Binary SolidâLiquid Phase Diagram of Phenol and t-Butanol: An

Laboratory Experiment pubs.acs.org/jchemeduc

Binary Solid−Liquid Phase Diagram of Phenol and t‑Butanol: An Undergraduate Physical Chemistry Experiment Xinhua Xu,* Xiaogang Wang,* and Meifen Wu Department of Chemistry, Tongji University, 1239 Siping Rd., Shanghai 200092, People’s Republic of China S Supporting Information *

ABSTRACT: The determination of the solid−liquid phase diagram of a binary system is always used as an experiment in the undergraduate physical chemistry laboratory courses. However, most phase diagrams investigated in the lab are simple eutectic ones, despite the fact that complex binary solid− liquid phase diagrams are more common. In this article, the cooling curves of phenol−t-butanol mixtures are measured and the phase diagram of this system is found to give three eutectic points and two congruent melting points. The laboratory provides the students a practical way to investigate a complex system with their own efforts and teamwork spirit.

KEYWORDS: Upper-Division Undergraduate, Physical Chemistry, Laboratory Instruction, Collaborative/Cooperative Learning, Hands-On Learning/Manipulatives, Alcohols, Phases/Phase Transitions/Diagrams, Phenols, Thermal Analysis, Thermodynamics

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M

ost of binary solid−liquid phase diagrams investigated in the undergraduate physical chemistry laboratory courses are simple eutectic diagrams (i.e., with one single eutectic point). In addition to binary alloy systems, binary systems of organic compounds have been described, such as phenol−ptoluidine,1 triphenylmethane−stilbene,2 naphthalene−biphenyl,3,4 and naphthalene−p-dichlorobenzene.5 The eutectic temperatures of a variety of binary polycyclic aromatic compounds,6 aromatic compounds7 and carboxylic acids8 have also been studied. However, the complex binary phase diagrams with congruent or incongruent melting points, which are more common than the simple eutectic ones, are still seldom observed in laboratory teaching. A complex binary phase diagram of 2,4,7-trinitrofluorenone (TNF)−trans-stilbene is presented by Williams et al.,9 in which the formation of congruently melting addition compound is observed with the microscope by means of mixed fusion. We sought to find a suitable experiment of complex phase diagram for the undergraduate laboratory course. Atkins10 mentions that t-butanol gives a complex freezingpoint curve with phenol, showing two maxima and three minima. Two compounds, P2A and PA3 (P: phenol, A: tbutanol), are formed. The data of composition−freezing-point temperature of this binary system, measured in 1894 and 1897 by Paterno et al., can be consulted.11 However, the two sets of data are not consistent with each other. A phase diagram of phenol−t-butanol with two compounds P2A and PA2 is also given by Macdougall.12 The uncertainties in the literature are presented as a challenge for the students to investigate this phase diagram by themselves. © 2014 American Chemical Society and Division of Chemical Education, Inc.

EXPERIMENTAL OVERVIEW

This experiment was one of the experiments in a comprehensive chemistry laboratory course during a 17-week semester (see Supporting Information) and was designed for 8 h lab periods. Because there are two congruent melting points in this phase diagram, the time requirement of this experiment was increased by three times in comparison with the experiment of simple eutectic diagrams. In order to finish the experiment in the scheduled time, division as well as cooperation among the students was needed. Each time the experiment was performed, ten students in each of five laboratory groups, working in pairs, were organized into a research team. All the students in a team cooperated in measuring the experimental data and worked out the phase diagram together. Fifty students participated in this experiment in a semester. Therefore, five complete phase diagrams of phenol−t-butanol system were obtained by the five teams. The results among the research teams were compared and discussed by the students. The laboratory experience and lessons about thermal analysis were shared within them. In comparison with the verification experiment of a definite phase diagram, the students’ enthusiasm was stimulated by exploring a phase diagram without definite literature data. Published: May 9, 2014 929

dx.doi.org/10.1021/ed400598s | J. Chem. Educ. 2014, 91, 929−933

Journal of Chemical Education

Laboratory Experiment

Table 1. Properties of Compounds Used in This Work

a

Compound

Molar Massa M (g·mol−1)

Melting Pointa T* (°C)

Enthalpy of Fusiona ΔfusH(kJ·mol−1)

Entropy of Fusiona,b ΔfusS (J·K−1·mol−1)

t-Butanol Phenol

74.121 94.111

25.69 40.89

6.70 11.51

22.42 36.65

Data are from ref 13. bCalculated from the data of ΔfusH and by the equation ΔfusG = ΔfusH − TΔfusS = 0

Figure 1. Device for experiment of freezing-point depression (A) and schematic drawing of experimental assembly (B).

