**Purposes**

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.

**Principle**

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,

**Is the pressure,**

*p***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:

then

or

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.

**Apparatus**

1. An isoteniscope

2. A vacuum pump

**Vacuum pump (right) and desiccators (middle)**

**Chemicals**

1. Absolute Ethanol (A.R.)

**Procedure**

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**

**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**

**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

**Reference**

[1] 傅献彩, 沈文霞, 姚天扬. 物理化学, 上册欧4 版. 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. 58

^{th}ed. Ohio: CRC Press, 1977.
[4] 朱文涛. 物理化学. 北京：清华大学出版社，1995.