Vapor Pressures of Triethylamine + Chloroform and of Diethyl Ether + Chloroform at 298.15 K

YASH PAUL HANDA AND DAVID EDWARD JONES' Department of Chemistry, University of Otago, Dunedin, New Zealand

Received April 2, 1975

YASH PAUL HANDA and DAVID EDWARD JONES. Can. J. Chem. 53,3299 (1975). Vapor pressures of triethylamine + chloroform and of diethyl ether + chloroform were mea-

sured at 298.15 K with a calibrated quartz spiral. The ideal associated solution model has been used to determine the equilibrium constants and standard enthalpies of formation of the com- plexes from the thermodynamic properties derived from the vapor pressure measurements and from earlier calorimetric measurements. These complex formation constants are compared with values obtained by calorimetric and spectroscopic methods.

YASH PAUL HANDA et DAVID EDWARD JONES. Can. J. Chem. 53,3299 (1975). On a mesurk, a 298.15 K et tI l'aide d'une spirale en quartz calibrk, les tensions de vapeur de

mklange de trikthylamine + chloroforme et d'kther kthylique + chloroforme. On a utilisk le modele de la solution associk idkale pour dkterminer les constantes d'kquilibre et les enthalpies standards de formation des complexes partir des propriktks thermodynamiques obtenues a I'aide des mesures de tension de vapeur et aussi en faisant appel tI des mesures calorimktriques antkrieures. On compare ces constantes de formation de complexes avec celles obtenues par des mkthodes calorimktriques et spectroscopiques.

[Traduit par le journal]

Introduction The experimental investigation of liquid mix-

tures which form complexes has in the past been largely the domain of spectroscopic and other nonthermodynamic measurements. Often there is evidence for complex formation in the system of interest from solid-liquid phase equilibrium studies. Recently, thermodynamic measure- ments (1, 2) have been used with considerable success both to supplement the results obtained independently and to improve their precision.

In particular triethylamine (A) + chloroform (B) has been studied extensively and there is considerable spectroscopic and thermodynamic evidence for the chemical reaction

The molar excess enthalpies (3), partial molar enthalpies of solution (4) at 298 K, and the vapor pressures (2) at 283.15 K of this system have been used in conjunction with the ideal associated solution model to determine the equi- librium constant and standard enthalpy of for- mation of the complex AB. The same model has been used to explain the sign of the temperature coefficient of the molar excess volume for this

'Author to whom correspondence should be addressed.

system and other systems containing chloroform (5).

Complete application of the theory of the ideal associated solution model to triethylamine + chloroform has not been entirely satisfactory since the vapor pressures were measured with a mercury manometer (2) and restricted in the temperature range studied. We report here the vapor pressures of triethylamine + chloroform at 298.15 K measured with an apparatus which uses a calibrated quartz spiral as the pressure sensor.

Thermodynamic and spectroscopic evidence suggests that diethyl ether (A) + chloroform (B) mixtures consist of A, B, and the complex AB. Molar excess enthalpies of ethers + chloro- form at 298 K (6) have been used with the ideal associated solution model to determine the com- plex formation constants (7). Constant pressure liquid-vapor equilibrium results for diethyl ether + chloroform at higher temperatures were analyzed using the ideal associated solution model (7). The equilibrium constant at 298 K (obtained by extrapolation) was in good agree- ment with that obtained from calorimetric and spectroscopic measurements but the scatter in the experimental results precluded determina- tion of the standard enthalpy of formation of

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3300 CAN. J . CHEM. VOL. 53, 1975

the complex AB. We report measurements of the vapor pressures of diethyl ether + chloro- form at 298.15 K.

TABLE 1. Vapor pressures p of pure materials at 298.15 K

Material P (kpa) Reference Experimental

' Materials Chloroform and triethylamine were purified as re-

ported previously (2). Diethyl ether (Baker, anhydrous A.R. grade) was

shaken with one-tenth its volume of a 10% (by mass) NaHSO, solution intermittently for a period of 1 h. The

- -

Triethylamine 8.942 8.911

This work Copp and Findlay (lo),

extrapolated value Chun and Davison (1 1)

Chloroform 26.231 26.236 26.222

This work Philippe et al. (12) Boublik et al. (13) aqueous phase was withdrawn and theether washed with

a saturated NaCl solution containing 0.59, (bv mass) Diethyl ether 71.557 71 .622 71.317

