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Friday, September 20, 2013

Isoteniscope-The Determination of the Saturated Vapor Pressure of Liquid-Experiments in Physical Chemistry


1. Use the Clausius-Clapeyron equation to figure out the average molar enthalpy of vaporization of a liquid inside the measured temperatures.
2. To master measuring saturated vapor pressure of liquid.


    Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid.

The vapor pressure of any substance increases non-linearly with temperature according to the Clausius–Clapeyron relation:

Where R is gas constant, T is the temperature, p Is the pressure, Δvap Hm is the enthalpy of vaporization. 
When the change of the temperature is small, we can assume the enthalpy of vaporization is independent of temperature, this equation can be integrated as follows:


where A is Δvap Hm/R , B is a integral constant.

According to the equation, we can get a line if we make (ln p)-(1/T) figure, and the slope “m” should be –A = -Δvap Hm/R. And then we get: 

And the boiling point can be figured out by the figure. 


1. An isoteniscope

2. A vacuum pump

Vacuum pump (right) and desiccators (middle) 
3. A barometer


1. Absolute Ethanol (A.R.)


1. Stet up the apparatus as below:

2. Leak detection:
Turn off the H valve, and open the I, F, G valves. Turn on the vacuum pump until the pressure reaches 25~30 kPa and then turn off the I, F, G valves, and then turn off the pump. Wait for a minutes and check the value of the barometer. 

3. Heating:
Open the stirrer and adjust the voltage of the heater to about 160V.

An autotransformer

4. Degas:
When the temperature of the hot water bath is over 50℃, the ethanol inside the isoteniscope starts bubbling. Keep the temperature at about 52℃ for 5 minutes by adjusting the voltage of the heater.

5. The measurement of vapor pressure:
Open the H valve slowly let air defuse through the capillary, until the BC phase is matched and is stable at least 1 minute. Record the temperature of the hot water bath and the pressure of the barometer. Keep heating the water bath and do the same things 6~10 times after raise about 2 degrees every times.

6. Finish:
Turn on all the valves, turn off all the electronics, and clean up the table.

Report Sheet

Data Processing

Fitting with an exponential function, y = a*b^x , on the P-T figure.

Figure 1

Linear fitting on the ln(Pa)-1/T figure
Figure 2
From this fitting curve, we can get the enthalpy of vaporization by using the equation as follow: 

Δvap Hm = - (8.3144621 J / (K mol) ) x (-5172.43 K)
       = 43.0 kJ/mol 
We can also get the normal boiling point from the fitting curve.
y = 26.29912 - 5172.43*x
ln(101325 Pa) = 26.29912 – 5172.43x 
 x = 1/T
=>  T = 1/(((ln 101325) – 26.29912) /(.5172.43))
      = 350.12 K = 76.97 ℃

To compare with the literature datas

[1]  傅献彩, 沈文霞, 姚天扬. 物理化学, 上册欧4 . 北京:高等教育出版社, 1990:144.
[2]  清华大学化学系物理化学实验编写组. 物理化学实验. 北京:清华大学出版社, 1991.
[3]  Robert C. Wcast Handbook of Chemistry and Physics. Physics. 58th ed. Ohio: CRC Press, 1977.
[4]  朱文涛. 物理化学. 北京:清华大学出版社,1995.

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