Chapter 12 Intermolecular Attractions and the Properties of Liquids and Solids

Gases, Liquids and Solids

Intermolecular Attractions Intermolecular forces depend on distance –Gases have very small attractive forces –Solids/liquids have larger attractive forces since molecules are closer to each other Intermolecular forces - attractions between two molecules Intramolecular forces - chemical bonds that hold molecule together Intermolecular forces weaker than Intramolecular forces

van der Waals’ Forces HCl molecules –H and Cl atoms held tightly by covalent bond –Strength of chemical bond keeps molecule intact –Attractions between HCl molecules are weaker (4% as strong) –Attraction between molecules determine physical properties –Notice disorientation!

Dipole-dipole attractions HCl(g) - polar molecule with partial charges Polar molecules tend to line up so partial negative and near partial positive Still net attraction!! (Dipole-dipole!) Why weak? –Charges associated are only partial charges –Ordinary temperatures (Thermal energy) causes the dipoles to be somewhat misaligned reducing effectiveness of attractions

Hydrogen Bonds Important Dipole-dipole attraction when hydrogen bonds to very small, highly electronegative atom Think FON (HF, OH, and NH) Why Hydrogen bonding? –Ends of bond contain substantial positive and negative charges –Charges highly concentrated due to small size –Positive ends can get very close to negative of another molecule due to small size

Hydrogen Bonds in Water In Liquid water - molecules experience hydrogen bonds that continually break and re-form As water freezes, molecules become locked and participate in 4 hydrogen bond Resulting structure has larger volume than liquid water Ice cubes float in more dense liquid

London Dispersion forces Nonpolar molecules still have attraction (although weak) to hold substance together Fritz London, German Scientist –Nonpolar substances can still have attraction –Atoms constantly moving –Motion in one particle affects neighboring particles –Electrons repel and push away –At any given moment, the electron density of molecule can be unsymmetrical –At particular instant, instantaneous dipole!

Induced Dipoles As instantaneous dipole forms, causes electron density of neighbor to be unsymmetrical Also forms a dipole (called INDUCED DIPOLE) Always causes positive of one to be near negative of another

Very short lived attraction Dipoles vanish as they are formed but will form in other location Over period of time, there is a net, overall attraction but relatively weak Called London dispersion forces or instantaneous dipole-induced dipole attractions –Distinguished from permanent dipole- dipole

Strengths of London forces Measure using boiling point Polarizability –Measure of the ease the electron cloud is distorted –As volume of electron cloud increases, polarizability increases –As atom size increase, higher London forces Number –For molecules containing same elecments, London forces increase with number of atoms –BP hexane > BP propane Molecular Shape

Intermolecular forces and tightness of packing affect the properties of liquids and solids Compressibility and diffusion depend primarily on tightness of packing Most physical properties depend primarily on strengths of intermolecular attractions Rate of evaporating depends on surface area, temperature, and strengths of intermolecular attractions

Compressibility and Diffusion Compressibility - measure of the ability of substance to be forced into smaller volume Incompressible –Solids and liquids have no empty volume Diffusion –Quick in gases –Slow in liquids –Almost nonexistent in solids

Surface tension A property related to the tendency of a liquid to seek a shape that yields the minimum surface area The shape with minimum surface area is sphere Why? –Molecules within liquid surrounded by densely packed molecules –Whereas surface molecules have neighbors beside and below it –Surface molecules are attracted to fewer neighbors

A molecule at the surface has a higher potential energy than a molecule in the bulk of the liquid Remember a system becomes more stable when its potential energy decreases For a liquid, reducing surface area (reducing the number molecules at surface area) lowers potential energy Lowest energy achieved when liquid has smallest surface area Surface tension of a liquid is proportional to energy needed to expand surface area

Surface tension every day Water in rim “Invisible Skin” Soap, Pepper and Water

Wetting of surface by a liquid Wetting - spreading of liquid across a surface to form a thin film –To occur, the intermolecular attractive forces between the liquid and the surface must be of about the same strength as forces within liquid itself –Think glass coated –SURFACTANTS - drastically lower the surface tension of water –Water is “wetter”

Viscosity Viscosity - resistance to a change in form of a liquid “Internal friction” of material Factors –Temperature (Temp decreases; viscosity increases) –Molecular size –Tangling –Attractions –Acetone vs. Ethlyene glycol

Evaporation and sublimation Important factor: Change of State! Evaporation - for liquid, tendency to undergo change to gas Sublimation - solid to gas change of state

Evaporation and cooling Evaporation causes cooling effect Rate of evaporation per unit of surface area of a given liquid is greater at a higher temperature The weaker the intermolecular attractive forces, the faster the rate of evaporation at a given temperature

Change of state - substance is transformed from one physical state to another Physical equilibrium similar to chemical equilibrium Evaporation and condensation (change of vapor to liquid) cause equilibrium –Evaporation increases number of molecules in vapor –Condensation decreases number of molecules in vapor Melting Point - solid to liquid

Vapor Pressure Vapor pressure - the pressure that molecules exert when a liquid evaporates Equilibrium vapor pressure –Occurs in closed container –Rates of evaporation and condensation are equal –Concentrations of molecules in vapor remains constant and the vapor exerts constant pressure

Factors that affect Vapor Pressure VP is solely function of evaporation per unit area of liquid’s surface If rate large –Large conc of molecules in vapor state necessary for eq. –Also, VP is high, and evaporation rate high –As temp increases, rate and VP increase –Can use VP as indication of relative strengths of attractive forces in liquids

Boiling Point How would you check for boiling? –Increasing heat just increases amt of bubbles Any pure liquid remains at constant temperature at boiling point Why do liquids boil? –Bubbles contain vapor!! –As liquid evaporates, pressure pushes –Opposing force is pressure of atmosphere The temp at which vp of liquid is equal to prevailing atmospheric pressure

Why does water boil at lower temp in Denver than NY? Normal boiling point - boiling point of liquid at 1 atm Relates to intermolecular attractions –When attractive forces are strong, the liquid has low vp and therefore, must be heated to higher temp –High boiling points result from strong intermolecular attractions

Heating and cooling curves Heating Curve Application Heat at constant rate Diagram

Le Châtelier’s Principle When a dynamic equilibrium in a system is upset by a disturbance, the system responds in a direction that tends to counteract the disturbance and, if possible, restore equilibrium Heat + Liquid  vapor Position of equilibrium

Phase Diagrams Graphical representation of phase equilibria Triple point - all three phases exist Critical point

Phase Diagrams for Water