation of Harned-Robinson6 type pK, =
A* -.
T
- D* + C*T
(1)
to be obtained by the method of least squares from the data of pK, a t different temperatures, it is necessary to find experimentally the autoprotolysis constants (K,) of the solvent a t different temperatures. The autoprotolysis of an amphiprotic solvent is generally represented by the equation
+
+
SH SH SHz+ S(2) where SH denotes the solvent molecule, SHz+ the lyonium ion, and S- the lyate ion. The autoprotolysis constant K , of the solvent is then given by the equilibrium constant of the process 2, ie. K,=-- aSHz+aS- - a S H z + m s - y S aSH2
aSH2
(3)
where a denotes the activity, m the molality, and y the activity coefficient. The values of K, for the two glycols have been determined a t nine temperatures in the range 5-45" a t 5" intervals, using a cell of the type
Pt, Hz(g, 1 atm)jMS, MBr (solvent)/AgBr-Ag (A) where M denotes Ka or K. (1) H . S . Harned and L. D. Fallon, J. Amer. Chem. Soc., 61, 2374 (1939). (2) J. Koskikallio, 0. Sournen Kern., 30B, 38, 43, 111 (1957). (3) B . Gutbeaahl and E. Grunwald, J. A m e r . Chem. SOC.,75, 559, 665 (1953). (4) S. K . Banerjee, K. K . Kundu, and M. N. Das, J . Chem. Soc., A , 166 (1967). (5) H. S. Harned and R . A. Robinson, Trans. Faraday Soc., 36, 973 (1940).
The Journal of Physical Chemistrg, Vol. 74, N o . 13, 1970
K. E(.KUNDU, P. K. CHATTOPADHYAY, D. JANA,AND M. N. DAS
2634
Table I : Emf in Volts of Cell A for the Evaluation of pK, Values of Ethylene Glycol a t Different Temperatures
0.00396 0.00666 0.00860 0.0137 0.0193 0,0244 0.0248 0.0277 0.0328 0.0367 0.0489
7
5
10
15
20
25
30
35
40
45
0.8242 0.8232 0.8236 0.8239 0.8248 0.8245 0.8251 0.8245 0.8255 0.8255 0.8250
0.8253 0.8243 0.8248 0.8253 0.8260 0.8257 0.8263 0 8260 0,8268 0.8267 0,8262
0.8265 0.8255 0.8260 0.8265 0.8272 0.8269 0.8275 0.8270 0,8279 0.8279 0.8274
0.8279 0.8269 0.8276 0.8279 0.8288 0,8283 0,8289 0.8285 0.8292 0.8292 0.8290
0.8292 0.8285 0.8286 0.8292 0.8299 0.8297 0.8302 0.8297 0.8306 0.8306 0 8301
0 8305 0.8298 0.8300 0.8308 0.8315 0.8311 0.8318 0.8313 0.8317 0.8317 0.8317
0.8322 0.8310 0.8314 0.8321 0.8330 0.8323 0.8333 0.8324 0.8328 0.8328 0.8330
0.8330 0.8322 0.8326 0.8336 0.8343 0.8348 0.8346 0.8338 0.8344 0.8344 0.8345
0.8347 0.8340 0.8340 0.8350 0.8359 0.8355 0.8360 0.8350 0,8355 0.8356 0.8360
mar-
0.00516 0.00901 0.00116 0.0185 0,0252 0.0318 0.0323 0.0374 0.0428 0.0479 0.0670
---
Temp, OC
I
ms -
I
I
I
~
~~
~~
~~
~
~
Table 11: Emf in Volts of Cell A for the Evaluation of the pK. Values of Propylene Glycol at Different Temperatures
0.00086 0,00538 0.00470 0.00913 0.00990 0.0210 0.0227 0.0153 0.0117
0.00378 0.00627 0.01050 0.01090 0.0202 0.0292 0.0438 0.0677 0,0842
0.8130 0.8450 0.8290 0.8447 0.8323 0.8424 0.8354 0.8152 0.8046
0.8140 0.8467 0.8306 0.8472 0.8330 0.8440 0.8368 0.8162 0.8054
0.8151 0.8484 0.8322 0.8486 0.8344 0.8455 0.8381 0.8173 0.8063
0.8162 0.8501 0.8337 0 3500 0.8357 0.8470 0.8396 0.8183 0.8071
0.8173 0.8518 0.8353 0.8514 0.8370 0.8485 0.8409 0.8193 0.8080
,
0.8184 0.8536 0.8368 0.8528 0.8383 0,8501 0.8423 0.8204 0.8088
0.8195 0 8554 0 8383 0.8542 0.8396 0.8516 0.8437 0.8215 0.8096 I
I
+ AgX(s) + SH
0.8206 0.8670 0,8399 0.8556 0.8409 0.8532 0.8451 0.8225 0.8104
0.8217 0 8587 0.8414 0.8570 0.8423 0.8547 0.8465 0.8236 0.8112 I
The general experimental procedure has been described earlier.4+8
l/zHz(g)