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EXPERIMENTS

The inner tube and its attachments (stirrer, thermometer, spacer, etc.) can be taken out and washed clean after finishing the lab. These components of the apparatus can be easily assembled by the students in the lab. In general, the outer jacket tube fixed in the commercial instrument and the corresponding circulating system of coolant will remain undisturbed, although they can also be easily disassembled and reassembled by the students (see the Supporting Information).

Chemicals

Phenol and t-butanol were purchased from Sinopharm Chemical Reagent Co., Ltd. in analytical reagent grade and used without further purification. Apparatus

The range of temperature measured for the cooling curves of the mixtures is 0 to 50 °C as per the properties of the pure components used in this work (Table 1).13 Therefore, one apparatus for the experiment of freezing-point depression (shown in Figure 1A and B) can be used. The device consisted of two cylindrical glass tubes arranged concentrically and immersed in a stainless steel Dewar containing a coolant. To avoid the occurrence of a great extent of supercooling arising from the obvious difference of temperature between the samples and coolant, the apparatus was modified by replacing the outer tube and cold bath in Dewar with a double-layer jacket glass tube connected to the circulating coolant from a cryostat. Thus, the coolant temperature could be conveniently monitored close to the sample temperature in the process of cooling. The jacket tube was fitted into the apparatus by a holder on the top plate; thus, the functions of the apparatus (sample stirring, temperature measuring, and data transmitting) were maintained. A ground glass joint centered the inner tube (sample tube) in the outer jacket tube, and a Teflon spacer centered a sheathed Pt-100 resistive thermometer (Class B) in the inner tube (Figure 1B). The data of the sample temperatures were acquired by computer and the temperature−time curves were obtained simultaneously.

Procedure

An equivalent volume mixture of ethylene glycol and water was used as the coolant. The coolant, at a constant initial temperature (about 20 °C or higher), was circulated through the outer jacket tube from the cryostat. A total sample mass of 25−30 g was weighed into the sample tube and the Teflon spacer with a stirrer was mounted on the upper end of the tube. The solid components were liquefied by heating the tube in a water bath at 45 °C in a fume hood. The sample tube was then fitted to the outer jacket tube in the apparatus and the sheathed Pt-100 resistive thermometer was inserted into the sample. The sample was stirred and cooled down to about 20 °C, at which temperature most samples of different compositions could be maintained in liquid state. For the samples of mass percent phenol below 5% or above 90%, the initial temperature of the coolant should be higher than 20 °C. Then the temperature setting of the cryostat was turned down to −5 °C, the coolant was cooled down gradually and dropped the temperature of the sample simultaneously. The temperature of coolant was always about 5−6 °C lower than that of the sample; thus, only a small extent of supercooling was observed for most of the samples. 930




The cooling curves of the binary mixtures with various compositions were recorded by the computer. Transition temperatures of the samples were determined by observation of the changes in the slope of the plot of temperature versus time, as described in laboratory textbooks.1−3

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HAZARDS Although t-butanol is less hazardous than most chemicals, phenol and its vapors are highly irritating to the skin, eyes, and mucous membranes in humans. Students must wear protective gloves, clothing, and eyewear in the lab. An essentially closed sample vessel was used to minimize the release of gaseous organic materials into the lab environment. Heating the sample above 35 °C must be conducted in a fume hood. Recent material safety data sheets (MSDS) should be consulted. All used chemicals must be collected in labeled waste containers.

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Figure 2. Solid−liquid phase diagram of phenol−t-butanol.

RESULTS About 100 students in ten research teams have carried out this lab during the spring semesters of 2012 and 2013. The experimental results from the transition temperatures of phenol−t-butanol system measured by one of the ten teams are presented in Table 2 and plotted in Figure 2. There are three minima and two maxima on the liquidus curves, as reported in the literature. These data are representative of the other teams, and the deviations of the results observed among the teams are reasonable and acceptable.

