Molar excess enthalpies of acetonitrile + chloroform and of acetonitrile + chloroform-d, at 298 K

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

Received February 10, 1977

YASH PAUL HANDA and DAVID EDWARD JONES. Can. J. Chem. 55,2977 (1977). Molar excess enthalpies of acetonitrile + chloroform and of acetonitrile + chloroform-dl

were measured at 298 K. The results for the former system were discussed in terms of an ideal associated solution model and the deuterium isotope effect was discussed.

YASH PAUL HANDA et DAVID EDWARD JONES. Can. J. Chem. 55,2977 (1977). On a mesure, a 298 K, les enthalpies molaires d'exces de l'adtonitrile + chloroforme et de

l'acetonitrile + chloroforme-dl. On discute des resultats obtenus dans le premier systeme en termes d'un modele de solutions associees idkales et on discute de l'effet isotopique du deutt- rium.

[Traduit par le journal]

Introduction There has been a revival of interest recently

in the thermodynamic study of liquid mixtures of A and B in which association takes place. Often, the "physical" contribution, x:,,,,, and the "chemical" contribution, X z ,,,,, to the molar excess function XE are assumed inde- pendent (there is experimental evidence for this assumptioii (I)) i.e.

[l l E Xz = Xz,phys + Xm,chem

IfEXE,,hy, is negligibly small compared with X,,,,,, or, alternatively, if XE, ,,,,, can be esti- mated (see, for example, ref. 2), the mixture can be treated as an ideal associated solution. The application of this model to calorimetric mea- surements on mixtures with AB association has been reviewed (3).

Triethylamine + chloroform and diethyl ether + chloroform have been exhaustively investi- gated by thermodynamic methods as a result of independent evidence for AB association in these systems. The earlier calorimetric studies in the former system at 298 K (4, 5) have been sup- plemented by vapour pressure measurements at 283 K (6) and 298 K (7) and by measurements of the molar excess volumes at 298 and 308 K (8). The ideal associated solution model has been extended and applied to these results. Similarly,

'Present address: Department of Chemistry, Wright State University, Dayton, OH, U.S.A. 45431.

'To whom correspondence should be addressed. Present address: Environment Ontario, P.O. Box 487, Bracebridge, Ont., Canada POB 1CO.

the ideal associated solution model has been applied (3) to the results of calorimetric mea- surements at 298 K (9), to vapour pressure measurements at 298 K (7), and to the results (10) for the temperature dependence of the molar excess volumes of diethyl ether f chloro- form (7).

For the system acetonitrile + chloroform vapour pressures at 313 K (ll) , constant pres- sure vapour liquid equilibria (12) and molar excess volumes at 293 and 313 K (13) have been measured. Spectroscopic measurements on this system (14, 15) have been interpreted in terms of AB association and the values of the equilib- rium constants at 299 and 310.5 K have been calculated. However, it appears that the AB associated model is too simple and that a model which includes self-association (see accompany- ing paper) is required to interpret the thermo- dynamic results. We have measured the molar excess enthalpies of acetonitrile + chloroform at 298 K.

The deuterium isotope effect on thermo- dynamic measurements in liquid mixtures has been reviewed (16). The molar excess enthalpies of triethylamine + chloroform and of tri- ethylamine + chloroform-dl at 298 K (17) have been interpreted in terms of a larger value of the equilibrium constant for the former system and a more exothermic enthalpy of AB association for the latter, assuming no difference in the physical contributions in the respective systems. No deuterium isotope effect was detected from comparison of the molar excess enthalpies of

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2978 CAN. J. CHEM. VOL. 55, 1977

diethyl ether -t- chloroform and of diethyl ether $ chloroform-d,, and this may be attri- buted to a compensation of the respective differences in the equilibrium constants and the standard enthalpies of AB association (17). In order to assess the deuterium isotope effect in this study, we report the molar excess enthalpies of acetonitrile $ chloroform-dl at 298 K.

Experimental Materials

Chloroform (B.D.H. Analar) was purified as reported previously (7).

