Phase Diagram of Liquid-Vapor Equilibrium in a Binary System - Experiment of Physical Chemistry - Lu Le Laboratory

Purpose

1.     To construct a phase diagram and find the temperature and composition of the azeotrope.

2.     To study the liquid-vapor equilibrium of a binary system.

Principles

        In an idealized system the boiling point of a mixture of two soluble liquids is always between the boiling points of these two pure components. Figure 1-(a) is a typical ideal system. The upper curve is called the dew point curve representing the changing composition of the vapor phase. The lower curve is known as bubble point  curve representing the composition of liquid phase.

Figure 1.

        A number of homogeneous liquid do not obey Raoult’s lawand as a result the phase diagrams would be different. Large positive deviations from Raoult’s law can result in minimum-boiling azeotrope (Fig. 1-(b)) while large negative deviations can lead to maximum-boiling azeotrope (Fig. 1-(c)). In such systems, there exists a particular composition at which both of the liquid phase and the vapor phase have the same composition. This is called an azeotrope mixture.

        A liquid-vapor phase diagram of a binary system can be constructed by using a reflux apparatus. When a mixture of two soluble liquids is heated to a boiling point, the vapor phase is condensed and trapped in the pocket below the condenser. Under equilibrium conditions, the trapped condensate represents the vapor phase while the liquid remaining in the reservoir represents the liquid phase. The composition of the vapor and the liquid will be determined through the use of a calibration curve of refractive index as a function of compositions. An Abbe refractometer is used in this experiment.

Chemicals

1.     Cyclohexane  (A.R.)

2.     Ethanol 99.9% (A.R.)

Apparatus

1.     Ebullimeter

2.     Abbe refractometer

3.     Pipet and sucker

4.     Thermostatic water bath

Procedure

Drawing the Calibration Curve

1.     Prepare 0%、10%、30%、69.5%、90.0%、96.0%、100%(w/w) ethanol-cyclohexane mixtures with an analytical balance to nearest 0.0001g.

2.     Determine the refractive indexes of these mixtures with an Abbe refractometer and prepare the calibration curve of composition-refractive index.

Determine the Composition of Liquid Phase and Gas Phase

1.     Flow the cooling water into the condenser of the ebulliometer.

2.     Turn the heater on and adjust he voltage to about 20~30V until reflux begins. When the liquid is boiling, control the reflux in the lower portion of the condenser (~2cm).

3.     After the thermometer reading is constant, record the temperature and the the pressure of the atmosphere. Then stop heating the mixture.

4.     Suck drops of liquid phase and gas phase(at the bottom of the condenser) out with pipets. Determine the composition by measuring refractive index and using the calibration curve.

5.     Draw a composition –temperature diagram.

   

Experimental Record

Label	Cyclohexane (g)	Ethanol (g)	Mass Fraction of Cyclohexane (MFC)	nD24.1℃
0%	0.0000	-	0.00000
10%	0.1015	0.9024	10.11057	1.3659
30%	0.3344	0.7231	31.62175	1.3777
69.5%	0.7038	0.2986	70.21149	1.4036
90.0%	0.9130	0.1069	89.51859	1.4188
96.0%	0.9559	0.0492	95.10497	1.4236
100%	-	0.0000	100.00000	1.4270

Mass Fraction of Cyclohexane (MFC)	Tb (℃)	nD24.1℃ of liquid phase	nD24.1℃ of gas phase
0%	78.08	1.3610	1.3614
10%	76.18	1.3627	1.3693
30%	66.83	1.3767	1.3981
69.5%	64.11	1.3966	1.4034
90.0%	64.68	1.4155	1.4056
96.0%	67.58	1.4237	1.4080
100%	79.65	1.4250	1.4250

   

Analysis

(1)  Calibration Curve：

Equation	y = Intercept + B1*x^1 + B2*x^2
Adj. R-Square	0.99985
		Value	Standard Error
Index of refraction	Intercept	1.36116	2.83E-04
Index of refraction	B1	4.68E-04	1.64E-05
Index of refraction	B2	1.95E-06	1.58E-07

Calibration Curve

y = 1.36116 + (4.68E-04) x + (1.95E-06) x2

        

(2) Determination of the Composition of Gas Phase and Liquid Phase：

Mass Fraction of Cyclohexane (MFC)	Tb (℃)	nD24.1℃of liquid phase	MFC of Liquid Phase	nD24.1℃ of gas phase	MFC of Gas Phase
0.0%	78.08	1.3610	-0.3%	1.3614	0.5%
10.0%	76.18	1.3627	3.2%	1.3693	16.3%
30.0%	66.83	1.3767	29.6%	1.3981	62.6%
69.5%	64.11	1.3966	60.5%	1.4034	69.9%
90.0%	64.68	1.4155	85.6%	1.4056	72.8%
96.0%	67.58	1.4237	95.6%	1.4080	76.0%
100.0%	79.65	1.4250	97.1%	1.4250	97.1%

(3) Draw the phase diagram with smooth curve

Azeotrope	64.21 ℃
Percentage Weight of Cyclohexane in the Azeotrope	69.9%

        (4) Data Analysis

Experimental Data
Azeotrope	64.21 ℃
Percentage Weight of Cyclohexane in the Azeotrope	69.9%

Literature
Azeotrope	64.90 ℃
Percentage Weight of Cyclohexane in the Azeotrope	69.5%

Percentage Error
Azeotrope	-1.0%
Percentage Weight of Cyclohexane in the Azeotrope	0.6%

References

[1]  傅獻彩, 沈文霞, 姚天揚. 物理化學, 上冊歐4 版. 北京:高等教育出版社, 1990:144.

[2]  清華大學化學系物理化學實驗編寫組. 物理化學實驗. 北京：清華大學出版社, 1991.

[3]  Robert C. Wcast Handbook of Chemistry and Physics. Physics. 58th ed. Ohio: CRC Press, 1977.

[4]  朱文濤. 物理化學. 北京：清華大學出版社，1995.

[5] Ponton, Jack (September 2001). "Azeotrope Databank" (Queriable database). The Edinburgh Collection of Open Software for Simulation and Education, Edinburgh University. Archived from the original on 24 April 2007. Retrieved 24 March 2007.