Results and Discussion
Similarly, in eq 3, the term usH2is usually dropped out, being assumed unity, but that should be correct only for sufficiently dilute solutions. The Emovalues used for the Ag-AgBr electrode at different temperatures were determined by the authors recently in this laboratory.*?g The emf data and the corresponding molalities of the lyate ion ms- and the halide ion mBr- are presented in Tables I and 11. The pK, values obtained from the extrapolations are given in Table 111. The values given in Table I11 can be expressed in the form of equations of Harned-Robin~on~ type, obtained by the method of least squares, as follows: for ethylene glycol
The emf of cell A is given by
Substituting the values of U S H ~ + by K,asH2/ms-7s from eq 3 and writing mBr-YBr- for aBr-9 we have by rearranging the terms in eq 4
F(E - EOAg-AgBr) 2.303R T
+ log ms -
mBr- -
(5)
The term log (YBr-asH)/Ys- becomes zero a t zero ionic strength ( p = 0), since by convention the terms become unity and USH also becomes unity for the pure solvent when p = 0. A plot of the quantity on the lefthand side, which may be denoted by pK,', against p should, therefore, yield pK, on extrapolation to p = 0. In eq 4 the term asH is very often assumed to be unity and dropped out for all concentrations, since the solutions used are generally dilute enough. The USH term should, however, occur in eq 4,since the reaction taking place in cell A involves the solvent as shown below The Journal of Physical Chemistry, Vol. 74, No, IS, 1070
--j
SHz+(solution)
pK, = 3487'25 ~
T
+ X-(solution) + Ag(s)
+ 0.927 + 0.01079T
(6)
+ 0.01120T
(7)
and for propylene glycol
pK, = 3372'00 + 2.571
T
(6) K. K. Kundu and M. N. Das, J . Chem. Eng. Data, 9, 87 (1964). (7) K. K. Kundu and M. N. Das, ibid., 9, 82 (1964). (8) K. K. Kundu, P. K. Chattopadhyay, D. Jana, and M. N. Das, J . Chem. Eng. Data, in press. (9) K. K. Kundu, D. Jana, and M. N. Das, J . Phys. Chem., 74, 2626 (1970).
2635
SELF-IONIZATION OF ETHYLENE AND PROPYLENE GLYCOLS Table 111: pK, Values of Ethylene and Propylene Glycols a t Different Temperatures Temp,
r _ _ _ _ _ _O _ C -_____ 7 __
Ethylene glycol Propylene glycol
5
10
15
20
25
30
35
40
45
16.47 17.81
lfi , 3 0 17.64
16 14 17.50
15.99 17.35
1,;. 84 17.21
15.71 17.08
15.57 16.96
15 44 16,83
15.32 16.73
Table IV: Free Energy, Enthalpy, Entropy, and Heat Capacity Changesa (in Molal Scale) Accompanying the Autoprotolysis of the Glycols and Water at Different Temperatures Temp, OC
__I______________-_____I
10
5
20
15
25
30
35
40
45
21.78 11.42 -34.2 -29.9
21.95 11.27 -34.7 -30.4
22.13 11.11 -38.2 -30.9
22.30 10.96 -35.7 -31.4
23.70 10.73 -42.8 -31.0
23,92 10,56 -43.3 -31.6
24.13 10.41 -43.8 -32.1
24.35 10.24 -44.4 -32.6
19.19 13.28 -19.5 -47.3
19,29 13.04 -20.2 -48.1
19.39 12.80 -21.0 -48.9
19,50 12.56 -21.8 -49.7
Ethylene Glycol
AGO AH' A S"
ACpo
21,12 12.00 -32.2 -28.0
20.96 12.14 -31.7 -27.5
21.28 11.86 -32.7 -28.5
21.44 11.71 -33.2 -29.0
21.81 11.57 -33.7 -29.4
Propylene Glycol
AG AHo
AX O ACpo
22,87 11.32 -40.8 -29.0
22,67 11.47 -40.3 -28.5
23.08 11.18 -41.3 -29.5
23.28 11.03 -41.8 -30.0
23,49 10.88 -42.3 -30.5 Water
AGO AH" AX O AC," a
18,83 14,20 -16.4 -44.2
18.75 14.42 -15.6 -43.4
18.91 13.98 -17.1 -45.0
19.00 13.75 -17.9 -45.8
19.09 13.52 -18.7 -46.6
AGO and AH" values are in kcallmol and ASo and AC," values are in cal/mol, "C.
