Transcript II.A. Phase Changes

Unit 10- States of Matter
III. Changes of State
A. Phase Changes
A. Phase Changes
 Evaporation
• molecules at the surface gain enough
energy to overcome IMF
 Volatility
• measure of evaporation rate
• depends on temp & IMF
A. Phase Changes
Boltzmann Distribution
p. 477
temp
# of Particles
volatility
IMF
volatility
Kinetic Energy
A. Phase Changes
 Dynamic
Equilibrium
• trapped molecules reach a balance
between evaporation & condensation
A. Phase Changes
p.478
Pressure
• pressure of vapor above
a liquid at equilibrium
• depends on temp & IMF
• directly related to volatility
temp
v.p.
v.p.
 Vapor
IMF
temp
v.p.
A. Phase Changes
 Boiling
Point
• temp at which v.p. of liquid
equals external pressure
• depends on Patm & IMF
• Normal B.P. - b.p. at 1 atm
Patm
b.p.
IMF
b.p.
A. Phase Changes
 Melting
Point
• equal to freezing point
IMF
 Which
m.p.
has a higher m.p.?
polar
• polar or nonpolar?
• covalent or ionic?
ionic
A. Phase Changes
 Sublimation
• solid  gas
• v.p. of solid equals
external pressure
 EX:
dry ice, mothballs,
solid air fresheners
B. Heating Curves – sketch
this in your notes
Gas - KE 
Boiling - PE 
Liquid - KE 
Melting - PE 
Solid - KE 
B. Heating Curves
 Temperature
Change
• change in KE (molecular motion)
• depends on heat capacity
 Heat
Capacity
• energy required to raise the temp of 1
gram of a substance by 1°C
• “Volcano” clip - water has a very high
heat capacity
B. Heating Curves
 Phase
Change
• change in PE (molecular arrangement)
• temp remains constant
 Heat
of Fusion (Hfus)
• energy required to melt 1 gram of a
substance at its m.p.
B. Heating Curves
 Heat
of Vaporization (Hvap)
• energy required to boil 1 gram of a
substance at its b.p.
• usually larger than Hfus…why?
 EX:
sweating,
steam burns,
the drinking bird
C. Phase Diagrams
 Show
the phases of a substance at
different temps and pressures.
C. Phase Diagrams
The
diagram on the last slide
shows the phase diagram for water
• Each region represents a pure phase
• Line between regions is where the
two phases exist in equilibrium
• Triple point is where all 3 curves
meet, the conditions where all 3
phases exist in equilibrium!
Phase changes by Name
Pressure (kPa)
Critical
Point
Temperature (oC)
C. Phase Diagrams
With
a phase diagram, the
changes in mp and bp can be
determined with changes in
external pressure
What are the variables
plotted on a phase diagram?
C. Johannesson