Chemistry 212
Experiment 9
HEAT OF VAPORIZATION OF WATER
LEARNING OBJECTIVES
The learning objectives of this experiment are to

explore the relationship between the temperature and vapor pressure of water.

determine the molar heat of vaporization of H2O.
BACKGROUND
The molar heat of vaporization of water, Hvap , is defined as the amount of heat needed to
convert a mole of liquid water to water vapor at the boiling point and one atmosphere pressure.
It is possible to estimate the heat of vaporization by measuring the vapor pressure of water at
different temperatures using the method described below.
Approximately 2 mL of air is trapped in an inverted graduated cylinder immersed in a water bath.
The water bath is cooled to a temperature close to 0C and then allowed to warm gradually. As
heating takes place, temperature and volume measurements are recorded. The number of moles
of water vapor in the gas phase changes with temperature, but the amount of "trapped" air
remains constant, assuming there is a negligible change in the solubility of air in water as
temperature changes.
If the number of moles of "trapped" air in the inverted cylinder (nair ) is known, the partial
pressure of air (Pair ) can be calculated at each temperature. When this value is subtracted from
the atmospheric pressure, the vapor pressure of the water PH2 O at each temperature will be
obtained.
Equation 1.
PH2 O = Patm  Pair
The number of moles of air, nair , can be found by measuring the volume, temperature, and
pressure of the trapped gas at a temperature near 0C, where the water vapor content is less than
one percent and can be neglected.
Equation 2.
n air =
Patm V1
R T1
The molar heat of vaporization can be estimated by means of the Clapeyron equation.
Equation 3.
P
Patm
T
R
C
P
=
Patm
  H vap 1
+ C
R
T
 H vap 1
+ C
R T
Po
is the vapor pressure of water (atm)
is the reference pressure (atm)
is the absolute temperature (K)
is the gas constant, 8.3145 J mol-1 K-1
is a constant that depends on the particular liquid
ln
where
and
ln
P
= 
The vapor pressure is determined at a series of temperatures. ln(P/Po) is plotted against 1/T to
give a straight line with a slope of -Hvap /R.
SAFETY PRECAUTIONS
Safety goggles must be worn at all times in the lab.
BEFORE PERFORMING THIS EXPERIMENT
You will need a LabWorks program capable of collecting temperature readings and keyboard
values and sending them to the Spreadsheet. See your instructor as to how to proceed.
EXPERIMENTAL PROCEDURE
1. Calibrate your thermistor using an ice/water mixture for the lower temperature
calibration point and hot water (approximately 80 oC) for the upper temperature
calibration point.
2. Design a program to read volume entries from the keyboard and temperature.
Line 2. Read keyboard label as Volume
Line 3. Read thermister1 label as Temperature
Line 4.
Line 5. Delay 0 sec
3. Go to Acquire and test the program.
4. Using two rubber bands, fasten the thermistor to the 10 mL graduated cylinder as shown
in Figure 1. Leave about 1.5 cm between the tip of the thermistor and the opening of the
tube. Make sure the rubber bands are not placed between the 1.6 and 7.0 mL markings so
as to not obscure any volume readings.
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Figure 1.
5. Place about 300 mL of crushed ice in the large beaker and then fill it up with distilled
water to about 500 mL. Stir the slush mixture until the temperature has dropped to
approximately 0C and then remove any excess ice. Place the beaker on the hot plate but
do not turn the heat on yet!
6. Fill the graduated tube with distilled water until it is about 2 cm from the top. Cover the
top with your finger and invert to see if the air space is 2.2 mL. Adjust by adding or
taking out some water until the space is about 2.2 mL.
7. Cover the top with your finger and invert the tube. Submerge the filled graduated cylinder
in the large beaker filled with ice water and remove your finger. An air sample of about 2
mL should be trapped. Clamp the thermistor to the ring stand, making sure the tube is
submerged and resting on the bottom of the beaker. Add more water to the beaker, if
necessary, until the bath water level is more than 1 cm above the inverted tube. This is
done to ensure that the trapped air in the tube is surrounded by water.
8. Turn on the stir bar and stir gently. Take a volume reading of the trapped air when the
temperature is near 0C. If the temperature is not near 0C, add more ice to bring the
temperature down. Make sure you are reading the bottom of the water meniscus.
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9. Turn hot plate on low and warm water gradually. Record temperature and air volume
measurements often (Every 0.1 to 0.2 mL). Continue until temperature reaches
approximately 85 oC.
10. Obtain the barometric pressure reading (Patm) and record on the data sheet.
DATA ANALYSIS
1.
Use Equation (2) to calculate the number of moles of "water less" air (nair). The
temperature and volume readings needed are those obtained for the first data point (V1 and
T1); R = 0.08206 L atm mol-1 K-1.
Note: for each volume reading, one must subtract 0.15 mL to correct for the inverted
meniscus.
2.
In the Spreadsheet, calculate PH2 O , according to Equation (1). Note that Pair depends on
temperature, and is determined by means of Equation (2). The formula used for this
calculation, PH2O = Patm - Pair , must include the corrected volume in liters and temperature
in Kelvin.
Note: Bold items below are constants that need to be typed into the formulas
Column
A
B
C
D
E
F
G
H
Formula
=B1+273.15
=(A1-0.15)/1000
= Patm – (nair*0.08206*C1/D1)
=E1/Patm
=ln(F1)
=1/C1
Label
Volume
Temperature
Kelvin
Volume Corrected
PH2O
PH2O over Patm
Ln of PH2O over Patm
Inverse K
Units
mL
o
C
K
L
atm
none
none
K-1
3.
Plot a linear regression graph of ln ( PH2 O /Patm) (column G) versus 1/T (column H) .
4.
From the slope of your line determine Hvap for H2O. (slope = -Hvap/(8.314 J K-1 mol-1))
5.
Collect class data for the experimentally determined value of the heat of vaporization of
water. The literature value for Hvap for H2O = 37 kJ/mole.
6.
Write a laboratory report according to the general form of the last several reports. (Purpose,
Results, Conclusion and graph from which Hvap was determined). See laptop formal lab
report format on web.
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Data Sheet
HEAT OF VAPORIZATION OF WATER
Barometric pressure reading (Patm)
atm
Lowest temperature reading (T1)
K
Initial volume at lowest temp (V1)
L
Moles of “water less” air (see Eq. 2) _______________
Hvap for water (slope = -Hvap/R)
_______________
% Error________________
Average Hvap for class
_______________
% Error________________
Calculations
Show calculation for Hvap of water.
Questions/Problems
1.
Explain why the volume of air in the graduated cylinder is measured at 0C rather than at a
higher temperature.
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