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Chem 101 week 13 ch11
 

Chem 101 week 13 ch11

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    Chem 101 week 13 ch11 Chem 101 week 13 ch11 Presentation Transcript

    • IntermolecularForcesChapter 11Intermolecular Forces,Liquids, and Solids
    • IntermolecularForces© 2009, Prentice-Hall, Inc.States of MatterThe fundamental difference between states ofmatter is the distance between particles.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.States of MatterBecause in the solid and liquid statesparticles are closer together, we refer to themas condensed phases.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.The States of Matter• The state a substance isin at a particulartemperature andpressure depends ontwo antagonistic entities:– the kinetic energy of theparticles;– the strength of theattractions between theparticles.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Intermolecular ForcesThe attractions between molecules are notnearly as strong as the intramolecularattractions that hold compounds together.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Intermolecular ForcesThey are, however, strong enough to controlphysical properties such as boiling andmelting points, vapor pressures, andviscosities.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Intermolecular ForcesThese intermolecular forces as a group arereferred to as van der Waals forces.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.van der Waals Forces• Dipole-Dipole interactions• Hydrogen bonding• London dispersion forces
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Ion-Dipole Interactions• Ion-dipole interactions (a fourth type of force),are important in solutions of ions.• The strength of these forces are what make itpossible for ionic substances to dissolve inpolar solvents.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Dipole-Dipole Interactions• Molecules that havepermanent dipoles areattracted to each other.– The positive end of one isattracted to the negativeend of the other and vice-versa.– These forces are onlyimportant when themolecules are close toeach other.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Dipole-Dipole InteractionsThe more polar the molecule, the higheris its boiling point.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.London Dispersion ForcesWhile the electrons in the 1s orbital of heliumwould repel each other (and, therefore, tendto stay far away from each other), it doeshappen that they occasionally wind up on thesame side of the atom.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.London Dispersion ForcesAt that instant, then, the helium atom is polar,with an excess of electrons on the left sideand a shortage on the right side.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.London Dispersion ForcesAnother helium nearby, then, would have adipole induced in it, as the electrons on theleft side of helium atom 2 repel the electronsin the cloud on helium atom 1.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.London Dispersion ForcesLondon dispersion forces, or dispersionforces, are attractions between aninstantaneous dipole and an induced dipole.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.London Dispersion Forces• These forces are present in all molecules,whether they are polar or nonpolar.• The tendency of an electron cloud to distort inthis way is called polarizability.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Factors Affecting London Forces• The shape of the moleculeaffects the strength of dispersionforces: long, skinny molecules(like n-pentane tend to havestronger dispersion forces thanshort, fat ones (like neopentane).• This is due to the increasedsurface area in n-pentane.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Factors Affecting London Forces• The strength of dispersion forces tends toincrease with increased molecular weight.• Larger atoms have larger electron cloudswhich are easier to polarize.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Which Have a Greater Effect?Dipole-Dipole Interactions or Dispersion Forces• If two molecules are of comparable sizeand shape, dipole-dipole interactionswill likely the dominating force.• If one molecule is much larger thananother, dispersion forces will likelydetermine its physical properties.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.How Do We Explain This?• The nonpolar series(SnH4 to CH4) followthe expected trend.• The polar seriesfollows the trendfrom H2Te throughH2S, but water isquite an anomaly.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Hydrogen Bonding• The dipole-dipole interactionsexperienced when H is bonded toN, O, or F are unusually strong.• We call these interactionshydrogen bonds.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Hydrogen Bonding• Hydrogen bondingarises in part from thehigh electronegativityof nitrogen, oxygen,and fluorine.Also, when hydrogen is bonded to one of thosevery electronegative elements, the hydrogennucleus is exposed.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Summarizing Intermolecular Forces
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Intermolecular Forces AffectMany Physical PropertiesThe strength of theattractions betweenparticles can greatlyaffect the propertiesof a substance orsolution.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Viscosity• Resistance of a liquidto flow is calledviscosity.• It is related to the easewith which moleculescan move past eachother.• Viscosity increaseswith strongerintermolecular forcesand decreases withhigher temperature.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Surface TensionSurface tensionresults from the netinward forceexperienced by themolecules on thesurface of a liquid.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase Changes