This work Ambrose et al. (14) Boublik et al. (13),

extrapolated value

, - . - NaOH. Diethyl ether was then washed initially with 4 saturated NaCl solution containing a small amount of H2S04, and subsequently with two portions of saturated NaCl solution. Storage of diethyl ether overnight over lithium aluminum hydride was followed by refluxing for 2 h in the presence of lithium aluminum hydride and then by fractional distillation in a column of 15 theoretical plates at a reflux ratio of 20. Dry and oxygen free nitro- gen was bubbled through during the treatment with lithium aluminum hydride and the distillation. The distil- lation apparatus was wrapped in black cloth to exclude light.

All materials were thoroughly degassed by vacuum sub- limation (8) and then by successive freezing (using a Dry Ice - acetone mixture), pumping, and distillation cycles. The degassed liquids were distilled under vacuum and the middle fractions collected in preweighed ampoules. The ampoules were sealed under vacuum and stored in the dark.

Constant values of the vapor pressure of a given liquid sample resulted when successive amounts of material were distilled from it and this is good evidence for the purity of each of the liquids used.

lar to one used previously (9). A quartz spiral mounted in a pressure gauge (Texas Instruments Model 145) was used as the pressure sensor. The quartz spiral was calibrated with a precision Hg manometer over the pressure range covered by each system studied. The equilibrium cell was thermostatted to 298.15 K with a probe and temperature controller (Tronac Model 40).

Material was added to the equilibrium cell from am- poules on a line external to the thermostat bath. For each system two sets of measurements were done in order to cover the entire mole fraction range with some overlap about the midregion of the mole fraction scale.

Table 1 gives a comparison of the measured and litera- ture values of the vapor pressures of triethylamine, chloro- form, and diethyl ether at 298.15 K.

Results The vapor pressures p of triethylamine +

chloroform and of diethyl ether + chloroform at 298.15 K are given in Table 2. Errors in the

Vapor Pressure Measurements Vapor pressures of mixtures of known total composi-

tion and volume were measured in a vacuum system simi-

TABLE 2. Vapor pressures of [xA(C2HS),N + XBCHCI,] (I) = bA(C2H5),N + yBCHC13] (g) and of [X.,(C~H~)~O + xBCHC13] (1) = + yBCHC13] (g) at 298.15 K

P &Pa) P &Pa)

XB y, Calculated Experimental x, y, Calculated Experimental

Triethylamine + chloroform 8.847 0.5211 0.7699 9.055 0.6023 0.8594 9.658 0.7085 0.9321

10.576 0.8374 0.9761 11.920 0.9281 0.9918

Diethyl ether + chloroform 61.911 0.5520 0.3445 52.092 0.6346 0.4797 43.575 0.7218 0.6321 37.189 0.8182 0.7885 34.242 0.9138 0.9023

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HANDA AND JONES: VAPOR PRESSURES 3301

TABLE 3. Molar mass Mk, molar volumes VkO, and second virial coefficients Bkk of the pure fluids at 298.15 K

Material Mk (g mol - ') Vkoa (cm3 mol- ') - Brkb (cm3 mol- ')

Triethylamine 101.191 140.3 Chloroform 119.378 80.7 Diethyl ether 74.122 104.7

TABLE 4. Coefficients in [3], the standard deviations of these coefficients, and standard deviations ~ ( p ) defined by [4]

gl g~ g3 &Pa)

Triethylamine + -3.632 -0.071 0.570 0.026 chloroform k0.012 f 0.023 f 0.034

Diethyl ether + -3.013 -0.060 0.306 0.072 chloroform kO.010 k0.021 f0.032

TABLE 5. Molar excess thermodynamic properties of [ x ~ ( C ~ H ~ ) ~ N + xBCHC13](I) and of [ X ~ ( C ~ H ~ ) ~ O + xBCHCI3](II)

at 298 K

- HmE (kJ mol- l)

mole fractions are less than k0.004 and those in the vapor pressures arising both from random and systematic errors in the cathetometer' and from fluctuations in the thermostat are k0.015 kPa or less.

