Name:_______________________________
Partner:_____________________________
JHS Regents Chemistry Laboratory
Vapor Pressure & Evaporative Cooling
Problem: How does vapor pressure effect evaporation?
Introduction:
When a liquid evaporates, the distance between its particles
increases tremendously, causing a significant increase in
their potential energy. The energy needed to evaporate 1 g of
a liquid, known as the heat of vaporization, is much greater
than the energy needed to cause a 1°C rise in the
temperature of a liquid. When a liquid evaporates, it takes
the energy needed for the process from its surroundings.
This is why sweating cools you and you may feel chilled
when you step from the shower. Some liquids cool your skin even faster than water. Did
you ever notice how cold it feels when the doctor cleans your skin with alcohol prior to
giving an (ouch!) injection? In this laboratory investigation, you will explore this
phenomenon.
Procedure:
1. Using the setup on your lab station, measure the temperature in
the room. Record the result in the data table on the next page.
2. Obtain three labeled beakers, each containing one of the following
liquids: propanone (C3H6O); ethanol (C2H5OH); and water.
3. Gently raise the beaker to the appropriate thermometer (match
the color tape) and allow the cotton to absorb the liquid.
7. Watch the temperatures on each of the thermometers for several
minutes (maximum 5 minutes). As the liquid evaporates, the
temperature will begin to drop.
8. Make note of the lowest temperature each thermometer reaches, and record it in the
data table on the data table.
9. Calculate the absolute value of the temperature change for each liquid. Record the
information on the data table.
10. Determine the normal boiling point of each liquid by looking at Table H, Vapor
Pressure of Four Liquids, on your reference tables. Record the boiling points in your data
table .
11. Prepare a graph of boiling point versus temperature change. Use the x-axis for boiling
point and the y-axis for temperature change. Plot the points and draw the best straight line
or curve through the points.
Data Table: Room Temperature: ______________0C
Liquid
Lowest Temp.
Reached
Temperature
Change
Normal Boiling
Point
Propanone
Ethanol
Water
Title: ____________________________________________________________
*Label your x-axis & y-axis
Understanding Vapor Pressure in Terms of Intermolecular Forces of
Attraction (IMF)
LIQUID
Propanone
Ethanol
Water
Ethanoic
Acid
STRUCTURE
IMF
NORMAL BOILING PT
Analysis Questions:
1. Using the three liquids you tested; explain the differences in temperature changes (data
table) and their normal boiling points in terms of intermolecular forces of attractions
(IMFs)
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2. Why does the temperature drop when the thermometer covered with wet cotton square
is left to dry?
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3. Which of the three liquids tested had the highest vapor pressure at room temperature?
___________________
Which had the lowest vapor pressure? ______________
How did you determine your choices?
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4. Describe the relationship between the normal boiling point of a liquid and how well it
cools by evaporation?
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5. Predict the lowest temperature a thermometer wet with isopropyl alcohol would reach if
its boiling point is 82.4°C. _____________________
Conclusion Questions:
1. At which temperature is the vapor pressure of ethanol equal to the vapor pressure of
propanone at 35°C?
(1) 35°C
(2) 60.°C
(3) 82°C
(4) 9
2. Which substance has the lowest vapor pressure at 75°C?
(1) water (2) ethanoic acid (3) propanone (4) ethanol
3. Which physical changes are endothermic?
(1) melting and freezing
(3) condensation and sublimation
(2) melting and evaporating
(4) condensation and deposition
4. The compound CO2(s) sublimes readily at 25°C. Which properties are usually associated
with a compound that undergoes this kind of change?
(1) high vapor pressure and high intermolecular attractions
(2) high vapor pressure and low intermolecular attractions
(3) low vapor pressure and high intermolecular attractions
(4) low vapor pressure and low intermolecular attractions
5. Ethane, C2H6, has a boiling point of -89°C at standard pressure. Ethanol, C2H5OH,
has a much higher boiling point than ethane at standard pressure. At STP, ethane is a gas
and ethanol is a liquid.
a. A liquid boils when the vapor pressure of the liquid equals the atmospheric pressure on the
surface of the liquid. Based on Table H, what is the boiling point of ethanol at standard
pressure? _____________________
b. Compare the intermolecular forces of the two substances at STP.
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6. At 50.°C and standard pressure, intermolecular forces of attraction are strongest in a
sample of
(1) ethanoic acid
(2) ethanol
(3) propanone
(4) water
7. A liquid’s boiling point is the temperature at which its vapor pressure is equal to the
atmospheric pressure. Using Reference Table H, what is the boiling point of propanone
at an atmospheric pressure of 70 kPa? _________________
8. Base your answers to questions a through c on the graph below, which shows the vapor
pressure curves for liquids A and B.
a. What is the vapor pressure of liquid A at 70°C? Your answer must include correct units.
________________________
b. At what temperature does liquid B have the same vapor pressure as liquid A at 70°C?
Your answer must include correct units. _______________________
c. Which liquid will evaporate more rapidly? Explain your answer in terms of intermolecular
forces.
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