+ 2.303RT log loo0 MSH
(9)
AH" has the same values on either scale for the solute. The AGx" values are expected to reflect in a general way the composite effects of the dielectric constant and the intrinsic acidity as well as the basicity of the solvents. Thus, AGN" values gradually increase as the dielectric constants of the solvents decrease, at least for the solvents of similar acidity and basicity. Since acetic acid as well as sulfuric acid is highly protogenic in nature, AGN" values are expectedly lower than those of the other solvents, while NH3 in spite of being a highly protophilic solvent has higher AG," values. It is well known that in many systems the free energy function is less discriminating than either the enthalpy or entropy function. This is chiefly because many of the effects associated with enthalpy functions get compensated with the corresponding effects associated with entropy functions. Presumably in view of this, Feakinsll is of the opinion that while free energy function is dominated by a contribution which does not reflect struc-
- 2.303R log 1000
(10)
(10) H. S. Harned and B. B. Owen, "Physical Chemistry of Electrolytic Solutions," 3rd ed, Reinhold Publishing Corp., New York,
The values of AGN" and TASN" for different solvents are also included in Table V. It may be noted that
1957, p 762. (11) F. Franks, Ed., "Physico-Chemical Processes in Mixed Aqueous Solvents," Heinemann Educational Books Ltd., London, 1967, p 148.
The corresponding equation for water is pK, =
4471.33 ~
T
- 6.085 + 0.01705T
(8)
The maximum deviations in pK, values obtained from the eq 6 and 7 and those found experimentally lie within =tO.Ol unit. The thermodynamic quantities AGO, AH", AS", and AC," for the self-ionization of the solvents were evaluated from the usual equationslO comprising the constants of eq 1. These values of AGO, AH", AS", and AC," for the glycols and water a t nine temperatures are given in Table IV. In Table V are collected the values AGO, AH", and TAS" at 25" for the different solvents so far studied. The values of standard free energy and entropy changes on the mole fraction scale, AGN" and ASN" were calculated by the equations AGN" = AGO
ASNO = AS"
~
MSH
The Journal of Physical Chemistry,Vol. 74, No.13, 1970
2636
E(.K. KUNDU, P. K. CHATTOPADHYAY, D. JANA, AND M. N. DAS
Table V : Thermodynamic Quantities" Accompanying Self-Ionization of Different, Solvents, at 25'
Waters Ethylene glycolb Methanol14 Propylene glycolb Ethanol14 Acetic acid14 Sulfuric acid16 Amnionia14 a
In kcal/mol.
19.09 21.61 22.71 23.49 25.63 1.7.21 4.73 41.31
23.85 24.91 26.79 26.54 29,28 20.54 7.48 46.16
13.53 11.57 11.20 10.88 11.50 5.70 0.23 26.70
-5.57 -10.04 -11.51 -12.61 -14.13 -11.51 -4.50 -14.61
-10.33 -13.34 -15.59 -15.66 -17.78 -14.84 -7.25 -19.46
78.30 36.76 32.63 31.00 24.30 6.30 101 22 (-340)
From the present study.
