1WHY?Why is water usually a liquid and not a gas? Why does liquid water boil at such a high temperature for such a small molecule?Why does ice float on water?Why do snowflakes have 6 sides?Why is I2a solid whereas Cl2is a gas?Why are NaCl crystals little cubes?Chapter 10Intermolecular Forces, Liquids, and SolidsIntramolecular vs. Intermolecular ForcesUp to this point we have studied only intramolecular forces.– These are the forces that hold atoms together to form molecules.We are now going to study intermolecularforces.– These are the forces betweenIons (Ion – Ion)Molecules and Ions (Ion – Dipole)Molecules and Molecules (Dipole – Dipole)In this chapter we will be talking a lot about dipole moments. So, it is a good idea to be familiar with that concept.Ion – Ion ForcesExtremely strong force– Coulomb’s LawThis helps explain why ionic solids have such a high melting point.NaCl, mp = 800 °CMgO, mp = 2800 °CIon – Dipole ForcesThis force arises because of the dipole charge on a compound interacting with the ion’s charge.For example:– Water is highly polar and can interact with positive ions to give hydratedions in water.Attraction Between Ions and Permanent DipolesWater is highly polar and can interact with positive ions to give hydratedions in water.2Ion – Dipole ForcesMany metal ions are hydrated. This is the reason metal salts dissolve in water.Ion – Dipole ForcesAttraction between ions and dipole depends on ion charge and ion-dipole distance. (Coulomb’s Law)If you had to guess which one out of the three forces is strongest?– Measured by ∆H for Mn++ H2O --> [M(H2O)x]n+OHHδ+δ-• • •OHHδ+δ-• • •OHHδ+δ-• • •Na+Mg2+Cs+-1922 kJ/mol -405 kJ/mol -263 kJ/molDipole – Dipole ForcesSuch forces bind molecules having permanent dipoles to one another.Influence of dipole-dipole forces is seen in the boiling points of simple molecules.Compd Mol. Wt. Boil PointN228 -196 oCCO 28 -192 oCBr2160 59 oCICl 162 97 oCDipole-Dipole ForcesSuch forces bind molecules having permanent dipoles to one another.Hydrogen BondingA hydrogen bond is an attraction between the hydrogen atom of an X – H bond and Y, where X and Y are atoms of hightly electronegative elementsA special form of dipole-dipole attraction, which enhances dipole-dipole attractions.H-bonding is strongest when X and Y are N, O, or FH-Bonding Between Methanol and WaterH-bond-δ+δ-δ3H-Bonding Between Two Methanol MoleculesH-bond-δ+δ-δH-Bonding Between Ammonia and WaterH-bond-δ+δ-δThis H-bond leads to the formation of NH4+and OH-Hydrogen Bonding in H2OH-bonding is especially strong in water because the O—H bond is very polar.Accounts for many of water’s unique properties.Hydrogen Bonding in H2OIce has open lattice-like structure.Ice density is < liquid.And so solid floats on water.Snow flake: http://www.its.caltech.edu/~atomic/snowcrystals/snow3x.jpgHydrogen Bonding in H2OIce has open lattice-like structure.Ice density is < liquid and so solid floats on water.Hydrogen Bonding in H2OH bonds ---> abnormally high specific heat capacity of water (4.184 g/K•mol). This is the reason water is used to put out fires, it is the reason lakes/oceans control climate, and is the reason thunderstorms release huge energy.4Hydrogen BondingH bonds leads to abnormally high boiling point of water.Effect of Hydrogen Bonding on Boiling PointsFORCES INVOLVING INDUCED DIPOLESHow can non-polar molecules such as O2 and I2dissolve in water?The water dipole INDUCES a dipole in the O2electric cloud. FORCES INVOLVING INDUCED DIPOLESSolubility increases with mass the gasFORCES INVOLVING INDUCED DIPOLESConsider I2dissolving in alcohol, CH3CH2OH.OH-δ+δI-IR-δ+δOH+δ-δI-IRThe alcohol temporarily creates or INDUCES a dipole in I2.FORCES INVOLVING INDUCED DIPOLESFormation of a dipole in two nonpolar I2molecules.A.K.A. London ForcesThe induced forces between I2molecules are very weak, so solid I2sublimes (goes from a solid to gaseous molecules).Induced dipole-induced dipole5FORCES INVOLVING INDUCED DIPOLESThe induced forces between I2molecules are very weak, so solid I2sublimes (goes from a solid to gaseous molecules).London Dispersion Forces relatively weak forces that exist among noble gas atoms and nonpolar molecules. (Ar, C8H18) caused by instantaneous dipole, in which electron distribution becomes asymmetrical. the ease with which electron “cloud” of an atom can be distorted is called polarizability.FORCES INVOLVING INDUCED DIPOLESThe magnitude of the induced dipole depends on the tendency to be distorted. Higher molec. weight ---> larger induced dipoles.Molecule Boiling Point (oC)CH4(methane) - 161.5C2H6(ethane) - 88.6 C3H8(propane) - 42.1C4H10(butane) - 0.5Boiling Points of HydrocarbonsNote linear relation between bp and molar mass.CH4C2H6C3H8C4H10Intermolecular Forces SummaryLiquidsIn a liquid• molecules are in constant motion• there are appreciable intermolec. forces• molecules close together• Liquids are almost incompressible• Liquids do not fill the container6LiquidsThe two key properties we need to describe are EVAPORATION and its opposite—CONDENSATION<---condensationevaporation--->LiquidVaporADD ENERGYBreak BondsForm BondsREMOVE ENERGYLiquids—EvaporationTo evaporate, molecules must have sufficient energy to break IM forces.Breaking IM forces requires energy. The process of evaporation is endothermic.Liquids—Distribution of EnergiesDistribution of molecular energies in a liquid.KE is propor-tional to T.0See Figure 13.12Minimum energy req’d to break IM forces and evaporateMolecular EnergyNumber of MolesHigher TLower TDistribution of Energy in a LiquidFigure 13.12LiquidsAt higher T a much larger number of molecules has high enough energy to break IM forces and move from liquid to vapor state.High E molecules carry away E. You cool down when sweating or after swimming.0Number of moleculesMolecular energyminimum energy neededto break IM forces and evaporatehigher Tlower TLiquidsWhen molecules of liquid are in the vapor state, they exert a VAPOR PRESSUREEQUILIBRIUM VAPOR PRESSUREis the pressure exerted by a vapor over a liquid in a closed container when the rate of evaporation = the rate of condensation.7Equilibrium Vapor PressureLiquid in flask evaporates and exerts pressure on manometer.Equilibrium Vapor PressureLiquidsEquilibrium Vapor PressurePrevious Figure: VP as a function of T.1. The curves show all conditions of P and T where LIQ and VAP are in