The liquid and vapor compositions and the calculated vapor pressures in Table 2 were de- termined from a set of equilibrium pressures, known total amounts of each component, and known total volume of the equilibrium cell line by using a modified form of Barker's analysis (15). The properties of the pure components listed in Tables 1 and 3 were used in the analysis.

The molar excess Gibbs free energy, GmE of a binary mixture of mole fractions xk (liquid) and yk (vapor) is given by

where k = A, B, and pkO and V> are respectively the saturated vapor pressure and the molar volume of liquid k at temperature T. The term 6 is given by 2BA, - BAA - BBB, where Bkj is a second virial coefficient.

An iterative technique (9) was used to deter- mine the liquid and vapor compositions and the coefficients in the series

3.. ... [3] G , ~ / ~ J inol-I = xAxB 1 gi(l - 2x,)'-l

i = l

with xB the mole fraction of chloroform, were

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3302 CAN. J. CHEM. VOL. 53, 1975

adjusted to minimize the difference in the ob- served and calculated pressures. Use of reason- able estimates (1 8) for BABY the cross virial coef- ficient of the mixture, had no significant effect on the calculated values for GmE, hence the re- sults given in Table 2 are obtained by setting 6 equal to zero.

The results for both systems were fitted satis- factorily using m = 3 in [3] and the coefficients gi and their respective standard deviations are listed in Table 4. Included in Table 4 are the values of o(p) the standard deviation in the va- por pressure given by

ties of triethylamine + chloroform and of di- ethyl ether + chloroform at 298 K.

Discussion With the notation given in Table 6 the equi-

librium constant K for [l] can be expressed as

[5] K = r( l - r)/(xAxB - r + r2)

where r = n/(N, + NB). Previously, McGlashan and Rastogi have

shown (19) that K can be related to the activities a, and a, by

where Ap is the deviation of the calculated from the observed vapor pressure and N is the num- ber of experimental points. The calculated liquid and vapor compositions and vapor pressures listed in Table 2 were determined using the values of gi given in Table 4.

The values of GmE obtained from our vapor pressure measurements at 298.15 K were com- bined with published values of the molar excess enthalpy, HmE at 298 K (3, 6) to give TSmE at 298 K, where SmE is the molar excess entropy. Table 5 lists the excess thermodynamic proper-

which is a contracted form of the more general expression

derived for systems in which there was an addi- tional equilibrium resulting in the formation of complex AB,.

Expressions for molar excess thermodynamic properties XmE of mixtures which form com- plexes have been derived (1, 2) using the ideal associated solution model with the following thermodynamic cycle (pe = 101.325 kPa)

L I I A X ~ I AX*

A(1) [NA - n,T,pe1 + B(1) [NB - n,T,pel + AB(1) [n,T,pe]

where X is any extensive thermodynamic property. The notation used is summarized in Table 6.

The molar X of mixing, AXm is related to the standard X of complex formation AXe by

[9] (NA + NB)AXm = nA Xe + AX*

Use of the ideal associated solution model al- lows representation of AX* by the appropriate ideal solution expression.

The equation for H~~ has been derived pre- viously (3) and applied to various systems (1) and will not be discussed here. The following ex- pressions were obtained for GmE and SmE (2)

[lo] GmE = RT[xA ln (1 - r/xA) + x, ln (1 - r/xB) - In (1 - r)]

[l 11 . SmE = r A p / T - R[xA In (1 - r / x d + x, In (1 - r/xB) - In (1 - r)]

The equation for VmE, the molar excess volume

has been applied and discussed elsewhere (5). The methods used to calculate Kand AHe have

been described (2) and can be summarized as follows: (i) the term (1 - a, - a,)/a,a, calcu- lated from the smoothed activities, which were in turn derived from the vapor pressure results, was plotted against a, as indicated by [7], (i i) [lo] was solved for r and the value of r obtained was substituted in [5] to obtain K, (iii) values of SmE derived from Table 5 were used in [l 11 and

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HANDA AND JONES: VAPOR PRESSURES

TABLE 6. Notation used for a liquid mixture in which [I] takes place

Stoichiometric NA NB amount

Stoichiometric X A XB

mole fraction

Equilibrium NA - n NB - n n amount

Equilibrium (XA - r)/(l - r, (XB - r)/(l - r, r/(l - r) mole fraction

TABLE 7. Complex formation constants for the triethylamine-chloroform complex at 298 K