Chloroform-dl (Merck, for spectroscopy) was purified by a procedure similar to that for chloroform (7) except that the initial shaking with water (to remove ethanol) was omitted. A high amplification nmr study indicated a purity of 99.7 molz , with chloroform being the only proton-containing impurity.

Acetonitrile (B.D.H. Analar) was refluxed over phos- phorous pent oxide for 4 h, and subsequently fractionally distilled in a dry and oxygen-free nitrogen atmosphere using a column of 15 theoretical plates at a reflux ratio of 20. The distillation apparatus was wrapped in black cloth to exclude light. The middle fraction was degassed by vacuum sublimation (18) and the degassed sample was stored over freshly activated molecular sieve (Union Carbide, type 13X) in a sealed ampoule. A high amplifica- tion nmr study indicated that the sample was at least 99.5 m o l z pure.

results were fitted by a least-squares computer program to the expression

where x, is the mole fraction of chloroform or of chloroform-dl. For both systems the results in Table 1 were adequately fitted with k = 3 in eq. 2. The parameters h i and their respective standard deviations are given for both systems in Table 2. Table 2 also gives the standard deviations u(H:) defined by

where HE(calcd), HZ(expt) are respectively the calculated (from the parameters h i given in Table 2) and experimental values of the molar excess enthalpies for N experimental values.

Discussion Both the magnitude and asymmetry of the HE

results suggest the inadequacy of the AB association model used to intermet the nmr

Calorimeter results. There is evidence (21) for silf-association The calorimeter was similar to that described by Larkin in acetonitrile for which T~~~~~~~~ constant is

and McGlashan (19) and has been described elsewhere (20). Tests with the I.U.P.A.C. recommended system 96.4 mol-l K-l . In the for n-hexane + cyclohexane have previously been carried Gf, are positive (1 1) which is not consistent with out and compared with literature values (22). an AB association model. Hence, it appears that

Results The molar excess enthalpies, HE of acetoni-

trile + chloroform and of acetonitrile -t chloro- form-d, at 298 K are given in Table 1. The

TABLE 1. Molar excess enthalpies HE of xACH3CN + xBCHCI~ and of xACH3CN + xBCDCI3 at 298 K

a more complex model is required to interpret the thermodynamic results for this system (see accompanying paper).

At 313 K Gfi;(x, = +) = 131 J mol-I and use of the Gibbs-Helmholtz equation gives GE(x, = 4) = 164 J mol-I and TSE = -974 J mol-' at 298 K.

From constant pressure liquid vapour equilib- rium results (12) it appears that Gfi; is slightly more positive at higher temperatures and con- sequently that H z changes sign. This is in accordance with the expectation that C;,, will be positive for mixtures in which AB association takes place. Consequently, measurements of C;,, would be of interest.

Both Vf and (aV:/aT), are negative (13), the former fact consistent with AB association and

0.6923 809.0 the latter as observed for triethylamine + 0.7444 735.6 0.8198 606.4 chloroform (8).

Although there is no significant deuterium

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HANDA AND JONES 2979

TABLE 2. Coefficients in eq. 2, the standard deviations of these coefficients, and standard devia- tions o(Hg) defined by eq. 3

System h~ hz h3 o(HE)/J mol- ' Acetonitrile + chloroform -3250k 11 1505k21 319252 4 . 8

Acetonitrile + chloroform-dl -3317216 1611+39 153k97 6 . 8

isotope effect on V: (13), the molar excess en- thalpies of acetonitrile + chloroform-d, are more exothermic (by 16 J mol- ' at xB = 4) than those of acetonitrile + chloroform. The differ- ence is beyond the experimental uncertainty (Table 2) and is therefore consistent with the idea that the strength of the hydrogen bond in the AB associated system i s increased on sub- stitution by deuterium. However, a different value of the physical contribution in the hydrogen and deuterium systems (23) may account for such a small difference in the values of H:. It appears that there will be some interest in similar experimental results for acetonitrile-d3 + chloro- form and acetonitrile-d3 + chloroform-dl.

Further treatment of the thermodynamic results for the binary systems formed by carbon tetrachloride, chloroform, and acetonitrile will be found in the following paper.

Acknowledgement Y.P.H. acknowledges the award of a Mellor

Post-Graduate Scholarship.

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