tural features of the system, the corresponding enthalpy and entropy functions contain important structural contributions. For this reason also Hills,ll Franks and Ives,12B a t e P and many others stressed the importance of examining these two functions also besides the less discriminating free energy function. Thus it is expected that the heats AHNO and entropies ASN" of the selfionization process should also involve the structural aspects of the solvents. The considerably less negative value of TASN" for water, as compared t o those for other hydroxylic solvents, has been explained by Jollyl4 in terms of considerable structure breaking of water caused by an ion when it enters the strongly hydrogenbonded structure of water. Gillespie, et a1.,'6 however, in explaining the still smaller TASN' value for the autoprotolysis of HzS04, held the view that the autoprotolysis of a protonic solvent like HzS04, acetic acid, or water, forming SH2+ and S- ions, does not have the same effect on the structure of the solvent as other ions do, since they differ from the SH molecule only by possessing one more or one less proton, and it is likely that they will cause little, if any, disruption of the structure. They further believe that the SHz+ and Sions will rather cause a strengthening of the hydrogen bonds around the ion and an increase in the "characteristic structure" of the solvent in bulk, involving a three-dimensional framework, resulting in a decrease in entropy. The larger negative TASNOvalues for weakly hydrogen-bonded solvents such as methanol and ethanol probably arise because the characteristic ions resulting from autoprotol ysis presumably cause a Considerable increase in the strength and the amount of hydrogen bonding. On the other hand, in the strongly hydrogenbonded solvents such as water and HzS04 the effect of the characteristic ions on the structure will be relatively much smaller, and hence the entropy changes accompanying the autoprotolysis are correspondingly small. I n the light of this concept, the entropy changes accompanying the autoprotolysis of ethylene and propylene glycols indicate that these dihydroxy alcohols are no doubt relatively more associated than the monohydroxy alcohols due to increased hydrogen bonding, but the existence of any such "characteristic structure" in The Journal of Physical Chemistry, Vola74, No. 18, 1970
the bulk is a t least less predominant in the glycolic solvents. As is well known, the unique distribution of charge centers in isolated water dipoles tends to impart a socalled hydrogen-bonded tetrahedral structure to water. The absence of suitable charge distribution in isolated dipoles of glycols perhaps cannot lead to any such characteristic structure in the solvents, though some polymeric chain-like two-dimensional structure due to hydrogen bonding might very well be present in the glycolic solvents. The fairly less negative values of TASN' in water are presumably related to the breakdown of such characteristic structure and not of the hydrogen-bonded polymeric chains. Since autoprotolysis involves charge separation, any conclusion regarding the chemical nature of solvents and their structural aspects that could be derived from the thermodynamic parameters accompanying the selfionization of the solvents should be largely masked, due to the electrostatic effect arising from the difference in dielectric constants of the solvents. Hence, the foremost task is to make an assessment of this electrostatic effect. I n spite of the well-known limitations in regard to dielectric saturation as well as the assumed independence of radii of solvated ions in different solvents, Born eq 11 is often used in computing the electrostatic part of the thermodynamic quantities, resulting from the difference in permittivity of the solvents, but because of limited knowledge regarding the proton solvation in solvents other than water, it would be difficult to use Born equation as such. If, however, one is allowed to assume the solvated ions to be a t least effectively spherical, it is possible to avoid the arbitrariness in choosing the radii of the ions in solvents by using the linear relation 12 predicted from simple Born equation a t different temperatures, as was first demonstrated by (12) F. Franks and D. J. G. Ives, Quart. Rev. Chem. SOC., 20,l (1966) (13) R. G. Bates in "Hydrogen-Bonded Solvent Systems," A. K. Covington, and P. Jones, Ed., Taylor and Francis, London, 1968. (14) W.L.Jolly, S. Amer. Chem. Soc., 7 4 , 6199 (1952). (15) R. J. Gillespie, E. A. Robinson, and C. Solomons, J . Chem. Soc., 4321 (1960). I
2637
SELF-IONIZATION OF ETHYLENE AND PROPYLENE GLYCOLS BaughanlBand later confirmed by La Mer and Brescia." According to the Born equation, electrostatic contribution to the standard free-energy change of the selfionization process (eq 2) is
where r+ and r- are the effective radii of the cation and the anion and D, is the macroscopic dielectric constant. Utilizing this simple Born equation, the standard heat change in self-ionization of the solvents can be represented by
+
AH" = ( A H " ) D ~ = ~
[
K e z (-l + - 1) 1 I + - T(dD,)] 2 r f r- D, D, d T
=
where CB is a constant, containing the radius factor, and (AH").,, represents the nonelectrostatic, i.e., chemical part of the enthalpy change, (AH")chem accompanying the self-ionization process. This (AHo)ohem would presumably incorporate the interaction energies due to ion-dipole, dipole-dipole, and other energy changes accompanying the self-ionization process except the electrostatic contribution arising from permittivity of the solvents. Equation 12 indicates that AH" should be a linear function of the quantity within the square brackets, provided that (AHo)chem is independent of temperature. Similarly, the entropy change is given by AS"
=
1 (AS")chem -/ CB[%
x
-1
dInD, dT
...
