Enthalpies of Mixing for Binary Liquid Mixtures of Monocarbonic Acids and Alcohols

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1 Enthalpies of Mixing for Binary Liquid Mixtures of Monocarbonic Acids and Alcohols R. H a a s e and R. L o r e n z Institut für Physikalische Chemie, RheinischWestfälische Technische Hochschule, Aachen, Germany Z. Naturforsch. 40a, (1985); received July 3, 1985 We present the results of calorimetric measurements of the molar enthalpy of mixing (molar excess enthalpy) 77E as a function of temperature and composition (described by the mole fraction.y of the alcohol) for 18 binary liquid systems consisting of an aliphatic monocarbonic acid (formic, acetic, propionic, butyric, valeric acids) and an aliphatic alcohol (methanol, ethanol, 1propanol. 2propanol, 1butanol, 2methyl2propanol). The experiments cover temperatures between and and the whole range of compositions (usually nearly 40 compositions at each temperature). There is a great variety of behaviour as far as the function HE(x) for T = const is concerned. Many systems show endothermic mixing ( / / E > 0), other systems exothermic mixing (HE < 0), again other systems partly endothermic, partly exothermic behaviour. There is one case (acetic acid 2methyl2propanol) where HE(x) changes its sign twice and the molar excess heat capacity exhibits unusually large negative values. Previous calorimetric work in this l a b o r a t o r y concerns the enthalpies of m i x i n g (excess e n t h a l p i e s ) for the binary liquid systems water acetic acid [1,2], f o r m i c acid acetic acid [3,4] as well as f o r m i c acid propionic acid a n d acetic acid I propionic acid [4], We now t u r n to a c o m p r e h e n s i v e study [5] of 18 binary liquid systems a m o n g the 30 possible c o m b i n a t i o n s of the type acid alcohol formed f r o m the c o m p o n e n t s f o r m i c acid, acetic acid, p r o p i o n i c acid, butyric acid, valeric acid (aliphatic m o n o c a r b o n i c acids) a n d m e t h a n o l, ethanol, 1propanol, 2propanol, 1butanol, 2methyl2propanol (aliphatic alcohols) at t e m p e r a t u r e s between and a n d over the whole range of compositions (usually nearly 40 c o m p o s i tions at each t e m p e r a t u r e ). T h e systems investigated are shown in Table 1. O u r calorimeter [5] is an i m p r o v e d construction.of a flowtype calorimeter described earlier [4], based on the principle of c o n t i n u o u s flow a n d of continuous c o m p e n s a t i o n of t e m p e r a t u r e changes by s i m u l t a n e o u s use of a n electric heater a n d a Peltiereffect device. The q u a n t i t y derived f r o m the m e a s u r e m e n t s is the molar enthalpy of m i x i n g ( m o l a r excess enthalpy) HE as a function of the t e m p e r a t u r e T and Table 1. s investigated with respect to molar excess enthalpy HE as a function of the mole fraction.v of the alcohol at temperature T. The column "Type" indicates the type of the HE (.v)diagramm: the signs and refer to positive values of HE (endothermic mixing) and to negative values of HE (exothermic mixing), respectively. T Formic acid 1propanol 2propanol 1butanol 2methyl2propanol Acetic acid methanol ethanol 1propanol 2propanol 1butanol 2methyl2propanol 2methyl2propanol 2methyl2propanol Propionic acid methanol 2propanol 1butanol 2methyl2propanol Butyric acid methanol Valeric acid methanol 2propanol Reprint requests to Prof. Dr. R. Haase, Institut für Physikalische Chemie, RheinischWestfälische TH, Templergraben 59, 5100 Aachen t 1butanol Type and and and and and and and I and and and / 85 / $ 01.30/0. Please order a reprint rather than making your own copy. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der MaxPlanckGesellschaft zur Förderung der Wissenschaften e.v. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License.

