P.5. ENTHALPY OF VAPORISATION OF WATER
FROM VAPOUR PRESSURE MEASUREMENT.

Objective

Introduction

	
			   Delta H vap
     		ln p =  -  ------------  + constant
			      R T

In this experiment, a sample of air is trapped over water, in an inverted measuring cylinder in a beaker. When the temperature of the apparatus is changed the number of moles of water vapour in the gas phase will vary according to the Clausius-Clapeyron equation, while that of air will remain constant.
The number of moles of air in the mixture can be found by reducing the temperature of the whole apparatus to about 5 C. At that temperature it can be assumed that the vapour pressure of water is so small that the volume of gas measured corresponds only to the air present.
The enthalpy of vaporisation can then be calculated from a plot of ln p(H2O) (the vapour pressure) versus 1/T.

Apparatus Required:

Procedure

1.   Fill a 10-cm3 graduated cylinder about 80% full with 
     distilled water.  Cover the top with a finger and quickly 
     invert and lower the cylinder into a tall beaker that has 
     been filled with tap water.  An air sample of 3 to 4 cm3 should 
     be trapped within the cylinder, record this volume and the 
     temperature.

2.   Add more water if necessary to the beaker to ensure that the 
     whole cylinder is surrounded by water.  Then heat with a Bunsen 
     burner to approximately 80 C.  During the heating, record the time, the 
     volume and the temperature at every 5 C.

3.   When the volume of trapped gas expands beyond the scale on the 
     cylinder, remove the burner and allow the water to cool slowly.
     When the gas begins to contract and the volume can be read again, 
     record the volume and temperature to the closest 0.1 cm3 and 
     0.5 C respectively.  Stir the water bath frequently to avoid 
     thermal gradients.  As the water cools, make additional T
     measurements at approximately 0.2 cm3 intervals down to 50 C.
     You should be able to record at least 15 readings.
     
4.   After the temperature has reached 50 C, cool the water 
     rapidly to less than 5 C by adding ice.  Record the air volume
     and the water temperature at 10 mins after reaching ca 5 C.  
     By then an equilibrium has been reached again.

5.   Obtain a value of the atmospheric pressure from the Demonstrator.

Calculations

1.   Correct all volume readings by subtracting 0.2 cm3 to compensate 
     for the inverted meniscus.  Using the measured values for volume and 
     temperature from step 4 and the atmospheric pressure, calculate the 
     number of moles n(air) of trapped air.  Assume that the vapour pressure of 
     water is negligible compared to atmospheric pressure at the low 
     temperature.

2.   For each temperature between 80 and 50 C calculate the partial 
     pressure of air in the gas mixture.

					n(air) RT
                          p(air) =    ------------
					    V
                                                                  
3.   Calculate the vapour pressure of water at each temperature:
                                            
                          p (H2O) =   p (atm)  - p (air)
                      
4.   Plot ln p(H2O) versus 1/T and draw the best straight line.
 
5.   Determine delta H(vap) from the slope and p(H2O) at the temperature, X, 
     given in class.  Determine the standard deviation of delta H (vap) by the 
     "box method".

Discussion Questions

(i)   Are there any assumptions other than those already mentioned 
      which are worth considering?

(ii)  Discuss the main sources of errors.

Return to Chemistry, UWI-Mona, Home Page