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Energy Changes Associatedwith Changes of StateThe heat of fusion is the energy required tochange a solid at its melting point to a liquid.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Energy Changes Associatedwith Changes of StateThe heat of vaporization is defined as theenergy required to change a liquid at itsboiling point to a gas.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Energy Changes Associatedwith Changes of State• The heat added to thesystem at the meltingand boiling points goesinto pulling themolecules farther apartfrom each other.• The temperature of thesubstance does not riseduring a phase change.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Vapor Pressure• At any temperature some molecules in aliquid have enough energy to escape.• As the temperature rises, the fraction ofmolecules that have enough energy toescape increases.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Vapor PressureAs more moleculesescape the liquid,the pressure theyexert increases.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Vapor PressureThe liquid and vaporreach a state ofdynamic equilibrium:liquid moleculesevaporate and vapormolecules condenseat the same rate.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Vapor Pressure• The boiling point of aliquid is thetemperature at whichit’s vapor pressureequals atmosphericpressure.• The normal boilingpoint is thetemperature at whichits vapor pressure is760 torr.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase DiagramsPhase diagrams display the state of asubstance at various pressures andtemperatures and the places where equilibriaexist between phases.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase Diagrams• The circled line is the liquid-vapor interface.• It starts at the triple point (T), the point atwhich all three states are in equilibrium.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase DiagramsIt ends at the critical point (C); above thiscritical temperature and critical pressure theliquid and vapor are indistinguishable fromeach other.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase DiagramsEach point along this line is the boiling pointof the substance at that pressure.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase Diagrams• The circled line in the diagram below is theinterface between liquid and solid.• The melting point at each pressure can befound along this line.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase Diagrams• Below the triple point the substance cannotexist in the liquid state.• Along the circled line the solid and gasphases are in equilibrium; the sublimationpoint at each pressure is along this line.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase Diagram of Water• Note the high criticaltemperature and criticalpressure.– These are due to thestrong van der Waalsforces between watermolecules.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase Diagram of Water• The slope of the solid-liquid line is negative.– This means that as thepressure is increased at atemperature just below themelting point, water goesfrom a solid to a liquid.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase Diagram of Carbon DioxideCarbon dioxidecannot exist in theliquid state atpressures below5.11 atm; CO2sublimes at normalpressures.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Phase Diagram of Carbon DioxideThe low criticaltemperature andcritical pressure forCO2 makesupercritical CO2 agood solvent forextracting nonpolarsubstances (likecaffeine)
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Solids• We can think ofsolids as falling intotwo groups:– crystalline, in whichparticles are in highlyordered arrangement.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Solids• We can think ofsolids as falling intotwo groups:– amorphous, in whichthere is no particularorder in thearrangement ofparticles.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Attractions in Ionic CrystalsIn ionic crystals, ionspack themselves so asto maximize theattractions andminimize repulsionsbetween the ions.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Crystalline SolidsBecause of theordered in a crystal,we can focus on therepeating pattern ofarrangement calledthe unit cell.
    • IntermolecularForces49A crystalline solid possesses rigid and long-range order. In acrystalline solid, atoms, molecules or ions occupy specific(predictable) positions.An amorphous solid does not possess a well-definedarrangement and long-range molecular order.A unit cell is the basic repeating structural unit of a crystallinesolid.latticepointUnit Cell Unit cells in 3 dimensionsAt lattice points:• Atoms• Molecules• Ions
    • IntermolecularForces50Seven Basic Unit Cells
    • IntermolecularForces51Three Types of Cubic Unit Cells
    • IntermolecularForces52Arrangement of Identical Spheres in a Simple Cubic Cell
    • IntermolecularForces53Arrangement of Identical Spheres in a Body-CenteredCubic Cell
    • IntermolecularForces54Shared by 8unit cellsShared by 4unit cellsA Corner Atom, a Edge-Centered Atom and aFace-Centered AtomShared by 2unit cells
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Crystalline SolidsThere are several types of basicarrangements in crystals, like the onesdepicted above.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Crystalline SolidsWe can determinethe empiricalformula of an ionicsolid by determininghow many ions ofeach element fallwithin the unit cell.