(13)
Hence, the plot of AS" against the quantity in square brackets in eq 13should also be linear. Thus, the chemical part of AH" as well as of AS" can be evaluated without any prior computation of the numerical value of the radius factor CB which is directly obtained from the slope of the linear plot. The data for water, ethylene glycol, and propylene glycol are available for the self-ionization process and the necessary parameters (in molal scale for the solute) for the two types of plots (AH" and AS") are given in Table VI. I n Figure 1 are shown the plots which are found to be good straight lines for all the three solvents. For each solvent, the slopes obtained from the two plots, given in Table VII, are in reasonable agreement, as required by eq 12 and 13, which adds to the confidence in the computed quantities. The values of ( A H o ) c h e m and ( A X o ) o ~ e m being graphically obtained in this manner, it is easy to evaluate ( A G " ) c h e m accompanying the self-ionization process. The thermodynamic quantities thus derived for each solvent, on a molal scale, are shown
I
*O
btb
8.0
-
(1
T dDa + -.Ds dT)
lo'
Figure 1. Variation of A H o us. us.
1 dD in water, ethylene glycol, and propylene glycol. Da2d T
- -'
in Table VI 11, where the corresponding data on a mole fraction scale are also included. Though these data (on mole fraction scale) are now free from the effect of dielectric constant of the solvents, they still incorporate the composite effect of intrinsic acidity and basicity of the solvents in the case of (AGo)ohem values and also that of the relative structural aspects of the solvents in the cases of (AHo)chem and (ASo)ohem values. From the computed results which are now free from the effect of dielectric constant, it can be seen that the relative ease of self-ionization of the solvents is as follows: water > ethylene glycol > propylene glycol. Now it has been indicated earliere that the order of acidity of water and glycols is as follows: water > glycols, and that of basicity: water < glycols. Again, it can be argued that due to the inductivity effect of methyl group in propylene glycol, the negative charge density on oxygen center should be larger than the corresponding quantity in ethylene glycol. As a result, the protonic character of H atom of the OH group in propylene glycol is less than that in ethylene glycol. Thus it is not unreasonable to infer that the order of acidity of the solvents is as follows: water > ethylene glycol > propylene glycol, and that of (16) E. C. Baughan, J . Chem. Phys., 7, 951 (1939). (17) V. K. La Mer and F. Brescia, J . A m e r . Chem. Soc., 62, 617 (1940).
T h e Journal of Physical Chemistry, Val. 74, No. 13, 1QYO
K. K. KUNDU, P. K. CHATTOPADHYAY, D. JANA, AND M. N. DAS
2638
Table VI: Necessary Parameters for the Plots of AH" us. 1/D.[1 AS" us. 1/DS2(dD,/dT)in the Glycols and Water
+ T/D,(dD,/dT)]
-_________ 5
10
15
and
Temp, OC25
20
7
_ _ _ I _ _ _ _ _
30
35
40
45
Ethylene Glycol
[l + O ,
D, _I
E1
E x
(""a)]
(g)x
103
106
-10.40
-11.31
-12.26
-13.25
-14.25
-15.35
-16.47
-17.65
-18.86
-12.35
-12.67
-13.00
-13.34
-13.69
-14.05
-14.42
-14.80
-15.18
Propylene Glycol
[
D, I l+O,
("a)]
dr
x
103
& (g)x
106
-21.38
-23.00
-24.68
-26.47
-28.32
-30.28
-32.33
-34.47
-36.72
-17.92
-18.49
-19.07
-19.69
-20.32
-20.98
-21.64
-22.33
-23.04
Water DS q l + ; ( g ) ] X l o ~
-3.38
-3.68
-3.99
-4.31
-4.65
-5.00
-5.37
-5.75
-6.16
& (2)x
-5.39
-5.50
-5.61
-5.72
-5.83
-5.95
-6.07
-6.19
-6.32
106
AH,'
solvent aggregates +nSH
Table VII: Slopesa of the Plots of AHo us. l/Da[l T/D,(dD,/dT)] and ASo vs. 1/Da2(dDa/dT) in the Glycols and Water
+
(depolymerization) (1)
+
SH % SH + (acidic ionization) (2) - AHaO H+ SH +SH2+ (basic ionization) (3) - AH4' SH2+) (n - 2)SH + (SH2+)Bo~v(S-)solv (solvation) (4)
+
AHe/
($)I
1 + 5, T dD
Da [i
Ethylene glycol Propylene glycol Water a
ASo/
& (2)
139 80 682
139 80 675
(S-
+
+
+
+
AHl0 AHSO - AH%"- AH4O = (AHI" - AH4') (AHtO - AH3'), negative sign before AHo values indicating exothermicity of the
Thus, (AH)Ochem
Values are in kcal/mol.
