Intermolecular Forces, Liquids, and Solids

Chapter 11 BLB 11thIntermolecular Forces, Liquids, and SolidsQuestions?Why does water have such anomalous properties?Why do you add salt to the water before cooking pasta?How can ice melt below 0C?How does antifreeze work?Answer: Solutions have different properties than pure solvents.11.2 Intermolecular ForcesThe attractive forces between molecules or between ions and moleculesMost properties of liquids and solids (and solutions) are due to the strength of the intermolecular forces present.Boiling pointMelting (freezing) pointHvapTypes of Intermolecular Forces:In order from strongest to weakest:Ion-dipole attraction of an ion and the partial charges of a polar molecule (hydration of ionic salts)Dipole-dipole attraction between two polar molecules (water, HF, alcohols)London Dispersion (induced dipole) instantaneous attraction between two nonpolar molecules due the polarizability of the electron cloud.Ion-dipole

Dipole-dipole

London dispersion

Hydrogen bondingSpecial type of dipole-dipole interactionOccurs between a H atom attached to an electronegative atom (N, O, F) on one molecule and a lone pair on an electronegative element on another molecule. (whew!)Common in water(!) and alcohols (COH) Very strong collectivelyHydrogen-bonding examples

Between same moleculeBetween different moleculesProtein structure(-helix)

DNA(double helix)Hydrogen-bonding between base pairs in DNA

Why is water so weird?High specific heat (only NH3 higher)High Hfus (only NH3 higher)Highest HvapHighest surface tensionHigher boiling point than expectedIce floats on water.

Answer: Strong network of hydrogen bondingHydrogen bonding in ice

Boiling Points (See Table 11.3)

11.3 Some Properties of LiquidsViscosity resistance to flowSurface tension a measure of the inward forces that must be overcome in order to expand the surface area of a liquidCohesive between moleculesAdhesive attraction between substance and a surface (glass water)Capillary action11.5 Vapor PressureIn an open container:liquid gas (evaporation)In a closed container:liquid gas (dynamic equilibrium)Vapor pressure pressure exerted by the vapor in equilibrium with the liquid or solid in a closed container at a given temperatureVapor Pressure

Vapor Pressure, cont.As T , vapor pressure .When vapor pressure = atmospheric pressure, boiling occurs.When pressure = 1 atm: normal boiling pointVolatile substances have a higher vapor pressure.Vapor Pressure

11.6 Phase DiagramsShow graphically the relationship between the different phases of a substance.Solid, liquid, gas regionsNormal melting and boiling pointsTriple pointCritical pointSlope of solid-liquid lineGeneric Phase Diagram

H2O vs. CO2

H2O