II.A. Phase Changes

1
Changes of State
H2O (g)
H2O (s)
H2O (?)
2
Phase Changes
3
Phase Changes

• Evaporation
• molecules at the surface gain enough energy to
  overcome IMF
• Volatility
• measure of evaporation rate
• depends on temp IMF

4
Phase Changes

• Equilibrium
• trapped molecules reach a balance between
  evaporation condensation

5
Phase Changes
p.478

• Vapor Pressure
• pressure of vapor above a liquid at equilibrium

v.p.

• depends on temp IMF
• directly related to volatility

temp
6
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

7
Phase Changes

• Melting Point
• equal to freezing point

• Which has a higher m.p.?
• polar or nonpolar?
• covalent or ionic?

polar
ionic
8
Phase Changes

• Sublimation
• solid ? gas
• v.p. of solid equals external pressure

• EX dry ice, mothballs, solid air fresheners

9
Phase Diagrams

• Show the phases of a substance at different temps
  and pressures.

10
Phase Changes Phase Changes Phase Changes 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.

11

• 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.

12
Heating Curves
13

• 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

14
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 1C
• Volcano clip -

water has a very high heat capacity
15
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.

16
Heating Curves

• Heat of Vaporization (?Hvap)
• energy required to boil 1 gram of a substance at
  its b.p.
• usually larger than ?Hfuswhy?

• EX sweating, steam burns, the drinking bird

17
Problem Solving

1. Draw heating curve
2. Mark starting and ending points
3. Calculate EACH step individually
4. Add energies for each step

• 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!

18
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
19
H2O (s) ? H2O (?) energy required ? 6 kJ/mol

• B-C 2. At 0 C a phase change begins
• Moving from left to right along LEG B, ice is
  melting to form liquid water
• Moving from right to left along LEG B, liquid
  water is freezing to form ice

The distance of LEG B along the Heat axis
(x-axis) is known as the Heat of Fusion d) Note
that temperature remains constant during a phase
change as energy is used to break inter-molecular
bonds.
0ºC
20
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
21
a) Moving from left to right along LEG D, water
is boiling to form water vapor b) Moving from
right to left along LEG D, water vapor is
undergoing condensation to form liquid water The
distance of LEG D along the Heat axis (x-axis)
is known as the Heat of Vaporization
D-E Liquid starts to vaporize, turning from
liquid to gas at 100oC. The temperature remains
constant as energy is used to break
inter-molecular forces.
H2O (?) ? H2O (g) energy required ? 41 kJ/mol
100ºC
22
E-F temperature starts to rise once all liquid
is vaporized. Gas particles gain kinetic energy.
steam
23
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.
24
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