Intermolecular Intermolecular Forces of Forces of AttractionAttraction

Attractive forces Attractive forces that cause atoms or that cause atoms or molecules to stick molecules to stick

togethertogether

MixturesMixturesElements or Elements or compounds blended compounds blended together but not together but not chemically chemically combinedcombined

MixturesMixtures Can be solid, liquid, or gaseous Can be solid, liquid, or gaseous

mixturesmixtures Solid mixture examples:Solid mixture examples:

Jewelry gold, most rocks, steel, alloysJewelry gold, most rocks, steel, alloys Liquid mixture examplesLiquid mixture examples

Beverages, sea water, solutionsBeverages, sea water, solutions Gaseous mixture examplesGaseous mixture examples

Air, car exhaustAir, car exhaust

MixturesMixtures Why do they form? What Why do they form? What

allows the parts to stay mixed? allows the parts to stay mixed? Why do they stay together? Why do they stay together?

The components of a mixture The components of a mixture are held together by are held together by IMF’sIMF’s – – intermolecular forces of intermolecular forces of attractionattraction

Intermolecular Intermolecular ForcesForcesAll intermolecular forces are:All intermolecular forces are:• attractive forcesattractive forces• betweenbetween molecules molecules• do not make do not make newnew compounds compounds• Makes the molecules “sticky”Makes the molecules “sticky”• weaker than true “bonds”weaker than true “bonds”

Types of IMF’sTypes of IMF’s London (dispersion) forcesLondon (dispersion) forces

all moleculesall molecules weakest interactionweakest interaction

dipole-dipole forcesdipole-dipole forces polar moleculespolar molecules

ion-dipole forcesion-dipole forces Between polar molecules and ionsBetween polar molecules and ions

hydrogen bondinghydrogen bonding H atoms w/ O, N, F not covalently bonded to itH atoms w/ O, N, F not covalently bonded to it strongest interactionstrongest interaction

Polar forcesPolar forces• ““Dipole - Dipole” interactionsDipole - Dipole” interactions• positive ends attract positive ends attract

negative ends of other negative ends of other moleculesmolecules

• about 1% as strong as a about 1% as strong as a covalent bondcovalent bond

Dipole - Dipole Dipole - Dipole interactionsinteractions

London forcesLondon forces• Momentary dipole-dipole Momentary dipole-dipole

interactioninteraction• ““instantaneous” dipole instantaneous” dipole

“induces” a dipole in a “induces” a dipole in a neighboring moleculeneighboring molecule

• brief attractive force brief attractive force resultsresults

Step 1Step 1

Step 2Step 2

Result...Result...

London forcesLondon forces• Strength depends on:Strength depends on:• size of electron cloudsize of electron cloud

– bigger = strongerbigger = stronger• number of atoms in moleculenumber of atoms in molecule

– more atoms = more more atoms = more electrons = strongerelectrons = stronger

Ion – dipole Ion – dipole forcesforces• Between polar molecules and ionsBetween polar molecules and ions

• Aqueous solutions of ionsAqueous solutions of ions• Positive ends of polar molecules Positive ends of polar molecules

attract negative ions attract negative ions • Negative ends attract positive Negative ends attract positive

ionsions• Only Only important in aqueous important in aqueous

solutionssolutions

Hydrogen Hydrogen BondsBonds• Special case of polar attractionSpecial case of polar attraction• H atom isH atom is

•1)bonded to high EN atom and 1)bonded to high EN atom and •2)attracted to a high EN atom it 2)attracted to a high EN atom it

is is notnot bonded to (N, O, F) bonded to (N, O, F)• 5 times stronger than regular 5 times stronger than regular

polar attractionspolar attractions

Hydrogen Hydrogen BondsBonds

Physical property Physical property effects of effects of IInterntermmolecular olecular FForces orces of of AAttractionttraction

……all come down to how all come down to how sticky the molecules are sticky the molecules are toward each other…toward each other…

The Key Idea…The Key Idea…

The stronger the The stronger the IMF’s, the tighter the IMF’s, the tighter the molecules cling to molecules cling to each other, the each other, the harder it is to harder it is to separate them.separate them.

The Key Idea…The Key Idea…

In other words, the In other words, the harder it is to harder it is to separate molecules, separate molecules, the stronger their the stronger their IMF’sIMF’s

What does that What does that mean…”separate the mean…”separate the molecules”?molecules”?

