Principles of Chemistry II © Vanden Bout! A.!! liquid iron ! B.!! solid iron! C.!! they are exactly the same! D.!! it depends on the temperature!Which has a lower Enthalpy?Principles of Chemistry II © Vanden Bout! A.!! liquid iron ! B.!! solid iron! C.!! they are exactly the same! D.!! it depends on the temperature!Which has a higher Entropy?Principles of Chemistry II © Vanden Bout! A.!! liquid iron ! B.!! solid iron! C.!! they are exactly the same! D.!! it depends on the temperature!Which has a lower Gibb's Free Energy?Principles of Chemistry II © Vanden BoutEquilibriaBalance between stability of lower Enthalpy (energy) & higher EntropyPhysical EquilibriaPhase transitions (no "chemistry")State with the lowest free energy is most stableG = H -TStherefore at high temperature the state with highest S will be the most stablePrinciples of Chemistry II © Vanden BoutVapor pressurelittle bit of liquidFigure Copyright Houghton Mifflin Company. All rights reservedPrinciples of Chemistry II © Vanden BoutLook at movieFigure Copyright Houghton Mifflin Company. All rights reservedPrinciples of Chemistry II © Vanden BoutComparing different liquidswhat matters is the free energy of the vapor compared to the liquidFor almost all substances the difference inENTROPY between the vapor and the liquid is the same!ΔSvap = 85 J K mol-1Therefore the diversity in liquids propertiesis dominatedby the ENTHALPY of vaporizationPrinciples of Chemistry II © Vanden BoutEnthalpies of VaporizationWater 40.65 kJ mol-1Ammonia 23.35 kJ mol-1Diethyl Ether 27.4 kJ mol-1 Methane 8.19 kJ mol-1Methanol 37.8 kJ mol-1Ethanol 38.5 kJ mol-1Propanol 47.5 kJ mol-1Butanol 51.6 kJ mol-1Principles of Chemistry II © Vanden Bout! A.!! it has a higher entropy ! B.!! it has stronger inter molecular forces! C.!! it has a lower molecular weight! D.!! it has a higher density!Why does butanol (C4H9OH) have a lower vapor pressurethan methanol (CH3OH)?Principles of Chemistry II © Vanden BoutIntermolecular forces lead to the enthalpy difference between the liquid and the vaporThe larger the IMF the larger the ΔHvapThe larger the ΔHvapthe smaller the vapor pressureThe the smaller the vapor pressurethe higher the boiling pointPrinciples of Chemistry II © Vanden BoutWhy is the boilng point of H2Te higher than H2Se?! A.! ! H2Te has a larger dipole! B.! ! H2Se has more dispersion forces! C.! ! H2Te has more dispersion forces! D.! ! Both A & C!Principles of Chemistry II © Vanden BoutBefore we get to boiling let'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!At equilibrium ! "G = 0 Therefore when a liquid is at its equilibrium vapor pressure ! Ggas" Gliq= 0 The Gibbs energy of the liquid at a given pressure is essentially equal to the Gibbs Energy at standard pressure as it is a very weak function of pressure (know from Thermodyanamcis that is beyond what is covered in General Chemistry) Therefore ! Gliq= Gliqo The Gibbs energy of the gas is strong function of pressure such that (again a known thermodynamic result) ! Ggas= Ggaso+ RT ln P Putting these two together we find ! "G = Ggas# Gliq= Ggaso+ RT ln P # Gliqo= 0"GVAPo= Ggaso# Gliqo= #RT ln P Where! "GVAPothe standard Gibb’s Energy for the reaction liq gas and is given by ! "GVAPo= "HVAPo# T"SVAPo Using all this we can relate the pressure to the temperature at equilibrium ! "GVAPo= "HVAPo# T"SVAPo= #RT ln P#ln P ="HVAPoRT#"SVAPoR This equation is essentially P as a function of T where all else is a constant. It is typically rearranged to look a two pressures P2, and P1 at two temperatures T2 and T1 ! lnP2P1" # $ % & ' = ()HVAPoR1T2(1T1* + , - . / This is the Claussius-Clapeyron Equation. It relates the vapor pressure at two temperature to the enthalpy of
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