Phase Changes

1
Phase Changes Phase Diagrams

• Intermolecular forces have a significant impact
  on the physical properties of compounds
• Boiling point
• Melting point
• Phase change
• A change in the physical state of a substance

Phase changes occur.
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Phase Changes Phase Diagrams

• You should know what phase change occurs for each
  of the following
• Melting
• Freezing
• Vaporization
• Condensation
• Sublimation
• Deposition

3
Phase Changes Phase Diagrams

• Melting
• Solid ? Liquid
• Freezing
• Liquid ? Solid
• Vaporization
• Liquid ? Gas
• Condensation
• Gas ? Liquid

4
Phase Changes Phase Diagrams

• Sublimation
• Solid ? Gas
• Deposition
• Gas ? Solid

5
Phase Changes Phase Diagrams

• Every phase change is accompanied by a change in
  the energy of the system.
• Heat of fusion (DHfus)
• Enthalpy change required to change a solid at its
  melting point to a liquid at the same temperature
• Heat of vaporization (DHvap)
• Enthalpy change required to change a liquid at
  its boiling point to a gas at the same temperature

6
Phase Changes Phase Diagrams

• Heat of Sublimation (DHsub)
• Enthalpy change required to transform a solid
  directly into the gas phase
• During a phase change, the two phases exist in
  equilibrium.
• At the melting point, solid and liquid phases are
  in equilibrium.
• At the boiling point, liquid and gas phases are
  in equilibrium.

7
Phase Changes Phase Diagrams

• The heat added to the system at the melting and
  boiling points goes into pulling the molecules
  farther apart from each other.
• The temperature of the substance does not rise
  during a phase change.

8
Phase Changes Phase Diagrams

• Example Calculate the enthalpy change
  associated with converting 12.0 g of ice at
  -15oC to steam (water vapor) at 115oC under a
  constant pressure of one atmosphere. The
  specific heats of ice, water, and steam are
  2.09J/g-K, 4.18 J/g-K, and 1.84 J/g-K,
  respectively. For water, DHfus 6.01 kJ/mol and
  DHvap 40.67 kJ/mol.

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Phase Changes Phase Diagrams

• A ? B Ice is heating from -15oC to 0.oC with no
  change in state
• Use specific heat of ice to determine q (from -15
  to 0.oC)
• qA? B

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Phase Changes Phase Diagrams

• B ? C Ice is melting with no change in T
• Use heat of fusion
• qB? C

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Phase Changes Phase Diagrams

• C ? D Water is heating from 0. to 100.oC with
  no change in state
• Use specific heat of water to determine q (from
  0. to 100. oC)
• qC? D

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Phase Changes Phase Diagrams

• D? E Water is boiling with no change in T
• Use heat of vaporization
• qD? E

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Phase Changes Phase Diagrams

• E ? F Steam is heating from 100. to 115oC with
  no change in state
• Use specific heat of steam to determine q (from
  100. to 115oC)
• qE? F

14
Phase Changes Phase Diagrams

• DHA? F
• 36.8 kJ

15
Phase Changes and Phase Diagrams

• Liquid-Vapor Equilibrium
• Molecules can escape from the surface of a liquid
  by evaporation.
• In a closed container, molecules in the gas phase
  cannot escape, and the pressure exerted by the
  vapor in the headspace begins to increase.

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Phase Changes and Phase Diagrams

• Liquid-Vapor Equilibrium
• The liquid and vapor reach a state of dynamic
  equilibrium
• liquid molecules evaporate and vapor molecules
  condense at the same rate.
• The vapor pressure reaches a constant value.
• Vapor pressure pressure exerted by a vapor when
  the liquid and vapor states are in dynamic
  equilibrium.

17
Phase Changes and Phase Diagrams

• Boiling point
• the temperature at which the vapor pressure of a
  liquid equals atmospheric pressure.
• Normal boiling point
• the temperature at which the vapor pressure of a
  liquid equals 760. torr or 1 atm

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Phase Changes and Phase Diagrams

• Example Use the graph of vapor pressure as a
  function of temperature to predict the normal
  boiling point of diethyl ether and its
  approximate boiling point at 200 torr and at 1000
  torr.
• Normal BP
• BP _at_ 200 torr
• BP _at_ 1000 torr

19
Phase Changes and Phase Diagrams

• Dynamic equilibria can exist between other states
  of matter.
• A phase diagram for a substance illustrates
• its physical state at various T and P
• the equilibria that exist between phases

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Phase Changes and Phase Diagrams

• The AB line is the liquid-vapor interface.
• It starts at the triple point (A)
• the point at which all three states are in
  equilibrium.

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Phase Changes and Phase Diagrams

• It ends at the critical point (B)
• the temperature (critical temperature) and
  pressure (critical pressure) above which the
  liquid and vapor are indistinguishable from each
  other

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Phase Changes and Phase Diagrams

• Each point along this line is the boiling point
  of the substance at that pressure.

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Phase Changes and Phase Diagrams

• The AD line is the interface between liquid and
  solid.
• The melting point at each pressure can be found
  along this line.

24
Phase Changes and Phase Diagrams

• 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.

25
Phase Changes and Phase Diagrams

• Example Given the phase diagram for water, which
  point represents
• the normal boiling point of water
• the normal melting point of water
• the critical point of water
• the triple point of water

26
Phase Changes and Phase Diagrams

• Example Given the phase diagram for carbon
  dioxide, which line represents the equilibrium
  between
• Solid and liquid?
• Liquid and gas?
• Solid and gas?

27
Phase Changes and Phase Diagrams

• Example Using this phase diagram for water,
  describe the changes in physical state that occur
  when water is maintained at 0oC while the
  pressure decreases from that at point 5 to that
  at point 1.
• 5
• 4
• 3
• 2
• 1

28
Phase Changes and Phase Diagrams

• Example Using this phase diagram for water,
  describe the changes in physical state that occur
  when water is kept at 1 atm pressure while the
  temperature decreases from that at point 9 to
  that at point 6.
• 9
• 8
• 7
• 4
• 6