Evaporation

1
Evaporation

• Model 1
• The complete evaporation of 5 mol of water is
  displayed in a rigid, closed container.
  Theenergy supplied to cause the phase changeis
  in the form of heat from a Bunsen burner. Each
  mole of water is depicted by a single Lewis
  structure of water.

D

• Critical Thinking Questions
• Label each container as either the liquid or gas
  phase.
• What is meant by the capital delta (D) in model
  1?
• Have the bonds of the water molecules been
  noticeably disturbed via the process depicted?
  Explain?
• Have the intermolecular forces of the water
  molecules been noticeably disturbed via the
  process depicted? Explain?

• Model 2
• A liquid, at 30ºC, held together by
  intermolecular forces. Three particular molecules
  have been singled out and have been numbered, as
  well as darkened, for clarity.

T 30ºC
3
2

• Critical Thinking Question
• Do all the molecules in this container have the
  same energy? Explain.

1
2
Evaporation

• Information
• It is reasonable to model the solid and liquid
  phase with a velocity distribution similar to
  the Maxwell-Boltzmann distribution, as shown in
  model 3.
• It is reasonable to expect the velocity, and thus
  kinetic energy,of molecules in the condensed
  phases to be distributed fromthe very slow (low
  energy) to the very fast (high energy).

• Model 3
• A Maxwell-Boltzmann distribution for a liquid.
  The dashed, vertical line displays the velocity
  (energy) at which the IM forces can be broken.

• Critical Thinking Questions
• In model 3, would a molecule with velocity to the
  right or left of the given vertical line have a
  higher energy?
• Suppose that molecules 1, 2, and 3 from model 2
  have avelocity that is to the right of the given
  line in model 3.
• Would molecule 1 vaporize? Why or why not?
• Would molecule 2 vaporize? Why or why not?
• Would molecule 3 vaporize? Why or why not?
• Information
• For a liquid molecule to evaporate, not only must
  the molecule have enough energy to overcome the
  IM forces attracting it, but it must also be at
  the surface of the container.

3
Evaporation

• Critical Thinking Questions
• To the figure in model 3, add a curve that
  represents the liquid after being heated to a
  temperature of 60ºC. Label the appropriate
  curves, one as T30ºC and the other as T60ºC
• According to your curve for the higher
  temperature system, are more or less molecules at
  sufficient energy to break the IM forces of the
  liquid state? Explain.
• Has increasing the temperature created more
  positions at the surface for the liquid
  molecules?
• Has increasing the temperature increased the rate
  of evaporation? Explain.
• Suppose I replace the molecules in model 2 ( )
  with another pure substance ( ) that has
  stronger intermolecular forces between molecules.
• Draw a dotted vertical line on model 3 that
  represents the minimum energy to overcome the IM
  forces.
• How would the rate of evaporation compare between
  the molecules and ?

4
Vapor Pressure

• Model 4
• A closed container of liquid is monitored over
  time. The temperatureinside is 25ºC.
  Initially, all the gas molecules were removed.A
  plot of pressure vs. time has been created from
  the data.

0.5
P (atm)
Time (sec)
20

• Critical Thinking Questions
• Why is the pressure zero inside the container at
  the beginning of the experiment depicted in model
  4?
• The container shown in panel (a) is immediately
  following the closing of the container. What
  does the upward pointing (or red) arrow
  represent?
• Panel (b) of model 4 introduces a downward facing
  (blue) arrow. What does this type arrow
  represent?
• What do the grey spheres represent in panels (b)
  and (c)?

5
Vapor Pressure

• Critical Thinking Questions
• Are there more upward or downward facing arrows
  in panel (b)? Where on the graph might represent
  panel (b)mark your estimate on the graph itself?
• Information
• The vapor pressure is the pressure created when
  the rate of evaporation is equal to the rate of
  condensation.
• The vapor pressure is a constant for a substance
  that depends only on the temperature.
• Critical Thinking Questions
• According to model 4, what is the vapor pressure
  of the liquid in the container at 25ºC?
• If the temperature were raised to 30ºC in model
  4, would the rate of evaporation increase,
  decrease, or stay the same? Justify our choice.
• Complete the following phrase and justify your
  choice as the temperature increases the vapor
  pressure ________.

6
Exercises

• Consider the following molecules all with similar
  mass
• Formaldehyde H2CO
• Methanol CH3-OH
• Ethane CH3-CH3
• All three are currently at a temperature and
  pressure at which they are in the liquid phase.
• Rank these three species from lowest to highest
  evaporation rate.
• Rank these three species from lowest to highest
  vapor pressure.