Pull them apart:Pull them apart:MeltingMeltingBoilingBoilingPhysically move them Physically move them apartapartPeel them away. Wipe Peel them away. Wipe them off, etc…them off, etc…

Boiling Point Boiling Point EffectsEffects

Formula Boilingpoint

Polar? IM force

HCl -85oC Polar Dipole-dipole

H2S -60.7oC Polar Dipole-dipole

H2O 100oC Polar H-bonding

Ar -185.7oC Nonpolar london

Boiling Point Boiling Point EffectsEffectsFormula Boiling

PointPolar? IM force

F2 -188.1oC Nonpolar London

Cl2 -34.6oC Nonpolar London

Br2 58.8oC Nonpolar London

I2 184.4oC Nonpolar London

Boiling Point Boiling Point EffectsEffectsFormula Boiling point Polar? IM force

CH4 -161.5oC Nonpolar London

C2H6 -88.6oC Nonpolar London

C3H8 -42.1oC Nonpolar London

C4H10 -0.5oC Nonpolar London

C5H12 36.1oC Nonpolar london

Intermolecular Intermolecular Forces of Forces of AttractionAttraction

Attractive forces Attractive forces that cause atoms or that cause atoms or molecules to stick molecules to stick

togethertogether

Types of IMF’sTypes of IMF’s London (dispersion) forcesLondon (dispersion) forces

all moleculesall molecules weakest interactionweakest interaction

dipole-dipole forcesdipole-dipole forces polar moleculespolar molecules

ion-dipole forcesion-dipole forces Between polar molecules and ionsBetween polar molecules and ions

hydrogen bondinghydrogen bonding H atoms w/ O, N, F not covalently bonded H atoms w/ O, N, F not covalently bonded

to itto it strongest interactionstrongest interaction

The Key Idea…The Key Idea…

The stronger the The stronger the IMF’s, the tighter the IMF’s, the tighter the molecules cling to molecules cling to each other, the each other, the harder it is to harder it is to separate them.separate them.

Solubility of a solid in Solubility of a solid in liquidliquid

General rule: General rule: Like dissolves LikeLike dissolves Like• Polar solutesPolar solutes dissolve in dissolve in polar solvents polar solvents

• SugarSugar (polar solute) in (polar solute) in HH22OO (polar solvent)(polar solvent)

• Ionic solidsIonic solids dissolve in dissolve in polar solventspolar solvents• NaCl NaCl (ionic solid) in (ionic solid) in HH22OO (polar solvent) (polar solvent)

• Nonpolar solutesNonpolar solutes dissolve in dissolve in nonpolar nonpolar solventssolvents

• Dried paintDried paint (nonpolar solute) in (nonpolar solute) in paint stripper paint stripper – turpentine– turpentine, etc., etc. (nonpolar solvent) (nonpolar solvent)

Colligative properties Colligative properties of Solutionsof Solutions-Properties of solutions that are affected -Properties of solutions that are affected

by the number of dissolved particles, by the number of dissolved particles, not the identity of the particlesnot the identity of the particles

Example: Example: vapor pressure loweringvapor pressure lowering

The more particles (with little tendency The more particles (with little tendency to vaporize) that are added to a to vaporize) that are added to a solvent, the lower the vapor pressure solvent, the lower the vapor pressure of the solution becomesof the solution becomes

Colligative properties Colligative properties of Solutionsof SolutionsDensity of a solutionDensity of a solutionThe presence of dissolved particlesThe presence of dissolved particles (whose (whose

mass is greater than the mass of the solvent mass is greater than the mass of the solvent particles)particles) in a solution causes the density in a solution causes the density of a solution to increaseof a solution to increase

• Sugar water is more dense than pure Sugar water is more dense than pure waterwater

• Sugary sodas are more dense than diet Sugary sodas are more dense than diet sodassodas• it takes more sugar molecules than it takes more sugar molecules than

artificial sweetener moleculesartificial sweetener molecules

Swimming in the Dead Swimming in the Dead SeaSea

Colligative properties Colligative properties of Solutionsof Solutions

Boiling point elevationBoiling point elevation::

The presence of dissolved particles tends The presence of dissolved particles tends to to increase the boiling pointincrease the boiling point of a liquid of a liquid

Salt added to water before boiling to Salt added to water before boiling to prepare food increases the prepare food increases the temperature the water boils at, and temperature the water boils at, and thus cooks the food faster, as the thus cooks the food faster, as the water is hotterwater is hotter

Colligative properties Colligative properties of Solutionsof SolutionsFreezing point depressionFreezing point depression

The presence of dissolved particles The presence of dissolved particles tends to tends to lower the freezing pointlower the freezing point of a of a liquidliquid

Salt or other substances are used to Salt or other substances are used to melt ice, because as they dissolve into melt ice, because as they dissolve into the ice, they lower the freezing point of the ice, they lower the freezing point of the ice, and thus even more of the ice the ice, and thus even more of the ice meltsmelts

CompressibilityCompressibility• gases are compressiblegases are compressible

– empty space between empty space between moleculesmolecules

– Weak IMF’s won’t keep Weak IMF’s won’t keep molecules togethermolecules together

• liquids, solids aren’tliquids, solids aren’t– closely packedclosely packed

DiffusionDiffusion• The intermingling of The intermingling of

molecules with each othermolecules with each other• gases = rapid gases = rapid much empty much empty

space space• liquids = slowly liquids = slowly more closely more closely

packed packed• solids = negligible solids = negligible locked in locked in

place place

Cohesion/AdhesionCohesion/Adhesion

Cohesion = molecules Cohesion = molecules sticking togethersticking together

Adhesion = molecules Adhesion = molecules sticking to a substratesticking to a substrate

ViscosityViscosity• The resistance of a The resistance of a fluid to flow.fluid to flow.

