Phase Changes
• Vaporization- The
process of changing
from a liquid to a gas.
• Endothermic- lower
energy liquid goes to
a higher energy gas.
• When this occurs only
at the surface the
process is called
evaporation.
Phase Changes
• Evaporation is how our body controls its
temperature. As high energy molecules
leave the surface of the water, the cooler
ones are left behind.
• A substance that evaporates very easily is
said to be volatile.
• condensation—opposite of vaporization.
When the energetic steam molecules
generated by your morning shower hurl
themselves across the bathroom and
collide with the cold mirror, they lose
energy and return to the liquid phase.
• Exothermic - goes from high energy gas
to a low energy liquid.
Phase Changes
• As temperature
increases the
amount of
evaporation
increases, if
evaporation is
taking place in a
closed container.
The vapor will exert
a pressure on the
liquid called vapor
pressure.
Phase Changes
Two things affect the vapor pressure:
•Temperature- increase temp increase vapor pressure
•Intermolecular forces- the weaker the force, the higher
the vapor pressure.
Equilibrium vapor pressure
• equilibrium vapor pressure—reached
when the rate of evaporation equals the
rate of condensation in a closed container.
• Moleucles leave and enter the liquid phase
at the SAME RATE.
Temperature and Vapor Pressure
• High Vapor Pressure =
Low IMF
• The liquid takes energy
from the surroundings
(warm room) and
vaporizes.
• Increase the
temperature and more
molecules have
sufficient KE to escape.
• Slope of the ln Pvapor versus 1/T tells the
∆Hvaporization. Water has a greater slope
than diethyl ether, therefore, it has a higher
∆Hvaporization.
Increase the temperature and more
molecules have enough energy to
escape
Vapor Pressure and Molar Mass
• In general, as molar mass increases,
vapor pressure DECREASES.
• This is due to polarizability.
• As MM increases, the number of electrons
increases. Greater number of electrons
increase the probability of an induced
dipole moment.
Clausius- Clapeyron equation
• If you know VP and ∆Hvaporization at one
temp, you can calculate VP at a different
temp and you know you want to do that.
 VP1  H vap  1 1 
 
  
ln 
R  T2 T1 
 VP2 
 VP1  H vap  1 1 
 
  
ln 
R  T2 T1 
 VP2 
Exercise 6
Calculating Vapor Pressure
The vapor pressure of water at 25°C is 23.8 torr, and the
heat of vaporization of water at 25°C is 43.9 kJ/mol.
Calculate the vapor pressure of water at 50°C.
Phase Changes
• Boiling Point- the
temperature where
the vapor pressure of
a liquid is equal to
atmospheric
pressure.
• To determine the
boiling point, a vapor
pressure curve can
be used
Phase Changes
• Sublimation- Changing directly from the
solid phase to the gas phase. Dry Ice,
Iodine, Smoke in your freezer are
examples.
Phase Changes That Remove
Energy
• Phase changes that release energy
• Freezing - Removing heat from liquid
molecules, allows them to slow down and
form intermolecular bonds.
• How does Melting Point and Freezing
point of the same substance compare?
Phase Changes Cont’d
• Condensation- Going from a gas to a
liquid. Bonds are forming. Energy must
be released.
• Deposition- going directly from a gas to a
solid. Snowflakes are an example
Supercooling
• Supercooled liquids can stay liquid
below melting point because doesn’t
achieve level of organization needed
to make solid.
Superheated
• A substance is at a temperature above its
BP, yet it remains a liquid. Usually
happens when heated very rapidly
[microwave oven] and bubbles form in the
interior with high internal pressures. They
often burst before reaching the surface.
Phase Changes Cont’d
Quick Review
If two liquids are at the same
temperature, which liquid:
• probably contains a nonpolar
compound?
• has would have a lower vapor
pressure?
• is more volatile?
• has a greater evaporation
rate?
• which has stronger attractive
forces?
Draw the vapor pressure curve
and label the lines for liquid A
and B.
Phase Diagrams
• Phase Diagrams:
• Normal Melting Point/freezing point – The
temperature at 1 atm crossing the liquid/solid
line.
• Normal Boiling Point – The temperature at 1
atm crossing the liquid/gas line.
• Triple point – The temperature and pressure
point when all three phases exist
• Critical point – The temperature and pressure
where the substance cannot exist as a liquid.
Phase Diagrams
PHASE DIAGRAMS
•
•
The AD line is the interface between liquid and solid.
– The melting point at each pressure can be found along this line.
– Below A the substance cannot exist in the liquid state.
Along the AC line the solid and gas phases are in equilibrium; the
sublimation point at each pressure is along this line.
Warming/Cooling Curves
q = mc∆T
q= mHv
q = mf