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Vapor Pressure and Changes of State
Heat of vaporization
• Enthalpy of vaporization• energy required to vaporize 1 mole of a liquid at a pressure of 1 atmHvap
Vapor pressure-in closed container
• Vapor molecules reform to a liquid
• condensation
• eventually rate of condensation = rate of evaporation
equilibrium
• When no further net change occurs in the amt of liquid or vapor b/c the two opposite processes exactly balance each other
No net change?
• System is highly dynamic on the molecular level!
• Means molecules are constantly escaping and entering the liquid
Vapor pressure
• Determined by intermolecular forces
• large IM forces = low vp
• the molecules need a lot of energy to escape
High vapor pressure
• Evaporate readily from an open dish
• volitile
Temperature?
• Vapor pressure for a given liquid increases significantly with temperature
• why?
graphs
• vp verses temperature
• nonlinear increase
• straight line by plotting ln(Pvap) versus 1/T (in K)
Straight line
• ln(Pvap) = -Hvap/R (1/T) + C
Hvap = enthalpy of vap
• R = universal gas const
• C = const for each liquid
Impt relationship
• Can find Hvap by measuring Pvap at several temps and evaluating slope
Two temps
• Can combine the eqn b/c C does not dept on temp in order to solve for Pvap at another temp
Equation• Ln(Pvap
T1) - ln(PvapT2) = Hvap/R (1/T2 - 1/T1)
• OR
• Ln(PT1/PT2) = Hvap/R (1/T2 - 1/T1)
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?
Changes of state
• What happens when a solid is heated?
• Heat solid --> melt to liquid --> liquid will boil to gas state
Heating curve
• Plot of time vs temp for a process where energy is added at a constant rate
Energy into ice
• Random vibrations of water molecules increase
• break from lattice and change to liquid
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)
0oC
• Temp is constant until all solid changes to liquid
• then temp will increase again
100oC
• Temp is constant until all the liquid changes to a gas
• physical changes
Melting point
• As the temp of the solid is increased, a point is eventually reached where the liquid and solid have identical vapor pressures
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
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
Phase Diagrams
• Represent the phases of a substance as a function of temperature and pressure
Phase Diagrams
• Shows which state can exist as given temp and pressure
• conditions of CLOSED system
Experiment 1
• Pressure is 1 atm
• initial- temp -20oC
• no vapor in cylinder (b/c at 0oC the vp is less than 1atm)
• Ice melts to liquid (still no vapor)
• at 100oC, vp is 1 atm and water boils
• changes until all steam
Experiment 2• Pressure is 2.0 torr
• ice is only component (-20oC, 2 torr)
• at -10oC, ice --> vapor
• sublimation (vp of ice = external pressure)
Experiment 3
• Pressure is 4.588 torr
• -20oC (ice only component)
• cylinder heated- no new phase until .0098oC
• TRIPLE point- solid and liquid have identical vp at 4.588 torr
• only at these conditions (.01oC) can all three states of water coexist
Experiment 4
• Pressure is 225 atm
• start with liquid water (300oC, 225 atm) b/c of high pressure
• Liquid changes to vapor as temp increases, but goes through intermediate “fluid” region which is neither true vapor or liquid
Critical temp
• temp above which vapor cannot be liquified no matter what pressure is applied
Critical Pressure
• Pressure required to produce liquification at the critical temp
Critical point
• Defined by critical pressure and temp (374oC, 218 atm)
• beyond this point is intermediate “fluid” region
Phase diagram for water
• Solid/liquid line has negative slope
• mp of water decreases as external pressure increases
Phase diagram for water
• Opposite of most substance b/c density of ice less than water at mp
Phase diagram for CO2
• Solid/liquid line has positive slope
• solid CO2 is more dense than liquid CO2
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