Enthalpy of Vaporization of Water
Purpose


To measure the vapor pressure of water at several temperatures.
To determine the molar heat of vaporization of water
Introduction
The enthalpy of vaporization represents the amount of energy required to expand
the volume of a liquid material to that of its gas. The magnitude reflects the strength of
the intermolecular forces.
The Clausius-Clapeyron equation relates the temperature variation of the vapor
pressure of a liquid to it Hvap.
ln P = -Hvap + C
RT
In this experiment the vapor pressure will be measured for a number of different
temperatures. Then ln P will be plotted versus 1/T and since the slope of the resulting
line is equal to -Hvap/R, the enthalpy of vaporization can be calculated.
A known quantity of air is trapped under water and kept at constant pressure. The
air quickly becomes saturated with water vapor. The temperature of the liquid water is
varied and the changes in the volume of the trapped gas mixture are recorded. This
volume change is caused by the thermal expansion of the constant amount of air and by
the variation in the amount of water vapor. From this data, the vapor pressure of water
can be calculated for each temperature.
Procedure
1. Fill a tall 1000 mL beaker with water.
2. Fill a 10 mL graduated cylinder to the 7 or 8 mL mark with water and invert in the
beaker of water. The cylinder should be completely submerged and the volume of
trapped air should be 4 to 5 mL.
3. Use a ruler to measure (in mm) the difference in height between the top of the water
in the beaker and the top of the water in the cylinder.
4. Heat the water to about 80C or until the air has expanded sufficiently to extend
beyond the graduated scale on the cylinder. Turn off the heat source.
5. Gently stir the water in the beaker and watch the air in the cylinder. As soon as the
air level reaches the 10 mL mark, record this volume to the nearest 0.1 mL and the
temperature to the nearest 0.1C.
6. Move the hot beaker to a tray so that the tray will collect any water that spills out.
Keep stirring the hot water, and record temperature and volume readings for each
two- or three-degree drop down to 50C.
7. Finally, rapidly cool the water temperature to under 5C with large quantities of ice.
Record the temperature and air volume at the lowest temperature.
Enthalpy of Vaporization of Water
Calculations
1. The 10 mL cylinder is graduated to give correct volume readings only when right
side up. This is due to the shape of the water meniscus. Subtract 0.2 mL from each
of your readings to correct for this effect.
2. Calculate the total pressure of the gas in the cylinder from the barometric pressure
and the difference in water levels between the top of the water in the beaker and the
top of the water inside the cylinder.
PT = Pbar + difference in levels/13.5
3. Calculate the moles of trapped air by using the volume of air present at the lowest
temperature. At this low temperature, the vapor pressure of water is negligible.
PTV=nairRT
4. For each temperature above the lowest one, calculate the partial pressure of air in
the cylinder, using the constant moles of gas calculated in # 3. PairV=nairRT
5. Calculate the vapor pressure of water at each temperature. Pwater= PT - Pair
6. Plot ln Pwater on the vertical axis versus 1/T on the horizontal axis. Use linear
regression to determine the equation of the line.
7. Calculate the value of Hvap.
8. Calculate the percent error.
Data
Barometric Pressure ___________
Difference in height ___________
Temperature
(C)
Volume
(mL)
Enthalpy of Vaporization of Water
Results
2
3
Corrected
Volume
(mL)
Total pressure in cylinder
Moles of trapped air
Temp
(K)
Pair
(atm)
6
7
8
Pwater
(atm)
1/T
(1/K)
ln Pwater
Regression Eqn
Hvap
Percent error
Questions
1. You have assumed the vapor pressure of the water below 5C to be negligible.
Look up the actual value and tell how its inclusion would have affected your
calculated results.
2.
Write out the long “two-point” form of the Clausius-Clapeyron equation. Why does
the graphical method of analysis give a better value for the enthalpy of vaporization
than does the form of the Clausius-Clapeyron equation using two temperature-vapor
pressure values?