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Lecture 30

CHEM101 Lecture Notes - Lecture 30: Vapor Pressure, Dynamic Equilibrium, EnthalpyPremium


Department
Chemistry
Course Code
CHEM101
Professor
Yoram Apelblat
Lecture
30

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CHEM101Lecture30ChangesofState
Vaporization:
● Moleculesintheliquidphasehaveadistributionofspeedsandkineticenergy
● HighenergymoleculescanovercometheIMFandescapetheliquidphase,whichleads
tovaporization.
● Enthalpyofvaporization=
○ energyneededtoevaporateacertainamountofliquid.(kJ/mol)
● Enthalpyofcondensation:Thereverseprocess
○ H
cond
= H
vap
Δ − Δ 
● Whilemoleculesareevaporating(rateofvaporization),somearecondensing(rateof
condensation)
● 
● Atacertaintemperature,thetworateequate.Thisiscalleddynamicequilibrium
● Equilibriumiswhenthepressureofvaporgraphplateaus.
VapourPressure:
● Vapor=Gasincontactwithitsliquidorsolid
● ClosedvesselDynamicequilibriumbetweentheliquid/solidandthevapour
● Vapourpressureisthepressureexertedbythevapourthatisindynamicequilibrium
withitsliquidorsolid.
● Vaporpressuredependsontemperatureandisconstantatagiventemperature.
Boiling:
● Boilingoccurswhenthevapourpressureequalstheatmosphericpressure.
● Normalboilingpoint=whenvaporpressureis1atm(760torr)
● Whenyougotoahigherelevation,waterboilsatalowertemperaturethan100degrees
Celsius.
● Thehigherthevaporpressure,themorevolatilethecompoundis.
○ ThismeansthattheIMFforcesinliquidsareweak,whichmakesiteasyfor
moleculestoescapetheliquidphase
■ Thismeanstherearemoremoleculesinthevaporphase
■ Thisalsomeansthatthereisahighvaporpressure.
○ Ex.N2=extremelyvolatile
○ C3H8isveryhighvolatility,andboilsat42.TheV.Pis7.5atm
○ Waterisnotveryvolatile,withavaporpressureof10^3torr.
ChangesofState:
● Ifasolidisheated,theatomicormolecularvibrationscandisruptthecrystalstructure
andcausemelting(fusion)
● Normalmeltingpointiswhensolidandliquidhavethesamevaporpressure.Normal
totalpressureis1atm.
● Triplepoint:Apointwhereall3phasescancoexist.
● Criticalpoint:Apointatwhichtheliquidandthegasorvaporareindistinguishable.
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