Vapor Pressure and Changes of State

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Vapor Pressure and Changes of State
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Heat of vaporization

• Enthalpy of vaporization
• energy required to vaporize 1 mole of a liquid at
  a pressure of 1 atm ?Hvap

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Vapor pressure-in closed container

• Vapor molecules reform to a liquid
• condensation
• eventually rate of condensation rate of
  evaporation

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equilibrium

• When no further net change occurs in the amt of
  liquid or vapor b/c the two opposite processes
  exactly balance each other

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No net change?

• System is highly dynamic on the molecular level!
• Means molecules are constantly escaping and
  entering the liquid

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Vapor pressure

• Determined by intermolecular forces
• large IM forces low vp
• the molecules need a lot of energy to escape

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High vapor pressure

• Evaporate readily from an open dish
• volitile

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Temperature?

• Vapor pressure for a given liquid increases
  significantly with temperature
• why?

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graphs

• vp verses temperature
• nonlinear increase
• straight line by plotting ln(Pvap) versus 1/T (in
  K)

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Straight line

• ln(Pvap) -?Hvap/R (1/T) C
• ?Hvap enthalpy of vap
• R universal gas const
• C const for each liquid

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Impt relationship

• Can find ?Hvap by measuring Pvap at several temps
  and evaluating slope

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Two temps

• Can combine the eqn b/c C does not dept on temp
  in order to solve for Pvap at another temp

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Equation

• Ln(PvapT1) - ln(PvapT2) ?Hvap/R (1/T2 - 1/T1)
• OR
• Ln(PT1/PT2) ?Hvap/R (1/T2 - 1/T1)

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Solve

• The vp of water at 25oC is 23.8 torr and the
  ?Hvap at 25oC is 43.9 kJ/mol. What is the vp at
  50. oC?

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Changes of State
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Changes of state

• What happens when a solid is heated?
• Heat solid --gt melt to liquid --gt liquid will
  boil to gas state

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Heating curve

• Plot of time vs temp for a process where energy
  is added at a constant rate

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Energy into ice

• Random vibrations of water molecules increase
• break from lattice and change to liquid

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Enthalpy of fusion

• energy added to break (or disrupt) the ice
  structure by breaking H-bonds
• enthalpy change that occurs to melt a solid at
  the melting point (kJ/mol)

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0oC

• Temp is constant until all solid changes to
  liquid
• then temp will increase again

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100oC

• Temp is constant until all the liquid changes to
  a gas
• physical changes

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Melting point

• As the temp of the solid is increased, a point is
  eventually reached where the liquid and solid
  have identical vapor pressures

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Normal melting pt

• The temp at which the solid and liquid states
  have the same vp under the conditions where the
  total pressure is 1 atm

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Normal boiling pt

• Temperature at which the vp of the liquid is
  exactly 1 atm
• boiling occurs when the vp of the liq is equal to
  the pressure of its environment

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PHASE DIAGRAMS
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Phase Diagrams

• Represent the phases of a substance as a function
  of temperature and pressure

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Phase Diagrams

• Shows which state can exist as given temp and
  pressure
• conditions of CLOSED system

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Experiment 1

• Pressure is 1 atm
• initial- temp -20oC
• no vapor in cylinder (b/c at 0oC the vp is less
  than 1atm)

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• Ice melts to liquid (still no vapor)
• at 100oC, vp is 1 atm and water boils
• changes until all steam

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Experiment 2

• Pressure is 2.0 torr
• ice is only component (-20oC, 2 torr)
• at -10oC, ice --gt vapor
• sublimation (vp of ice external pressure)

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Experiment 3

• Pressure is 4.588 torr
• -20oC (ice only component)
• cylinder heated- no new phase until .0098oC

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• TRIPLE point- solid and liquid have identical vp
  at 4.588 torr
• only at these conditions (.01oC) can all three
  states of water coexist

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Experiment 4

• Pressure is 225 atm
• start with liquid water (300oC, 225 atm) b/c of
  high pressure

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• Liquid changes to vapor as temp increases, but
  goes through intermediate fluid region which is
  neither true vapor or liquid

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Critical temp

• temp above which vapor cannot be liquified no
  matter what pressure is applied

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Critical Pressure

• Pressure required to produce liquification at the
  critical temp

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Critical point

• Defined by critical pressure and temp (374oC, 218
  atm)
• beyond this point is intermediate fluid region

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Phase diagram for water

• Solid/liquid line has negative slope
• mp of water decreases as external pressure
  increases

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Phase diagram for water

• Opposite of most substance b/c density of ice
  less than water at mp

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Phase diagram for CO2

• Solid/liquid line has positive slope
• solid CO2 is more dense than liquid CO2

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Phase diagram for CO2

• Triple point at 5.1 atm and -56.6oC
• Critical point at 72.8 atm and 31oC
• at 1 atm CO2 sublimes

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