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Published byJemimah Goodman Modified over 9 years ago
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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 H vap
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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(P vap ) versus 1/T (in K)
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Straight line ln(P vap ) = - H vap /R (1/T) + C H vap = enthalpy of vap R = universal gas const C = const for each liquid
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Impt relationship Can find H vap by measuring P vap 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 P vap at another temp
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Equation Ln(P vap T1 ) - ln(P vap T2 ) = H vap /R (1/T 2 - 1/T 1 ) OR Ln(P T1 /P T2 ) = H vap /R (1/T 2 - 1/T 1 )
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Solve The vp of water at 25 o C is 23.8 torr and the H vap at 25 o C is 43.9 kJ/mol. What is the vp at 50. o C?
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Changes of state What happens when a solid is heated? Heat solid --> melt to liquid --> 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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0oC0oC Temp is constant until all solid changes to liquid then temp will increase again
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100 o C 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 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 -20 o C no vapor in cylinder (b/c at 0 o C the vp is less than 1atm)
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Ice melts to liquid (still no vapor) at 100 o C, 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 (-20 o C, 2 torr) at -10 o C, ice --> vapor sublimation (vp of ice = external pressure)
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Experiment 3 Pressure is 4.588 torr -20 o C (ice only component) cylinder heated- no new phase until.0098 o C
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TRIPLE point- solid and liquid have identical vp at 4.588 torr only at these conditions (.01 o C) can all three states of water coexist
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Experiment 4 Pressure is 225 atm start with liquid water (300 o C, 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 (374 o C, 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 CO 2 Solid/liquid line has positive slope solid CO 2 is more dense than liquid CO 2
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Phase diagram for CO 2 Triple point at 5.1 atm and -56.6 o C Critical point at 72.8 atm and 31 o C at 1 atm CO 2 sublimes
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