Vapor Pressure of A Pure Liquid

Contents

• Purposes and Demands • Principle• Apparatus and Reagent • Procedure• Data Records and Processing • Questions• Attentions • Demonstrated

Purposes and Demands Understand vapor pressure of a pure liquid and gas-liquid equilibrium. Acquaint the relationship of vapor pressure and temperature by Clausius-Clapeyron equation.

Determine saturated vapor pressure of ethanol by precision digital pressure gage under different temperature. Master the vacuum technique for determination.

Determine the molar heat of vaporization and boiling point by the method of illustration.

Principle

• When a pure liquid is placed in an evacuated bulb, molecules will leave the liquid phase and enter the gas phase until the pressure of the vapor in the bulb reaches a definite value. This pressure is called the vapor pressure of the liquid at a given temperature.

The equilibrium vapor pressure is independent of the quantity of liquid and vapor present as long as both phases exist in equilibrium with each other at the specified temperature.

Temperature is the main factor to the vapor pressure.

As the temperature increases, the vapor pressure increases up to the critical point.

Then the liquid may be heated up

to a temperature at which the vapor pressure is equal to the external pressure.

At this point, vaporization will occur by the formation of bubbles in the interior of the liquid as well as at the surface.

It is quite the boiling point of the liquid

at the specified external pressure. The heat consumed to vaporize 1 molar liquid is called the gasification latent heat(ΔHv).

Clausius-Clapeyron equation is known as

Where P is vapor pressure of liquid, T stands for absolute temperature and R is gas constant of 8.314J·K － 1·mol － 1.

R

Hm

303.2

气化

We can see that if the vapor are a perfect gas and ΔHv are independent of temperature, then a plot of lgP versus 1/T will be a straight line f which the slope o will determine ΔHv.

Apparatus and Reagent

• Vapor pressure measuring device

• thermostatic bath• Precision digital pressure gage • buffer gas tank• Thermometer

Sucking pump• Ethanol

Sketch-maps of Equipments

DF-AF precision digital pressure gageThe front panel of the device

• 1)Units key: click to choose a measuring unit.

• 2)Zero key: click to deduct pressure at zero level.

• 3)Reset key: click to restart-up.

• 4)Display screen: display pressure data.

5)Indicator light: display the unit of the pressure.

• 1)   Electrical outlet• 2)   Facet contacted with host computer• 3)   Facet contacted with pressure canal• 4)   Pressure-regulating 5) Fuse

Procedure

• 1.Assemble the equipments as shown in the Sketch-maps of equipments above.

• 2.Charging-up ethanol to isoteniscope and the bulb is about two-third full. Startup sucking pump.

• 1.     Check gas tightness:• 1)Do the gas leakage test to the buffer

gas tank: Open the valve 2 and tight the valve2, t

hen startup the sucking pump. When the pressure is up to the range of 100kpa-200kpa, stop sucking and close air inlet.

If the pressure falls down by less than 0.01kpa/s the system has good airtightness, or else must workout the problem before next procedure.

• 2) Check gas tightness of the inching part:

Open the valve 1 when the pump, air inlet and valve 2 are not open. Regulate the pressure of inching part less than the one of the overhead tank by valve 1.

It shows that the inching part has good airtightness if the pressure falls down by less than +0.01kPa/4s. Otherwise resolve the question before your experiment.

• 3) Check gas tightness of the determined part:

Open valve 2 when valve 1 is not open. After pressure in inching part is equal to gas tank, close valve 2. Release pressure moderately.

Observe the pressure reading. It shows that it has good tightness if the pressure change is less than or equal to 0.01kpa/4s. If not, find out and resolve the problem before next step.

• 4.     Determination step Set up the temperature of the water ba

th at 30℃. Start up sucking pump and circumfluence equipment. Open valve 2, carefully admit air to the ballast bulb through the U tube. Then close valve 2 and open valve 1.

After 3-4 min, adjust the two valves until the levels of the liquid in both arms of the U tube are the same. Read and record the temperature and the pressure.

Repeat the same procedure with the pressure discrepancy between two experiments in the range of 67pa.

• Repeating the above procedure, determine difference vapor pressure respectively at temperature of 30ºC ， 35ºC ， 40ºC ， 45ºC ， 50ºC.

• Release pressure in the gas tank and disassemble the equipments. Stop running reflux and cut down the current.

Data Records and Processing

• 1) Records• Room temperature: • Atmospheric pressure:

 tH （ in ）cmHg

t1

30℃

    

   

t2

40℃

    

     

t3

50℃

    

 

Processing

1.     Proofread atmospheric pressure base on the equation

Pp （ 273.15/K ） =P(T)[1-1.63×10-4(t-273.15K)]

2 、 Vapour pressure (Pp) of ethanol at difference temperatures:

t℃

T/K273+t

/K℃

1/TT-1×10-3/K-1

Δh0

MPa

P 乙醇MPa

lgP 乙醇

30℃

           

40℃            

50℃            

Draw a plot of lgP versus 1/T and figure out the slope.m=-ΔHv2.303RlgP=lg101.325=

From the plot we obtain 1/T ＝And boiling point T=

Questions

• 1. What condition is Clausius-Clapeyron equation used in practice?

• 2. What’s the result if air in the U tube has not extracted completely?

• 3. Did the method can be used on determining solution’s vapor pressure?

• 4. What’s the use we make from buffer gas tank?

Attentions

• 1. The system must be good tightness.

• 2. Determination must be after liquid boiling up for 3-4 min.

• 3. Release a little pressure in order to avoid the liquid is boiling too rapidly.

• • 4.Regulate the temperature accuratel

y.

• 5. Release pressure carefully and avoid the liquid in the U tube run to ballast bulb.

• 6. U tube must be under the level of the water bath.

• 7. Examine carefully the two valves 1 and 2, and see if they have leakage.

• 8. Revolve the valves moderately and don’t use too big strength.

•  

• 9. Determine data after pressure reading is steady.

• 10. If air run into the U tube, you need to vacuumize it again before start determination.