The data of the initial slopes at each of t-butanol and phenol ends of the diagram follow the equation for the temperature variation of the solubility of a pure solid in an ideal solution 1 1 R = − ln x T T* ΔfusH (1) where T* is the freezing point of pure A, T is the temperature at which pure solid A is in equilibrium with solution, x is the mole fraction of A, and ΔfusH is the enthalpy of fusion for A. The entropy of fusion can be obtained by the equation

Table 2. Experimental Data of Mole Fraction, Mass Percent of Phenol, Freezing-Point Temperature (Tf), and Eutectic Temperature (Te) of Phenol−t-Butanol System

ΔfusG = ΔfusH − T A*ΔfusS = 0.

(2)

Using the data in Table 2, the linear relation between ln x and T−1 is followed approximately for t-butanol and phenol (Figure 3). It is found for t-butanol (A) 1 = 0.00335 − 0.00173ln xA (3) T and for phenol (P) 1 = 0.00319 − 0.00117ln x P (4) T where xA and xP are the mole fractions of t-butanol and phenol, respectively. The enthalpies and entropies of fusion, as well as

a

Calculated with the data of mass percent phenol in Table 2 and molar mass of the compounds in Table 1. bThe data used for calculation of enthalpy and entropy of fusion are in gray background shading.

Figure 3. Linear fitting between ln x and T−1 for t-butanol and phenol. 931




Table 3. Properties of Pure Components Calculated with the Experimental Data Compound

Melting Point T* (°C)

Enthalpy of Fusion ΔfusH (kJ·mol−1)

Entropy of Fusion ΔfusS (J·K−1·mol−1)

t-Butanol Phenol

25.4 40.3

4.8 7.1

16.1 22.6

here is a good undergraduate lab for the students to acquire personal experience with a complicated solid−liquid phase transformation.

the freezing points of pure components, are calculated by eqs 1 and 2 and listed in Table 3. The properties of the eutectics and the compounds with a congruent melting point read from the phase diagram are listed in Table 4. The compositions of the eutectics and the

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CONCLUSIONS An experiment is presented to determine a complex phase diagram with three eutectic points and two congruent melting points for undergraduate physical chemistry laboratory course. The binary solid−liquid system of phenol−t-butanol is investigated with a modified common instrument in physical chemistry lab. The materials needed are readily available, inexpensive, and relatively safe. We believe that this experiment provides a suitable practice illustrating the complexity of heterogeneous equilibrium, and the undergraduate students benefit from the cooperative involvement in this laboratory.

Table 4. Properties of the Eutectics and the Compounds with a Congruent Melting Point Compounds with a Congruent Melting Point

Eutectics Property

I

II

III

I

II

Mass Percent Phenol Mole Fraction Phenol Eutectic Temperature Te (°C) Congruent melting point Temperature Tc (°C)

17.0 0.139 4.9

60.0 0.542 4.0

80.1 0.760 13.0

38.7 0.332

72.1 0.671

17.1

15.5

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ASSOCIATED CONTENT

S Supporting Information *

Timing of the experiment, student handout, typical cooling curves of phenol−t-butanol mixtures, coolant materials, hazards and disposal. This material is available via the Internet at http://pubs.acs.org.

compounds determined in this work are in agreement with the literature.11,12 But the deviations of the corresponding temperatures between measured data and literature are observed. The molecular representations of the compounds with a congruent melting point can be calculated as follows: Compound I: PA2 or (C6H5)OH·2(CH3)3COH; phenol:tbutanol = 0.332:0.668 = 1:2.01 ≈ 1:2. Compound II: P2A or 2(C6H5)OH·(CH3)3COH; phenol:tbutanol = 0.671:0.329 = 2.04:1 ≈ 2:1. The formation of compound PA3, as mentioned by Atkins,10 is not observed in this work. We think that it might be a mistake by Atkins, as he draws the conclusion from the data measured by Paterno et al., in which it can be observed clearly that the forms of the compounds are PA2 and P2A.11,12 Besides these, the phase diagrams of t-butanol with a series of phenol derivatives are determined by Kremann and Wlk,14 which show that only PA2 and P2A compounds are formed and no PA3 observed. The extensive association between phenol and tbutanol is also discussed by Bigelow.15 Only if the students made great effort to extract the original literature, would they have clear knowledge of the compounds before the lab. Therefore, it was impressive and exciting for the students when they identified the congruent melting points on the liquidus curves and calculated the molecular representation of the compounds. The compounds formed with the simple integer ratios of the two pure components aroused a sense of beauty in the students’ mind. Understanding of phase diagrams is critical in material science. Unfortunately, phase diagrams are often shortchanged in a physical chemistry course.16 Experiments of binary solid− liquid phase diagrams are found in numerous laboratory outlines, textbooks, and teaching papers, but only numbered ones involve the complex phase diagrams applied in undergraduate lab curricula.1,9,17 A lot of organic solid−liquid phase diagrams with congruent/incongruent melting points have been investigated,18 but most of them are not suitable for undergraduate lab teaching. Thus, the experiment described

