ENTHALPY OF VAPORIZATION OF WATER
The purpose of this lab is to determine the vapor pressure of water at various temperatures, and from these
data to calculate ∆Hvaporization for water.
Procedure
Bring two 600 mL beakers of water to a boil on your hot plate. Fill a tall 1000 mL beaker nearly full with
hot tap water. Fill a small graduated cylinder about 1/2 with tap water, seal it with thumb or finger, and
invert it into the beaker of water. The volume of trapped air should be around 5 mL (if it isn’t, try again).
Use clamps or string to support the inverted cylinder in a vertical position, not touching the beaker.
Suspend a thermometer near the cylinder.
Add boiling water to the large beaker to bring the water to about 75 °C. Make sure the inverted cylinder
is completely submerged. Put the beaker on the stir plate and stir gently as you read and record the
temperature and trapped air volume (bottom of the meniscus) about every 5 °C until the water has cooled
to about 50 °C.
Now you need to add cold water and eventually ice to bring the temperature down to less than 5 °C,
without disturbing the trapped air and preferably without flooding the lab. You can move the beaker
(without stirrer!) into the sink and run cold water in it; you can set up a siphon and siphon out warm water
as you add cold water: devise your own clever method for replacing the warm water with cold water
without messing up the trapped air. Bottom line: get the temperature down below 5 °C, and take your
final reading of temperature and trapped air volume. Read and record barometric pressure.
Analysis and Discussion
If we assume that the vapor pressure of water is negligible at <5 °C, then at that temperature the trapped
bubble is all air, and the moles of air can be calculated from ideal gas law (assuming Ptrapped air = Pbarometric).
Watch the units of R.
Now use ideal gas law to calculate Pair for each temperature in the 50 – 75 °C range, using n = nair. The
difference between Pbarometric and Pair should be the water vapor pressure at that temperature.
The Clausius-Clapeyron equation can be expressed as
ln P = - H(1/T) + B ln P
R
where P is vapor pressure, ∆H is heat of vaporization, and B is a constant. This equation has the form of a
straight line. Calculate ln Pwater vapor and 1/T (Kelvins, of course) for each reading between 50 and 75 °C,
and construct a plot of ln P vs. 1/T (be careful to put the variables on the correct axes, so the slope
corresponds to –∆H/R). From the slope, calculate the value of ∆Hvaporization. Be careful to use the correct
value of R (in Joules), and convert the ∆H value to kJ/mol.
In your journal, you can make a plot (not a sloppy draft) to represent the data, then use your graphing
calculator to determine an accurate slope value from which to calculate ∆Hvaporization.
The accepted value for ∆Hvaporization for water in the temperature range of this experiment is about 43
kJ/mol. Calculate your % error and discuss your results.