• The stronger the The stronger the IMF’s, the higher the IMF’s, the higher the viscosityviscosity

Surface TensionSurface Tension• Intermolecular attractions at the Intermolecular attractions at the

surface are only inward towards surface are only inward towards the substancethe substance

Surface TensionSurface Tension, , continuedcontinued• Molecules at the surface of a Molecules at the surface of a

liquid act as a “skin”liquid act as a “skin”• the greater the IMF’s, the the greater the IMF’s, the

more pronounced the effectmore pronounced the effect• liquids seek to lower their liquids seek to lower their

surface tensionsurface tension• water drops are sphericalwater drops are spherical

Surface TensionSurface Tension, , continuedcontinued

Surface WettingSurface Wetting

• The spreading of a liquid The spreading of a liquid across a surfaceacross a surface

• must have adhesion must have adhesion cohesioncohesion

GasolineGasoline – a nonpolar – a nonpolar hydrocarbonhydrocarbon

• Low surface tensionLow surface tension– only London forcesonly London forces

• low cohesionlow cohesion– spreads easily across most spreads easily across most

surfacessurfaces• does not effectively “wet” does not effectively “wet”

glassglass

HH22OO – – a polar molecule that H- a polar molecule that H-

bondsbonds

• High surface tensionHigh surface tension– hydrogen bondinghydrogen bonding

• High cohesive forcesHigh cohesive forces• High adhesive forcesHigh adhesive forces

– will H-bond with O’s in glasswill H-bond with O’s in glass• Good wetting agent for Good wetting agent for

glassglass

• Water beads on greasy Water beads on greasy glassglass– eliminates H-bonding to eliminates H-bonding to

glassglass– low adhesionlow adhesion

SurfactantsSurfactants, used in , used in soaps and detergents, soaps and detergents, drastically lower the drastically lower the surface tension of watersurface tension of water–soapy water spreads more soapy water spreads more easily over dirty surfaceseasily over dirty surfaces

VaporizationVaporization

• The change of a liquid The change of a liquid → gas → gas is called is called vaporizationvaporization• an example of separating an example of separating

moleculesmolecules• EvaporationEvaporation: happens at the : happens at the

exposed surface of a liquidexposed surface of a liquid• BoilingBoiling: happens within the : happens within the

liquid itselfliquid itself

EvaporationEvaporation• Liquids exert vapor pressure Liquids exert vapor pressure

due to the evaporated liquiddue to the evaporated liquid

• The The stronger the IMF’sstronger the IMF’s, the , the slower the evaporation rateslower the evaporation rate, , the the lower the vapor pressurelower the vapor pressure for a given temperaturefor a given temperature

EvaporationEvaporation

• The higher the T of a The higher the T of a substance, the more substance, the more molecules with enough energy molecules with enough energy to overcome cohesive to overcome cohesive forces/surface tension and forces/surface tension and evaporateevaporate

• As liquid T As liquid T , vapor P , vapor P

Minimum E to evaporate

Abundance

coolest warmest

Energy

Temperature is ~ the average KE of a sample

What happens...What happens...• To the evaporation rateTo the evaporation rate

– if the temperature = the if the temperature = the boiling point ?boiling point ?

• All the molecules, on All the molecules, on average, have enough average, have enough energy to vaporizeenergy to vaporize– the liquid the liquid boilsboils

What happens...What happens...• If the vapor pressure If the vapor pressure

equals the atmospheric equals the atmospheric pressure ?pressure ?

• All the molecules, on All the molecules, on average, have enough average, have enough energy to vaporizeenergy to vaporize–the liquid the liquid boilsboils

T vs Vapor T vs Vapor PressurePressure

Temperature

Vap Pressure

1.00 atm

100oC

How can you boil a How can you boil a liquid?liquid?1)1) Heat it to its boiling pointHeat it to its boiling point

vapor P = air Pvapor P = air P

2)2) Reduce the air P above itReduce the air P above it

to equal the vapor P of the to equal the vapor P of the liquid at that temperatureliquid at that temperature

• air P = vapor Pair P = vapor P all done!all done!

Intermolecular Intermolecular Forces of Forces of AttractionAttraction

Forces of attraction Forces of attraction between moleculesbetween molecules