2 948 R. Haase and R. Lorenz Binary Liquid Mixtures of Monocarbonic Acids and Alcohols Fig. 1. Molar excess enthalpy H E as function of the mole Fig. 2. Function H E (x) for carbonic acids lbutanol at fraction x of the alcohol for binary liquid mixtures of car. bonic acids and 2propanol at. Table 2. Parameters a k of (1) for the indicated systems (a: standard deviation). T a 0 a, a 2 a 3 a 4 a 5 a 6 Jmol 1 Jmol 1 Jmol 1 Jmol 1 Jmol 1 Jmol 1 J mol 1 J mol" Formic acid 1Propanol Propanol Butanol Methyl2propanol Acetic acid Methanol Methanol Ethanol Propanol Propanol Propanol Butanol Butanol Methyl2propanol Methyl2propanol Methyl2propanol

3 R. Haase and R. Lorenz Binary Liquid Mixtures of Monocarbonic Acids and Alcohols 949 Table 2. (Continued) T «o «2 «3 a4 a 5 "6 er J mol 1 J mol" 1 J mol 1 JmoF 1 JmoF 1 J mol 1 J mol 1 J mol 1 Propionic acid Methanol Methanol Propanol Propanol Butanol Butanol lmethyl2pro panol Butyric acid Methanol Valeric acid Methanol Methanol Propanol Propanol Butanol* Butanol * Use parameters only for.v < 0,8! Fig. 3. Function H E (x). for acetic acid alcohols at Fig. 4. Function H E (.v) for carbonic acids methanol at.

4 950 R. Haase and R. Lorenz Binary Liquid Mixtures of Monocarbonic Acids and Alcohols Cp J mol" 1 " OA Fig. 6. Molar excess heat capacity Cf (x) for acetic acid 2methyl2propanol at (derived graphically from H e, see Figure 5). of the mole fraction x of the alcohol, the pressure P being irrelevant. We fit the experimental data to the power series m H le = x (1 x) X a k (1 2 x)'* = x (1 x) *=0 (1) [fl 0 fli(l2jc)... fl m (l2.vh, where the parameters a k only depend on the temperature. We need up to 7 constants (a 0, a\,..., a 6 ) at each temperature to fit our results (see Table 2, where the standard deviation a is also given). In some cases we may derive the molar excess heat capacity (at constant pressure) m v (2) ' Cp = (S/7 e /6 T)p,, = x (1 X) X b k (1 2 x) k k = 0 with [see (1)] b k = da k /dt (3) from experimental values of the function H E (T, x). Figures 1 to 5 and 6 give examples of measured values of H E and Cf, respectively. In the last case (Cf for acetic acid 2methyl2propanol at ) the data have been fitted to (2) using the parameters [without reference to (3)] b 0 = 64.4 J " 1 mol" 1, b,= 24.7 J" 1 mor', b 2 = 6.2 J"' mol 1, by = 13.3 J _ 1 mol" 1, the standard deviation being 0.2 J " 1 mol 1.

5 951 R. Haase and R. Lorenz Binary Liquid Mixtures of Monocarbonic Acids and Alcohols We now briefly discuss the functions H E (x) and Cp(.\) for given temperatures. In most cases (see Table 1 and Figs. 1 to 5) the process of mixing the liquid components is endothermic (H E > 0) throughout. However, systems containing formic acid show nearly throughout exothermic behaviour (/7 E < 0). In systems consisting of methanol and carbonic acids (the system formic acid I methanol being too exothermic to be susceptible to measurements) the function 77 E (x) is positive for small values of x (endothermic region), passes through zero, and is negative for large values of.v (exothermic region), the last region becoming smaller with higher carbonic acids and disappearing in mixtures with valeric acid. Finally, the system acetic acid 2methyl2propanol exhibits an extraordinary behaviour (see Figures 5 and 6): In a certain range of temperatures, the function H E (x) changes the sign twice, while the function C E (x) has unusually large negative values. [1] R. Haase, P. Steinmetz, and.h. Dücker, Z. Naturforsch. 27 a, 1527 (1972). [2] R. Haase and M. Pehlke, Z. Naturforsch. 32 a, 507 (1977). [3] R. Haase, H.J. Jansen,. Puder, and B. Winter, Z. Naturforsch. 34 a, 659 (1979). [4] R. Haase, H.J. Jansen, and B. Winter, Z. Naturforsch. 38 a, 1400 (1983). [5] R. Lorenz, Dissertation, RheinischWestfälische Technische Hochschule, Aachen 1985.