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.