Method K -AHe(kJ mol-I)

Spectroscopic 3-4.7" 17" Calorimetric 4.8 k 2.5" 14.2 k 1.1" Partial molar 3.3 + 2.1b 15.8 f 2.3b

enthalpies Equation 7 2.50 k 0.01

(slope = 0.16 + 0.02) Equation 10 2.6 + 0 .4 Equation 11 3.0 k 1.0 16.4 + 1.0

"Reference 3, K value of 4.7 based on n.m.r. measurements in c-C6H12 solution. bReference 4.

i r values were adjusted to give the most constant ~ value of AHe over the entire mole fraction range. The values of K and AHe (where determined)

at 298 K for triethylamine + chloroform are given in Table 7. These values are compared with values previously reported and obtained by calorimetric or spectroscopic methods. With the exception of those values of K derived directly from the vapor pressure measurements there is satisfactory agreement. The low values of K which result from the use of [7] and 1101 may be attributed to failings in the ideal associated solu- tion model. A reasonable estimate of the 'physi- cal' contribution to GmE would increase the value of K.

Bertrand (20) has discussed the effect of an estimation of the physical contribution on the results obtained for triethylamine + chloro- form.' The values of K and AHe derived by ap- plication of this extension of the ideal associated solution model to the calorimetric results (3, 4)

1 I

'We are indebted to one of the referees for bringing this paper to our attention.

are respectively 2.0 f 0.5 and - 20.4 + 2 kJ mol-'. These values are claimed to be in better agreement with those derived from spectroscopic studies (Table 7). Use of the same values for the weighting and interaction parameters intro- duced by Bertrand gives a physical contribution to GmE of approximately 200 Jmol-' and K/Kid = 1.4 where Kid is given by [5]. The ap- proximations involved in using (i) the solubility parameters to estimate the interaction parameter and (ii) the same weighting and interaction parameters as used for the calorimetric results indicate the qualitative nature of these calcula- tions. The range of K values and their respective uncertainties in Table 7 includes the possible variation in K from an estimate of the physical contribution to XmE.

The values of K (and AHe where determined) at 298 K for diethyl ether + chloroform are summarized in Table 8. For comparison values previously obtained by spectroscopy, calorimetry, and liquid-vapor equilibrium are also listed.

Application of Bertrand's procedure (20) for estimation of the physical contribution to GmE

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CAN. J. CHEM. VOL. 53. 1975

TABLE 8. Complex formation constants for the diethyl ether - chloroform complex at 298 K

Method K -AHe (kJ mol-l)

Spectroscopic 1.46 f 0.04, 3.76 f 0.10" Calorimetric 1.46 _+ 0.37" 14.5 + 1.2 Liquid-vapor 1.4"

equilibrium Equation 7 1.91 + 0.01

(slope = 0.12 f 0.02) Equation 10 1.9 + 0 . 3 Equation 11 2 .0 k 0 . 5 12.6 + 0 . 3

OReference 7, K values of 1.46 and 3.76 based on n.m.r. measurements in CC14 and c- C,H,, solutions respectively.

(using the solubility parameters to estimate the interaction parameter) gives qualitatively the same results as obtained for triethylamine + chloroform. In view of the magnitude of GmE and the effect of an estimation of the physical contribution on the value of K, the values of K obtained directly from the vapor pressure re- sults using [7] and [lo] are unexpectedly large and in direct contrast with the corresponding results for triethylamine + chloroform.

Furthermore, the values of (aVmE/a~), are of opposite sign for diethyl ether + chloroform (21) and for triethylamine + chloroform (5). In the former system (avmE/aT), is positive, as ex- pected for a system in which there is complex formation. The term involving the variation in extent of complex formation with temperature is necessarily positive and for diethyl ether + chloroform is larger in magnitude than that which includes the temperature dependence of AVO, the molar volume of complex formation.

NOTE ADDED IN PROOF: Becker et al. (22) reported vapor pressure measurements for diethyl ether and chloroform at 298.15 K and found K = 1.742 from application of the ideal associated solution model to their results.

We acknowledge the assistance of Dr. D. V. Fenby and the financial support of the New Zealand Universities Grants Committee. Y.P.H. acknowledges the award of a Mellor Postgraduate Scholarship.

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