    • IntermolecularForces57Number of Atoms Per Unit Cell1 atom/unit cell(8 x 1/8 = 1)2 atoms/unit cell(8 x 1/8 + 1 = 2)4 atoms/unit cell(8 x 1/8 + 6 x 1/2 = 4)
    • IntermolecularForces58Relation Between Edge Length and Atomic Radius
    • IntermolecularForces59Closet Packing: Hexagonal and Cubichexagonal cubic
    • IntermolecularForces60Exploded Views
    • IntermolecularForces61When silver crystallizes, it forms face-centered cubic cells. Theunit cell edge length is 409 pm. Calculate the density of silver.d =mVV = a3= (409 pm)3= 6.83 x 10-23cm34 atoms/unit cell in a face-centered cubic cellm = 4 Ag atoms107.9 gmole Agx1 mole Ag6.022 x 1023atomsx = 7.17 x 10-22gd =mV7.17 x 10-22g6.83 x 10-23cm3= = 10.5 g/cm3
    • IntermolecularForces62An Arrangement for Obtaining the X-ray Diffraction Patternof a Crystal.
    • IntermolecularForces63Extra distance = BC + CD = 2d sinθ = nλ (Bragg Equation)Reflection of X rays from Two Layers of Atoms.
    • IntermolecularForces64X rays of wavelength 0.154 nm are diffracted from a crystal at anangle of 14.17o. Assuming that n = 1, what is the distance (in pm)between layers in the crystal?nλ = 2d sin θ n = 1 θ = 14.17oλ = 0.154 nm = 154 pmd =nλ2sinθ=1 x 154 pm2 x sin14.17= 314.0 pm
    • IntermolecularForces65Types of CrystalsIonic Crystals• Lattice points occupied by cations and anions• Held together by electrostatic attraction• Hard, brittle, high melting point• Poor conductor of heat and electricityCsCl ZnS CaF2
    • IntermolecularForces66Types of CrystalsCovalent Crystals• Lattice points occupied by atoms• Held together by covalent bonds• Hard, high melting point• Poor conductor of heat and electricitydiamond graphitecarbonatoms
    • IntermolecularForces67Types of CrystalsMolecular Crystals• Lattice points occupied by molecules• Held together by intermolecular forces• Soft, low melting point• Poor conductor of heat and electricitywater benzene
    • IntermolecularForces68Types of CrystalsMetallic Crystals• Lattice points occupied by metal atoms• Held together by metallic bonds• Soft to hard, low to high melting point• Good conductors of heat and electricityCross Section of a Metallic Crystalnucleus &inner shell e-mobile “sea”of e-
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Metallic Solids• Metals are not covalentlybonded, but theattractions betweenatoms are too strong tobe van der Waals forces.• In metals valenceelectrons are delocalizedthroughout the solid.
    • IntermolecularForces70Crystal Structures of Metals
    • IntermolecularForces71Types of Crystals
    • IntermolecularForces72An amorphous solid does not possess a well-definedarrangement and long-range molecular order.A glass is an optically transparent fusion product of inorganicmaterials that has cooled to a rigid state without crystallizingCrystallinequartz (SiO2)Non-crystallinequartz glass
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Covalent-Network andMolecular Solids• Diamonds are an example of a covalent-network solid, in which atoms are covalentlybonded to each other.– They tend to be hard and have high meltingpoints.
    • IntermolecularForces© 2009, Prentice-Hall, Inc.Covalent-Network andMolecular Solids• Graphite is an example of a molecular solid,in which atoms are held together with van derWaals forces.– They tend to be softer and have lower meltingpoints.