Evaporation and Boiling Point

1
Evaporation and Boiling Point

• Chapter 14. Sections 14.4 and 14.5

2
Review

• What is the heat of fusion?
• What is the heat of vaporization?
• What phase change occurs with vaporization?

3
Evaporation and Vapor Pressure

• When water goes from being a liquid to a gas,
  this is called vaporization
• The water molecules have overcome the
  intermolecular forces in liquid water and escaped
  into gas or vapor form

4
Evaporation and Vapor Pressure cont

• When boiling happens heat energy is added to
  overcome the intermolecular forces
• How about evaporation? There isnt heat energy
  being added, but the molecules still overcome the
  intermolecular forces?
• In evaporation, there are still energy changes
  occurring.
• You have to remember that heat is only the
  AVERAGE kinetic energy of a substance

5
Evaporation and Vapor Pressure cont

• Some parts of the water have very low kinetic
  energy, however some have a lot.
• So the faster moving molecules can escape,
  evaporation.

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Evaporation and Vapor Pressure cont

• Now if this process occurs in an insulated
  container (no heat can flow in or out) then the
  process is endothermic because the average
  kinetic energy is being lowered (taken in)
• If the container is not being insulated the temp
  will remain the same, however the process is
  still endothermic because energy is still being
  taken in

7
Vapor Pressure

• In a sealed container, there may be slight
  evaporation at first, but it will soon stabilize
• In fact some of the vapor molecules will return
  to a liquid state
• This is called condensation.

8
Vapor Pressure cont

• At the point where no further changes occur in
  the amounts of liquid and vapor in a closed
  container, because the two processes equally
  balance each other, the system is in equilibrium
• The amount of a pressure of the gas at
  equilibrium is referred to as vapor pressure

9
(a) Measuring vapor of a liquid by using a
simple barometer. (b) The water vapor pushed the
mercury level down. (c) Diethyl ether shows a
higher vapor pressure than water.
10
Vapor Pressure cont

• The vapor pressure of certain liquids vary
• Liquid with high vapor pressure are said to be
  volatile
• The vapor pressure of a liquid at a given temp is
  said to be determined by the intermolecular
  forces
• The stronger the intermolecular forces, the lower
  the vapor pressure because it is hard for
  molecules to go to the gas phase

11
Vapor Pressure cont

• You can predict which molecule will have a high
  vapor pressure, by looking at the number of polar
  (or O-H) bonds
• Example H2O and CH3OH
• H2O has 2 O-H bonds
• CH3OH has 1 O-H bond
• So Water has more polarity and lower vapor
  pressure

12
Boiling point and vapor pressure

• When you look at boiling water, what do you
  notice?

13
Boiling point and vapor pressure cont

• When heat energy is added to water, there are
  already tiny air bubbles in the water.
• The water turns to a vapor inside the air bubbles

14
Boiling point and vapor pressure cont

• The bubble can only form when the water molecules
  pressure can be equal to or greater than the
  atmospheric pressure.
• So the vapor pressure of the water must be equal
  to the pressure outside of it

15
Boiling point and vapor pressure cont

• The lower the atmospheric pressure, the easier it
  is for water vapor to form.
• Does this tell you why water boils at lower
  temperatures at high elevations?

16
Boiling point and vapor pressure cont

• http//www.dac.neu.edu/physics/b.maheswaran/phy112
  1/data/ch04/anim/anim0405.htm

17
Homework

• Read pages 449-453, problems 1-3 on 453 and
  problems 28 and 31 on page 464