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Phase Changes

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Phase Changes & Phase Diagrams Intermolecular forces have a significant impact on the physical properties of compounds: Boiling point Melting point – PowerPoint PPT presentation

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Title: 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.
2
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.

9
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

10
Phase Changes Phase Diagrams
  • B ? C Ice is melting with no change in T
  • Use heat of fusion
  • qB? C

11
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

12
Phase Changes Phase Diagrams
  • D? E Water is boiling with no change in T
  • Use heat of vaporization
  • qD? E

13
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.

16
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

18
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

20
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.

21
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

22
Phase Changes and Phase Diagrams
  • Each point along this line is the boiling point
    of the substance at that pressure.

23
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
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