Principles of Chemistry II © Vanden BoutLet's look at howdifferent properties affect vapor pressurefirst all liquidthen comesto equilibriumwith liquid + vaporwith a pressure that isthe vapor pressurethen add more volumeAt equilibrium there is less liquid, but the same Pressure!P =0 P =vapor pressureP =vapor pressure Principles of Chemistry II © Vanden BoutWhich container has a higher pressure at equilibrium?! A.!! the 2 L container ! B.!! the 1 L container! C.!! they are exactly the same! D.!! it depends on the temperature!Quick QuizYou have two containers. one has a total volume of 2 L and one has a total volume of 1 L Into each you place 500 mL of liquid etherThey have the same temperature Principles of Chemistry II © Vanden BoutWhich container has a greater number of ether molecules in the gas phase at equilibrium?! A.!! the 2 L container ! B.!! the 1 L container! C.!! they are exactly the same! D.!! it depends on the temperature!Another QuestionYou have two containers one has a total volume of 2 L and one has a total volume of 1 LInto each you place 500 mL of liquid ether Principles of Chemistry II © Vanden BoutIf we stress the system it adjust to get back to equilibriumLet’s imagine a container with some liquid ethanol,when we look at the gas above the liquidIf Pethanol > Pethanol,eq then there are “too many” molecules in the gas phase. The gas will condense until you get to equilibriumpartial pressure at equilibrium = vapor pressureactual partial pressure of ethanolIf Pethanol < Pethanol,eq then there are “too few” molecules in the gas phase. The liquid will evaporate until you get to equilibriumIf Pethanol = Pethanol,eq the number molecules in the gas phase is “just right” The system is at equilibriumIf all the liquid evaporates before getting to equilibrium then the pressure will never get up to the equilibrium valuePrinciples of Chemistry II © Vanden BoutVapor Pressure is determined primarily from ΔvapHΔvapH depends on the intermolecular forcesVapor Pressure is independent of volumeNeither the volume of the gas or liquid matterTo have the equilibrium both gas and liquid must be presentVapor Pressure is a strong function of temperatureThe Vapor Pressure is the PARTIAL PRESSURE OF THAT SUBSTANCE! Principles of Chemistry II © Vanden BoutFigure Copyright Houghton Mifflin Company. All rights reservedVapor Pressure as a function of Temperature Principles of Chemistry II © Vanden Bout! lnP2P1" # $ % & ' = ()HvapR1T2(1T1" # $ % & ' How Does Vapor Pressure Change with T?The Claussius-Clapeyron EquationRelates the vapor pressure P1 at temperature T1to the vapor pressure P2 at temperature T2 Principles of Chemistry II © Vanden BoutFigure Copyright Houghton Mifflin Company. All rights reservedPlot of lnP vs 1/Tslope = (-ΔHvaporization)/RPrinciples of Chemistry II © Vanden BoutWhat is the boiling point?Definition: The boiling point is temperatureat which the vapor pressure is equal to prevailing pressureWhat is the normal boiling point?Definition: The normal boiling point is temperatureat which the vapor pressure is equal to 1 atmBoilingNote: at the boiling point the partial pressure of the substance is equal to the total pressure. It cannot get any higher (its the whole thing). At higher temperatures the liquid would have a higher vapor pressure. To achieve this you need to have a higher total pressure. Principles of Chemistry II © Vanden BoutGiven that the vapor pressure of water is 24 Torr at 25°C and that the normal boiling point of water is 100 °C, what is the enthlapy of vaporization of water?Doc Cam Principles of Chemistry II © Vanden BoutfreezingΔH < 0ΔS < 0fusion (melting)ΔH > 0ΔS > 0sublimationΔH > 0ΔS > 0depositionΔH < 0ΔS < 0condensationΔH < 0ΔS < 0vaporizationΔH > 0ΔS > 0The different phase transitionsSolidLiquid GasfreezingΔH < 0ΔS < 0fusion (melting)ΔH > 0ΔS > 0sublimationΔH > 0ΔS > 0depositionΔH < 0ΔS < 0condensationΔH < 0ΔS < 0vaporizationΔH > 0ΔS > 0 Principles of Chemistry II © Vanden BoutPhase TransitionsFigure Copyright Houghton Mifflin Company. All rights reservedHeatall solidsolid/liq equilibrium (heat goes into phase change)all liquid (heat goes into temperature change)liq/gas equilibrium (heat goes into phase change)all gas (heat goes into temperature change)Principles of Chemistry II © Vanden BoutPrevious ThermoSingle Phaseenergy into goes into temperature changeat constant Pq = ΔH = CΔTyou need heat capacity of specific phase (sol,liq, gas)be careful with the units of heat capacity.you may need to multiply by a number of moles or a mass!Phase Changeenergy into goes into phase change (the two phases have the same free energybut one is higher in enthalpy)q = ΔHtransition for example q =ΔHfus for melting Principles of Chemistry II © Vanden BoutPhase TransitionsFigure Copyright Houghton Mifflin Company. All rights reservedHeatq = ΔHΔTq = ΔH = C ΔTslope = ΔT/ΔH = 1/C Principles of Chemistry II © Vanden BoutPhase TransitionsFigure Copyright Houghton Mifflin Company. All rights reservedHeatq = ΔHlength is simplyΔHvaporization Principles of Chemistry II © Vanden Bout! A.!! solid ! B.!! liquid! C.!! gas! D.!! no way to know!Phase DiagramsTemperatureThe diagram on the right shows different phases for a compound as a function of temperature and pressureWhat is the phase labeled "A"?PressureABCPrinciples of Chemistry II © Vanden BoutImportant PointsSolid LiquidGasEquilibrium LinesTwo Phases Triple PointThree Phasesin
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