Lab 9: Enthalpy Vaporization of Water
Modified from Dr. Watson (Maine-Endwell High School)
Investigator:
Date:
PURPOSE
To determine the enthalpy of vaporization of water using the Clausius-Clapeyron method.
INTRODUCTION
This experiment is designed to find the vapor pressure of water at temperatures between 50°C and
80°C. A graph of the natural logarithm of vapor pressure versus the reciprocal of absolute temperature
allows the calculation of the enthalpy of vaporization.
A sample of air is trapped in an inverted graduated cylinder that is immersed in a tall beaker of
water. As the water in the beaker is heated to about 80°C, the air in the graduated cylinder expands and
becomes saturated with water vapor. Temperature and volume are recorded. The total air and water vapor
pressure inside the cylinder is equal to the barometric pressure plus a small correction for the pressure
exerted by the depth of the water above the trapped air. As the water in the beaker is allowed to cool, the
volume of air contracts and less water vapor is present at the lower temperature. The temperature and
volume are recorded every 5°C between 80°C and 50°C. Next, the beaker is cooled with ice to a
temperature close to 0°C. At this temperature the vapor pressure of water is so low that it can be assumed
that all of the gas in the graduated cylinder is air.
The moles of air molecules in the cylinder can be found by using the volume of dry air present at
the temperature near 0°C and the ideal gas equation. Knowing the moles of air in the container, the partial
pressure of air can be calculated at each temperature, and the vapor pressure of water can be obtained by
subtracting the pressure of air from the total pressure inside the cylinder.
The Clausius-Clapeyron equation is a mathematical expression relating the variation of vapor
pressure to the temperature of a liquid. It can be written:
ln P = (-Hvap/RT) + C
where ln P is the natural logarithm of the water vapor pressure, Hvap is the enthalpy of vaporization of
water, R is the gas constant (8.314 J/mol K), T is the temperature in Kelvin, and C is a constant which
does not need to be evaluated. It can be seen that this equation fits the straight line equation y = mx + b. If
a graph is made of ln P versus 1/T, the heat of vaporization can be calculated from the slope of the line.
Lab 9: Enthalpy Vaporization of Water
Modified from Dr. Watson (Maine-Endwell High School)
Investigator:
Date:
thermometer
1 L beaker
h
water
inverted 10 mL graduated cylinder
air
water
Figure 1. Diagram of Apparatus
PROCEDURE
1. Refer to Figure 1. Fill a 10-mL graduated cylinder about two-thirds full of water. Close the top
with your finger and quickly invert and lower the cylinder in a 2 L beaker half filled with water.
Add water to the beaker, until the water level extends above the cylinder
2. Heat the assembly with a Bunsen burner until the temperature is about 80°C. The air inside the
cylinder should not expand beyond the scale on the cylinder. If it does, remove the cylinder (use
tongs) and start again with a smaller initial volume of air. Record the temperature, the volume of
air (to 0.01 mL) in the cylinder, and difference in height between the top of the water in the
beaker and the top of the water in the cylinder, h, in mm (use a ruler). Be sure to continuously stir
the water in the beaker to ensure an even distribution of heat.
3. Cool the beaker (continue stirring) until the temperature reaches 75°C, and wait several minutes
for the temperature to equilibrate. Again, record the temperature, volume of gas in the cylinder,
and ‘h’. You may add some ice or ice water to the beaker to speed up the cooling slightly, but try
to keep the volume of water in the beaker about the same by removing as much volume as you
add.
4. Cool in 5°C increments to 50°C, recording temperature, volume of gas in the cylinder, and ‘h’ at
each temperature.
5. After the 50°C readings, cool the beaker rapidly to about 0°C by adding ice. Record the
temperature, volume of gas in the cylinder, and ‘h’.
6. Record the barometric pressure in mmHg.
Lab 9: Enthalpy Vaporization of Water
Modified from Dr. Watson (Maine-Endwell High School)
Investigator:
Date:
ANALYSIS
Note: This analysis is best performed using an “Excel” or similar spreadsheet program to perform and organize the calculations.
1. There is a small error in the measurement of the volume of air caused by using the upside-down
graduated cylinder because the meniscus is reversed. Correct all volume measurements by
subtracting 0.20 mL from each volume reading.
2. For each temperature, 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 flask, h. The pressure inside the cylinder is slightly greater than the
atmospheric pressure. This increased pressure can be calculated by using the measured difference
in water depth, h, and multiplying by the conversion factor that the pressure exerted by 1.00
mmHg is the same as that exerted by 13.6 mmH2O. This factor results from the fact that the
density of mercury is 13.6 times that of water.
Pcylinder = Patmosphere + [h(mmH2O) x (1.00 mmHg/13.6 mmH2O)]
3. Calculate the moles of trapped air, nair, by using the volume of air present near 0°C and the ideal
gas equation. At this low temperature you can assume that the vapor pressure of water is
negligible, so almost no water vapor is present in the cylinder. Be careful with units.
4. For each temperature between 50°C and 80°C, calculate the partial pressure of air in the cylinder,
Pair, again using the ideal gas equation:
Pair = nairRT/V
5. Calculate the vapor pressure of water at each temperature:
Pwater = Pcylinder - Pair
6. Plot ln Pwater on the vertical axis versus 1/T on the horizontal axis. Draw the best fit straight line.
Determine the slope of the line, and calculate the value of Hvap of water using the ClausiusClapeyron equation.
7. Compare your result to the accepted value for the enthalpy of vaporization of water (40.67
kJ/mol).
DISCUSSION
1. What is vapor pressure and why does it change with temperature?
2. What is enthalpy of vaporization?
3. The assumption was made that the vapor pressure of water is negligible at a temperature close to
zero. Find the actual vapor pressure of water at your low temperature and comment on the
validity of the assumption.
Levinson, G. S. J. Chem. Ed., 1982, 59, p 337