Vapor Pressure and Changes of State

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 atm Hvap 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(PvapT1) - 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) 0C o

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

100 C o 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

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