CYCLE 5
Developing Ideas
ACTIVITY 5 HW: Vapor pressure of other liquids-
-KEY
Name: ________________________________ Date: _______________ Group: ______
Is evaporation the same for all liquids?
Simulator Exploration #1: How does the evaporation of methanol
compare to the evaporation of water?
You might be wondering, do all liquids evaporate at the same rate? We can
use the simulator to study the evaporation of another liquid, methanol. In
order to answer some of the questions you will need to refer to your water
vapor data from Activity 5.
STEP 1. Go to the simulator index page and open Cycle 5 Activity 5 HW
Setup 1.
This setup is similar to Cycle 5 Activity 5 Setup 1 except that it shows a closed
container half-full of methanol at a constant temperature.
STEP 2. Run the simulator for 30 seconds and then Pause.
Sketch the Pressure vs Time graph for methanol vapor. Estimate the
equilibrium vapor pressure from the graph.
According to the graph, how does the
pressure of methanol vapor change in
the first 15 seconds? After 15 seconds?
Increases for 15 seconds, then levels off to a constant
pressure of about 0.16 atm.
© 2007 PSET
5-107
Cycle 5
Sketch the Density vs Time graph for
methanol
vapor.
Estimate
the
equilibrium density from the graph.
About 0.20 g/l
According to the graph, how does the density of methanol vapor change
in the first 15 seconds? After 15 seconds?
Increases for 15 seconds, then levels off to a constant density.
Like water, when methanol evaporates the vapor pressure and density are
transient (changing) for a short time until the equilibrium vapor pressure and
density are reached. While the evaporation process is similar for different
liquids, different liquids have different equilibrium vapor pressures.
Which liquid, water or methanol, has the higher equilibrium vapor
pressure at room temperature?
Methanol has a higher vapor pressure (0.16 atm) at room temperature (water has a vapor
pressure of about 0.03 atm).
Simulator Exploration #2: How does vapor pressure change when
methanol is heated?
STEP 1. Go to the simulator index page and open Cycle 5 Activity 5 HW
Setup 2.
This setup is nearly identical to Cycle 5
Activity 5 HW Setup 1 except that the
element below the container has been
switched to ‘Heater’ and a plot of
Pressure vs Temperature is included.
Run the simulator until the temperature
stops changing.
Sketch the Pressure vs Temperature
graph for methanol vapor.
5-108
Activity 5 Homework
Recall that the boiling point is the temperature at which the vapor pressure is
equal to the atmospheric pressure. On a vapor pressure vs temperature
graph, the slope is vertical at the boiling point. The normal boiling point is
defined as the boiling point of a liquid at sea level, where the atmospheric
pressure is 1 atm.
Which liquid has the higher normal boiling point —water or methanol?
Explain your reasoning.
Water has the higher normal boiling point. Water’s curve becomes vertical at 100 degrees, but
methanol’s curve becomes vertical around 70 degrees.
Simulator Exploration #3: How does the Small Particle Model explain the
differences between the vapor pressures of methanol and water?
STEP 1. Go to the simulator index page and open Cycle 5 Activity 5 HW
Setup 3.
This setup is similar to Cycle 5 Activity
5 Setup 3 except that it shows a closed
container half-full of methanol at a
constant temperature.
STEP 2. Run the simulator for 30-40
seconds, and then Pause the simulator.
Carefully sketch the # particles vs
time graph for methanol vapor.
Refer back to Cycle 5 Activity 5 Simulator Exploration #3. In what
way(s) are the methanol and water # particles vs time graphs similar
and different?
They are similar in that they both increase in the first 5-10 seconds, then level off. They are
different in that # particles is higher for methanol than for water throughout.
Carefully sketch the leaving rate vs time and entering rate vs time
graphs for methanol vapor.
5-109
Cycle 5
Refer back to Cycle 5 Activity 5 Simulator Exploration #3. In what
way(s) are the methanol and water leaving rate vs time and entering rate
vs time graphs similar and different?
They are similar in that leaving rate is constant while entering rate increases until it is the same
as leaving rate. They are different in that the leaving/entering rate for methanol (25/sec) is
greater than for water (6/sec).
Heating the liquid
STEP 3. Activate the Select tool, and
then double click on the element
beneath the container to open its
properties box. Select Heater, and
click OK. Run the simulator for 30-40
seconds, and then Pause the
simulator.
Sketch the # particles vs time
graph for methanol vapor while
heating the system.
Sketch the leaving rate vs time and entering rate vs time graphs for
methanol vapor while heating the system.
(Note change in scale to show the graph more easily.)
5-110
Activity 5 Homework
In terms of numbers of vapor particles, why does methanol have a
higher vapor pressure and density at a given temperature?
According to leaving rate, more particles are leaving the surface of the methanol than are
leaving the surface of water. This means that # particles of vapor above the liquid is greater for
methanol than for water. If there are more particles of vapor for methanol, then vapor pressure
will be higher than for Methanol.
In terms of equilibrium leaving/entering rates of particles at the liquid
surface, why would a small droplet of methanol evaporate faster than an
equal-volume droplet of water at a given temperature?
The leaving rate is much higher for methanol than for water. More methanol particles will leave
the surface and become vapor in a given time than is observed for water.
The differences in the vapor pressures and boiling points of water and
methanol are due to differences in the strengths of attractive electrostatic
forces between particles.
Consider the vapor pressures and boiling points of water and methanol.
Which has stronger attractive electrostatic forces between liquid
particles, water or methanol? Explain your reasoning.
Water has stronger attractive electrostatic forces between liquid particles. Since the leaving
rate is lower, and fewer particles escape the liquid surface as vapor, we might conclude that
the liquid particles are held together more strongly in water than in methanol.
5-111
Cycle 5
Summarizing Questions
S1. In Cycle 5 Activity 2 you placed water, ethanol, and hexane on a sheet of
wax paper and observed that water formed a bead and did not
noticeably evaporate, ethanol formed a puddle and slowly evaporated,
and hexane spread out and quickly evaporated.
a) Rank the liquids in order of increasing vapor pressure (lowest to
highest) at room temperature. Explain why you chose this order.
Water < ethanol < hexane; the bead of water was still present at the end of the
experiment, the ethanol puddle has mostly evaporated by the end of the experiment, the
hexane evaporated almost instantly
b) Rank the liquids in order of increasing boiling point (lowest to
highest). Explain why you chose this order.
Hexane < ethanol < water; liquids with higher vapor pressures at a given temperature
have lower boiling points (vapor pressure reaches atmospheric pressure at a lower
temperature)
c) Rank the liquids in order of increasing strength of attractive
electrostatic forces (lowest to highest) between particles. Explain why
you chose this order.
Hexane < ethanol < water; the lower the strength of attractive electrostatic forces, the
higher the vapor pressure and the lower the boiling point.
5-112