Transcript II.A. Phase Changes

H2O (g)
H2O (s)
H2O ()
Phase Changes
Phase Changes
 Evaporation
• molecules at the surface gain enough
energy to overcome IMF
 Volatility
• measure of evaporation rate
• depends on temp & IMF
Phase Changes
 Equilibrium
• trapped molecules reach a balance
between evaporation & condensation
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.
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.
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
Phase Changes
 Sublimation
• solid  gas
• v.p. of solid equals
external pressure
 EX:
dry ice, mothballs,
solid air fresheners
Phase Diagrams
 Show
the phases of a substance at
different temps and pressures.
Phase Changes
From
To
…is Called
Energy is:
Solid
Liquid
Melting
Absorbed
Liquid
Solid
Freezing
Released
Liquid
Vapor
Boiling or Vaporization Absorbed
Vapor
Liquid
Condensation
Released
Solid
Vapor
Sublimation
Absorbed
Vapor
Solid
Deposition
Released
•The temperature at which a substance melts/freezes at standard pressure (1 atm)
is known as the Normal melting point or Normal freezing point. For water, this is
0 °C.
•The temperature at which a substance boils/condenses at standard pressure is
known as the Normal boiling point or Normal condensation point. For water, this
is 100 °C.





The Triple point is the condition of temperature and pressure in at which all
three phases exist together at equilibrium. For water, this is 0.0099 °C and
0.006 atmospheres.
The Critical Temperature, Tc is the temperature beyond which the solid and
liquid phases of the substance cannot exist. Put another way, above the
critical temperature, the substance can only be found as a gas. For water, this
temperature is 373.99 °C.
The Critical Pressure, Pc is the pressure above which the substance cannot
exist as a gas. For water, this pressure is 217.75 atmospheres.
The Critical Point is the point defined by the critical temperature and the
critical pressure.
The slope of the line between the solid and liquid phase provides important
information about the substance:
• If the slope is negative (as it is for water), then the substance is more
dense as a liquid than it is as a solid.
• If the slope is positive (as it is for most substances), then the substance is
more dense as a solid than it is as a liquid.
Heating Curves
Gas - KE 
Boiling - PE 
Liquid - KE 
Melting - PE 
Solid - KE 
Every Substance has a
unique heating curve
•Shape
is the same
•Melting and Boiling points are
different
•Each phase has its own specific
heat capacity
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
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.
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
Problem Solving
Draw heating curve
2. Mark starting and
ending points
3. Calculate EACH step
individually
4. Add energies for
each step
1.

You must know the
following information:
• Cp (ice) = 2.06 J/goC
• Cp (water) = 4.184 J/goC
• Cp (steam) = 2.02 J/goC
This is ONLY for H2O.
Every compound has their
own numbers!
Heating and cooling curve for water heated at a constant rates.
A-B = Solid ice, temperature is increasing as HEAT energy is absorbed
Particles gain kinetic energy, vibration of particles increases.
Ice
H2O (s)  H2O () energy required  6 kJ/mol
B-C distance
= 2. At 0of°CLEG
a phase
change
The
‘B’ along
thebegins:
Heat axis (x-axis) is known as
•Moving
left to right along LEG ‘B’, ice is melting to form liquid water
the Heat
of from
Fusion
•Moving
right to leftremains
along LEG
‘B’, liquid
water is
to form
d) Note
thatfrom
temperature
constant
during
a freezing
phase change
ice
as energy is used to break inter-molecular bonds.
0ºC
C-D = temperature starts to rise once all the solid has melted.
Particles gain kinetic energy as heat absorbed by water is no longer
going toward changing the phase of the substance.
Liquid
water
a) Moving from left to right along LEG ‘D’, water is boiling
to form
water vapor
D-E
= Liquid
starts to vaporize, turning from liquid to gas at
oC. Thefrom
b) Moving
right to left
along LEG
‘D’, water
vaporisisused to
100
temperature
remains
constant
as energy
undergoing
condensation
to form liquid water
break
inter-molecular
forces.
The
of
LEG energy
‘D’ along
the Heat
axis
(x-axis) is
H2Odistance

H
O
required

41
kJ/mol
(g)
known()as the2Heat
of Vaporization
100ºC
E-F = temperature starts to rise once all liquid is vaporized. Gas
particles gain kinetic energy.
steam
Energy Requirements for changing state:
In ice the water molecules are held together by strong intermolecular
forces.
The energy required to melt 1 gram of a substance is called the heat
of fusion
( H fus) For ice it is 335J/g
The energy required to change 1 gram of a liquid to its vapor is
called the heat of vaporization
(Hvap ) For water it is 2260 J/g
H (delta H) is the change in energy or heat content.
What is specific heat capacity?
The amount of energy required to change the temperature of one
gram of a substance by 1C .
10 C
11 C
Another name for specific heat is a calorie
(1 calorie = 4.184 Joules)
Specific heat capacity of liquid water (H2O (L) ) is 4.18 J /gC.
Water (s) = 2.03 J/ g C
Water (g) = 2.0 J/ g C