Chemistry 123

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Chemistry 123

• Week of 23 January 2007

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Course info

• http//www.albion.edu/chemistry/abbethune/123/S08/
  default.htm

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Think about it
Which phase change occurs in each of the
following (vaporization, condensation,
sublimation, deposition, freezing, melting)

• The water level in an aquarium tank falls
  continuously (there is no leak)
• vaporization
• A black deposit of tungsten metal collects on the
  inside of a light bulb containing a tungsten
  filament.
• deposition
• Mothballs slowly become smaller and eventually
  disappear.
• sublimation
• Candle wax turns to liquid under the heat of a
  candle flame.
• melting (fusion)
• A mixture of scrambled eggs placed in a cold
  vacuum chamber slowly turns to a powdery solid.
• evaporation

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Think about it

• A cup of room temperature water is placed in a
  well-insulated cup. 
• After some of the water is allowed to evaporate,
  it is found that the temperature of the water in
  the cup has dropped below the room temperature. 
• Why?

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Refrigeration phase changes
Inside (refrigerant gas)
Cooling coils
Compressor (gas ? liquid)
outside
Expansion valve (liquid? gas)
http//www.howstuffworks.com/refrigerator4.htm
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DHvap and DHfus for several common substances
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Cooling curve for water
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Vapor pressure temperature
0
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Phase diagram, CO2
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Phase diagram, H2O
http//www.colorado.edu/physics/phys4230/phys4230_
sp02/mainPage5.html
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Cleaning clothes with CO2?

• Liquid carbon dioxide
• Great solvent (maybe too good)
• Cheap
• Abundant supply

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Reading the diagram

• What is the minimum temperature at which carbon
  dioxide will be in the liquid phase?
• -57C

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Reading the diagram

• At -57C, what is the lowest pressure that could
  be used in order to keep CO2 in liquid phase?
• 5.1 atm

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Reading the diagram

• It is more energy-efficient to clean clothes at
  room temperature (25C). 
• Will this require a higher or lower pressure of
  CO2 than T -57C?
• A higher pressure will be required (5 atm lt P lt
  73 atm)

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Application

• Beyond the critical point, a supercritical fluid
  (sc) exists.
• What is the max temperature and pressure at which
  the CO2 cleaning system could be used while still
  avoiding the scCO2 phase.
• T lt 31CP lt 73 atm

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Phase diagrams for CO2 H2O
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Pauling electronegativity (EN) scale
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Dispersion (London) Forces
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Intermolecular forces
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Think about it

• Which intermolecular forces exist between CH3Cl
  molecules?
• dispersion forces
• hydrogen bonding
• dipole-dipole forces
• dipole-induced dipole forces
• ion-induced dipole forces

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Dipole-dipole interaction
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Dipole-dipole interactions
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Think about it

• Which intermolecular forces exist between CH3OH
  molecules?
• dispersion forces
• hydrogen bonding
• dipole-dipole forces
• dipole-induced dipole forces
• ion-induced dipole forces

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Hydrogen bonding and BP
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Hydrogen bonding in water (ice)
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Hydrogen bonding in DNA
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Think about it

• Indicate the strongest intermolecular force that
  exists between H2O molecules and benzene
  molecules (C6H6)?
• dispersion forces
• hydrogen bonding
• ion-dipole forces
• dipole-induced forces
• ion-induced dipole forces

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Think about it

• Indicate the strongest intermolecular force that
  exists between CH3CH3 molecules and Na?
• dispersion forces
• hydrogen bonding
• ion-dipole forces
• dipole-induced forces
• ion-induced dipole forces

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Think about it

• Indicate the strongest intermolecular force that
  exists between H2O molecules and Na?
• dispersion forces
• hydrogen bonding
• ion-dipole forces
• dipole-induced forces
• ion-induced dipole forces

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Think about it

• Which of the following would you expect to be the
  most polarizable?
• Ca2
• K
• Ar
• Cl-
• S2-

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Think about it

• Consider HF and H2O.  These two molecules are
  both polar (HF has a larger dipole moment) and
  the same number of electrons. 
• Why does water boil at a much higher temperature
  than HF?