Principles of Chemistry II © Vanden Bout! A.!! liquid water ! B.!! gaseous water! C.!! they are exactly the same! D.!! it depends on the temperature!Which has a higher Enthalpy? Principles of Chemistry II © Vanden BoutWhat in the world is Enthalpy?If somehow you have forgotten last semester, then you need to know that Enthalpy is essentially EnergyTechnically, the change in enthalpy ΔHfor a process at constant pressure is equal to the heatConceptually it is easiest to think about it as the EnergyLow Enthalpy = Low Energy (stable, bottom of the hill)High Enthalpy = High Energy (top of the hill) Principles of Chemistry II © Vanden BoutWhy do different phases have different Enthalpies?Intermolecular Forces (IMF)EnthalpyLiquidMolecules Close TogetherIntermolecular Forces are AttractiveLower enthalpy (energy) compared tomolecules that are separatedGasMolecules Very far apartessentially no interactions no IMF = higher energy Principles of Chemistry II © Vanden BoutComparing the twoAttractive IMFLower HΔHvaporization = Hgas - Hliquid > 0remember: positive change in energy is energy into the systemWe need to put in energy to overcome the molecules attractions for each otherNo IMFHigher HPrinciples of Chemistry II © Vanden Bout! A.!! liquid water ! B.!! gaseous water! C.!! they are exactly the same! D.!! it depends on the temperature!Which has a higher Entropy? Principles of Chemistry II © Vanden BoutA quick review of EntropyThe entropy technically depends on the number of equivalent microstates of a system.How to deal with this qualitatively today?Entropy increases with increasing volumeEntropy increases with increasing temperatureEntropy increases with increasing the number of moleculesEntropy increases with going from a solid to a liquid to a gasThe Universe tends towards higher entropy Principles of Chemistry II © Vanden Bout! A.!! liquid water ! B.!! gaseous water! 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 BoutStability is governed by Free Energy It is the balance between lower Enthalpy (energy) & higher EntropyState with the lowest free energy (G) is most stableG = H -TStherefore at low temperature the state with lowest H will be the most stable(dominated by energy)at high temperaturethe state with the largest S will be the most stable(dominated by entropy)Principles of Chemistry II © Vanden BoutWhat if there were no IMF?If molecules were not attracted to each other they would all wander away from each otherand end up spread out all over the universeThis is entropyIMF hold them backthere are always some molecules that have enough energy to escapeat higher temperatures more molecules that can overcome the IMFThis is evaporation Principles of Chemistry II © Vanden BoutWe can measure how many molecules “escape”This is the vapor pressurelittle bit of liquidFigure Copyright Houghton Mifflin Company. All rights reserved Principles of Chemistry II © Vanden BoutFigure Copyright Houghton Mifflin Company. All rights reservedEvaporation is fixed.This depends on the IMF and the temperatureCondensation starts out slowSince there are initially no molecules in the gas phaseAs the number of molecules in the gas phase increases, the condensation rate increases Principles of Chemistry II © Vanden BoutVapor Pressures at 25°CWater 24 TorrDiethyl Ether 545 Torr Ammonia 7600 TorrMethanol 127 TorrEthanol 65 TorrPropanol 44 TorrButanol 7 TorrPrinciples of Chemistry II © Vanden Bout! A.!! butanol has a higher entropy ! B.!! butanol has stronger inter molecular forces! C.!! butanol has a lower molecular weight! D.!! butanol has a higher density!Why does butanol (C4H9OH) have a lower vapor pressurethan methanol (CH3OH) at 25°C? 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 point Principles of Chemistry II © Vanden BoutCompound VP (Torr) ΔHvap (kJ mol-1) Water 24 40.65Diethyl Ether 545 27.4Ammonia 7600 23.35Methanol 127 37.8Ethanol 65 38.5Propanol 44 47.5Butanol 7 51.6 Principles of Chemistry II © Vanden BoutWhy is the boiling 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!at the same temperature H2Te will have a lower vapor pressure, thus it will need to get to a higher temperature before it is equal to one atmosphere
View Full Document
Unlocking...