+
process.
Table VI11: Thermodynamic Quantities" Representing the Chemical Effects of Solvents Accompanying the Self-Ionization of Glycols and Water
Water Ethylene glycol Propylene glycol a
(AGo)ohem
(AGN')ohem
10.45 17.92 20.91
15.21 21 21 23.96
(AH')ohem = (AHNo)ohern
16,68 13.56 13.14
I
(TASo)chem
6.23 -4.36 -7.77
(TA8N')chern
1.47 -7.65 -10.82
All values are in kcal/mol.
basicity : water < ethylene glycol < propylene glycol. These inferences are, however, contrary to what has been suggested from our studies on proton-transfer equilibria,' but after all, the earlier conclusion was indeed too naive18 in view of too many complexities involved in the proton-transfer equilibria. Again (AHo)c~em can be thought to be equal to the Of the heat changes in the steps (being free from dielectric constant effects on ionization)
The Journal of Physical Chemistry, Vol. 74,No. 13,1970
Since steps 2 and 3 involve isolated molecules it can be assumed that the order of heat changes involved in those steps for different solvents would be parallel to that of free energy changes. Thus in view of the above contentions regarding the relative acidity and basicity of the solvents, the order of magnitudes of (AHz") and (18) F. Franks, Ed., in "Physico-Chemical Processes in Mixed Aqueous Solvents," Heinemann Educational Books Ltd., London,
1967.
PHOTOLYSIS AND RADIOLYSIS O F 3-n~ETHYL-2-BUTANON'E
2639
smaller values of (AH')chem and also (AH1' - AH4') for ethylene and propylene glycols indicate the absence of a similar type of characteristic structure in the bulk, in spite of larger association due to hydrogen bonding in glycols. The positive (TASNo)chemvalues for water further < (AHz' - AH3°)Eg< (AH2' substantiate the view that the heat changes for struc(AH,' - A H ~ O ) ' ~ ture disruption in water are so large that even the presence of order-producing characteristic ions like Again, it can be seen from the Table VI11 that the HsO+ and OH- is unable to bring about an overall resulting (AH')chem values are in the order: (AH')"chern order, and causes an overall disorder instead. The > (AHo)Egchem> (AHo)Pgohem. Thus it is expected that larger negative values of (TASN')chem for ethylene and the magnitudes of (AH1' - AH4') values should have propylene glycols, on the other hand, indicate that the following order perhaps owing to the absence of a similar characteristic structure in glycols, smaller amounts of heat changes are (AH1' - AH4')" >> (AHI' - AH4°)Eg>> actually involved in affecting the proper configurational (AH1' - AH4')'@: changes t o facilitate the self-ionization, and hence the order-producing characteristic ions are able t o bring and in these values of (AH1' - AH4') will be reflected about an overall order in glycols. Further analysis of the overall energetics due to structural changes acthe results should, however, await similar studies in companying self-ionization. This suggests that larger other amphiprotic solvents. heat changes are involved in bringing a proper conAcknowledgment. The work was done under a figurational change of water molecules to facilitate project financed by the National Bureau of Standards, self-ionization which is perhaps due t o the characteristic Washington, D. C. tetrahedral structure of water in the bulk, while the
(-AH3') in the solvents should be as follows: (AH,')" < (AHz')'~ and (-AH3')" < (-AH3°)Eg (- AH30)Pg,where the superscripts denote water, ethylene glycol and propylene glycol, respectively. It follows immediately from the above inequalities that
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