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AUTHOR INFORMATION

Corresponding Authors

*X. Xu. E-mail: [email protected]. *X. Wang. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge the contribution of the applied chemistry student of 2009 and 2010 in conducting this experiment. The SWC-LGD devices for the freezing-point depression were manufactured by Nanjing Sangli Electronic Equipment Factory and modified with the help from them. This work was supported by a grant from the Key Project Foundation of Shanghai Municipal Undergraduate Teaching Reform (No. 2013TJ04) and recognized as one of the fifth batch of excellent experiment development items by Department of Facility and Laboratory Management of Tongji University (No. 1380104064).

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REFERENCES

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(5) Blanchette, P. P. The Binary Liquid-Solid Phase Diagram of Naphthalene-p-Dichlorobenzene: An Physical Chemistry Laboratory Experiment. J. Chem. Educ. 1987, 64, 267−268. (6) Fu, J.; Rice, J. W.; Suuberg, E. M. Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures. In Advances in Crystallization Processes; Mastai Y., Ed.; InTech: Rijeka, Croatia, 2012; http://www.intechopen.com/books/advances-incrystallization-processes/phase-behavior-and-crystal-structure-ofpolycyclic-aromatic-compound-mixtures (accessed Apr 2014). (7) Gallus, J.; Lin, Q.; Zumbühl, A.; Friess, S. D.; Hartmann, R.; Meister, E. C. Binary Solid-Liquid Phase Diagrams of Selected Organic Compounds. A Complete Listing of 15 Binary Phase Diagrams. J. Chem. Educ. 2001, 78, 961−964. (8) Sharma, S.; Kumar, A.; Sinhg, V. Dependence of EutecticComposition on Densities of Components of Binary Eutectic Mixtures of Solid Carboxylic Acids. IOSR J. Appl. Chem. 2012, 3, 18−22. (9) Williams, K. R.; Collins, S. E. The Solid-Liquid Phase Diagram Experiment-Updated for the Physical Chemistry Laboratory. J. Chem. Educ. 1994, 71, 617−620. (10) Atkins, W. R. G. II.- Cryoscopic, Ebullioscopic, and Association Constants of Trimethylcarbinol. J. Chem. Soc., Trans. 1911, 99, 10−23. (11) Timmermans, J. The Physico-Chemical Constants of Binary Systems in Concentrated Solutions; Interscience Publishers, Inc.: New York, 1959; Vol. 2, pp 1022. Data sheets from Paterno are reproduced in Timmermans. (12) Macdougall, F. H. Thermodynamics and Chemistry; John Wiley and Sons, Inc.: New York, 1921; pp 229−231. (13) CRC Handbook of Chemistry and Physics, 86th ed.; Lide, D. R., Eds.; CRC Press: Boca Raton: FL, 2005; pp 3-406, 3-460, 6-134, 6135. (14) Kremann, R.; Wlk, O. Ü ber den Einfluβ von Substitution in den Komponenten binärer Lösungsgleichgewichte. XXI. Mitteilung. Die binären Systeme von Trimethylearbinol mit Phenolen, beziehungsweise Aminen. Monatsh. Chem. Verw. Teile Anderer Wiss. 1919, 40, 205− 236. (15) Bigelow, M. J. Tert-Butyl Alcohol as a Solvent for Cryoscopic Measurements. J. Chem. Educ. 1968, 45, 108. (16) Gerald, R. V. H. What to Teach in Physical Chemistry: Is There a Single Answer? In Advances in Teaching Physical Chemistry; Mark, D. E., Tracy, A. S., Eds.; American Chemical Society: Washington, DC, 2008; pp 11−27. (17) Walter, W. L.; Robert, P. K.; John, G. M. The Solid-Liquid Phase Equilibria of Two Component Systems. J. Chem. Educ. 1944, 21, 454−459. (18) Ott, J. B.; Goates, J. R. Summary of Melting and Transition Temperatures of Pure Substances and Congruent and Incongruent Melting Temperatures of Molecular Addition Compounds. J. Chem. Eng. Data 1996, 41, 669−677.

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