Announcements

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Announcements

• 1st HW answer key available Friday afternoon
• LTEF poster due by 400 pm 10/1
• 2nd HW due on 10/8 or 10/9
• Test on 10/11 or 10/12

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Change of State

• Vapor Pressure

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What is Vaporization?

• aka Evaporation
• Change of state liquid to gas

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Is Evaporation Exo- or Endothermic?

• Endothermicwhy?
• Must overcome the intermolecular forces
• DH(vap) for H2O is 40.7 kJ/mol

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Practical Uses for Vaporization

• Most of the energy from the sun is used to
  evaporate water rather than warm the planet.
• Evaporation cools the human body.

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What is the opposite of vaporization?

• Condensation
• Rate of evaporation vs. condensation

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Vapor Pressure

• Pressure of the vapor above a liquid when the
  rate of evaporation and condensation are equal.

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Measuring Vapor Pressure
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Measuring Vapor Pressure

• Different liquids have different vapor pressures.
  Why?
• How does temperature affect vapor pressure?

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Temperature vs. Vapor Pressure

• As T increases, vapor pressure increases
• What happens when T gets 100ºC?
• Boiling point T at which the vapor pressure
  equals atm pressure.

• For H2O
• T (ºC) P(torr)
• 0.0 4.58
• 25.0 23.76
• 40.0 55.32
• 70.0 233.7
• 90.0 525.8

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Graphing Pvap vs. Temperature
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Chemistry and Straight Lines
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Chemistry and Straight Lines

• Slope - DHVAP / R
• lnP -DH / R1/T b
• lnPT1 lnPT2 DH / R1/T2 1/T1

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Clausius Clapeyron Equation
lnPT1 / PT2 DH / R1/T2 1/T1
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Practice Problem

• The vapor pressure of water at 25ºC is 23.8 torr
  and the heat of vaporization is 40.7 kJ/mol.
  Calculate the vapor pressure of water at 50ºC.

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Heating Curve
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Practice Problem 2

• Calculate the enthalpy change associated with
  converting 1.00 mol of ice at 25.0 ºC to water
  vapor at 125 ºC and 1 atm pressure.
• c(ice) 2.09 J / g C DH(fus) 6.01 kJ/mol
• c(liquid) 4.18 DH(vap) 40.7 kJ/mol
• c(steam) 1.84

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Phase Diagrams
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Whats on the test?

• Multiple choice (20 points)
• Ordering (15 points)
• Free response (65 points)
• Heating curve calculations
• Phase diagram interpretations
• Clausius Clapeyron manipulations
• Unit cell calculations
• Short answer questions