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WOU CH 222 - Intermolecular Forces and Liquids and Solids

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1Chapter 11Intermolecular Forces and Liquids and SolidsScientists are interested in how matter behaves under unusual circumstances. For example, before the space station could be built, fundamental research into materials properties had to be undertaken.2In this chapter intermolecular forces will be used to explain (among other things):• How can carbon exist as the hardest (diamond) and one of the softest substances known (graphite) ?• Why do boiling points decrease with altitude?• Why is ice less dense than water?• What are the structures of crystals?• Why does dry ice (CO2) sublimate?Solids and LiquidsIntermolecular ForcesIon-dipole, dipole-dipole and H-bonding, dipole-induced dipole, induced dipole-induced dipoleLiquidsVapor pressure and temperature, Critical T & P, Surface tension and viscosityPhase DiagramsShow relation of solid, liquid, and gas phases with change in T and PSolidsUnit cells, metal structures, formulas and structures of ionic compounds, Molecular, network, and amorphous solidsProperties of SolidsLattice energy, heat of fusion, melting point3Summary of Intermolecular ForcesVan der Waal’s forcesDispersion forces1. Ion-dipole ForcesHHO••••+-+--+-++-+-δ+δ+δ-HHO••••δ+δ+δ-42. Dipole-Dipole Interactions+-+-+-+-+-+-The positive and negative ends of polar molecules interact with each other, resulting in a net force of attraction.The strongest type of dipole-dipole interaction involving a hydrogen on one molecule (attached to a F, O, or N) and either F, O, or N on another molecule.About 10 % as strong as an ordinary covalent bond so approximately 15-40 kJ/mol.5HHOδ+δ+δ-HHOδ+δ+δ-HHOδ+δ+δ-HHOδ+δ+δ-HHOδ+δ+δ-6adenine thyminewhite = hydrogenblue = nitrogenblack = carbonred = oxygenThe Double Helix of DNA is held together by hydrogen bondingThe polymer Nylon is also held together by hydrogen bonding7Why is the hydrogen bond considered a “special” dipole-dipole interaction?Decreasing molar massDecreasing boiling point3. Dipole-Induced Dipole InteractionsPolarizability - a measure of the extent to which the electron cloud of an atom or molecule can be distorted by an external electric charge.In general, larger atoms or molecules are more easily polarizable than smaller ones (more shells, etc), and so experience larger dipole-induced dipole interactions.DISPERSION FORCES894. Induced dipole - Induced dipole Interactions (London Forces)Even nonpolar molecules and uncombined atoms have attractive forces between them, otherwise they would never condense or solidify.Which straight-chain alkane (CnH2n+2) molecule has greater attractive intermolecular forces ?H -C -C -C -HH H HH H HH -C -C -C -C -C -C -HH H HH H HH H HH H HH -C -C -C -HH H HH H HH -C -C -C -C -C -C -HH H HH H HH H HH H HThe molecule with the longer chain because there are more points of “attachment” via London Forces.10Straight-Chain AlkaneEffect of Dispersion Forces on Melting Points of Nonpolar Compounds11Properties of LiquidsSurface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.Strong intermolecular forcesHigh surface tensionPhoto credit: Microsoft EncartaProperties of LiquidsCohesion is the intermolecular attraction between like moleculesAdhesion is an attraction between unlike moleculesAdhesionCohesion12Properties of LiquidsViscosity is a measure of a fluid’s resistance to flow.Strong intermolecular forcesHigh viscosityCrystals and SolidsCrystal structures covered in the laboratory. Read Sections 11.4 – 11.7, p. 446-462CsCl ZnS CaF213T2> T1EvaporationCondensationThe equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporationH2O (l) H2O (g)Rate ofcondensationRate ofevaporation=Dynamic Equilibrium14Enthalpy of Vaporization, ∆Hvap(Molar Heat of Vaporization)LiquidVaporvaporizationheat energy absorbed by liquid∆Hvap= amount of energy required to evaporate 1 mole of a liquid under constant pressureMolar Heat of Vaporization and Boiling Points15BeforeEvaporationAt EquilibriumThe tendency for a liquid to evaporate increases as -1. the temperature rises2. the surface area increases3. the intermolecular forces decrease16Vapor Pressure and TemperatureDiethyl ether - dipole-dipole interactionsWater - hydrogen bonding (stronger)17Boiling Point“If you have a beaker of water open to the atmosphere, the mass of the atmosphere is pressing down on the surface. As heat is added, more and more water evaporates, pushing the molecules of the atmosphere aside. If enough heat is added, a temperature is eventually reached at which the vapor pressure of the liquid equals the atmospheric pressure, and the liquid boils.”Normal boiling point - the temperature at which the vapor pressure of a liquid is equal to the external atmospheric pressure of 1 atm.Increasing the external atmospheric pressure increases the boiling pointDecreasing the external atmospheric pressure decreases the boiling point18Location Elevation (ft) Boiling Point H2O (oC)San Francisco sea level 100.0Salt Lake City 4400 95.6Denver 5280 95.0Mt. Everest 29,028 76.5 The Clausius-Clapeyron EquationRelationship between vapor pressure, T, and ∆HvapCRTHPvap+∆−=lnWe can use this equation to measure ∆Hvapfor a given compound if we know P and T at two different points:R = 8.314 J/mol·KC = constant that depends on the compound’s volatility19Problem 11.88 – Estimate the molar heat of vaporization of a liquid whose vapor pressure doubles when the temperature is raised from 85oC to 95oC.MeltingFreezingH2O (s) H2O (l)The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium20Liquid – Solid Equilibriumsolid liquid→→The melting point of the solid, or the freezing point of the liquid, is the temperature at which the two phases are in equilibrium.rate melting rate freezing→→(dynamic equilibrium)∆Hfusion= molar heat of fusion, energy in kJ required to melt one mole of solid21SublimationDepositionH2O (s) H2O (g)Molar heat of sublimation (∆Hsub) is the energy required to sublime 1 mole of a solid.∆Hsub = ∆Hfus+ ∆Hvap( Hess’s Law)Copyright ©

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