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Intermolecular Intermolecular Forces, Gases, Forces, Gases, and Liquidsand Liquids

Ch.13Ch.13

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GasesGases

Kinetic-Molecular Theory says Kinetic-Molecular Theory says molecules/atoms separatedmolecules/atoms separated

Little, if any, interactionsLittle, if any, interactions Not so in solids and liquidsNot so in solids and liquids

Examples:Examples: Big difference in volume between liquids Big difference in volume between liquids

& solids and gases& solids and gases Gases compressible, liqs & solids notGases compressible, liqs & solids not

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Intermolecular ForcesIntermolecular Forces

Various electrostatic forces that Various electrostatic forces that attract molecules in solids/liqsattract molecules in solids/liqs

Much weaker than ionic forcesMuch weaker than ionic forces About 15% (or less) that of bond About 15% (or less) that of bond

energiesenergies Remember, ionic bonds extremely Remember, ionic bonds extremely

powerfulpowerful Boiling pt of NaCl = 1465 Boiling pt of NaCl = 1465 °C!°C!

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Intermolecular ForcesIntermolecular Forces

Reason behind importance of Reason behind importance of knowing about IMF:knowing about IMF:

1) b.p. & m.p. and heats of 1) b.p. & m.p. and heats of vaporization (lvaporization (lg) and fusion (sg) and fusion (sl)l)

2) solubility of gases, liquids, and 2) solubility of gases, liquids, and solidssolids

3) determining structures of 3) determining structures of biochemicals (DNA, proteins)biochemicals (DNA, proteins)

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Remember dipole Remember dipole moments?moments?

Dipole moment = product of Dipole moment = product of magnitude of partial charges (+magnitude of partial charges (+//-) -) & their distance of separation& their distance of separation

= (1 Debye = 3.34 x 10= (1 Debye = 3.34 x 10-30-30 C x m) C x m) Important in IMFImportant in IMF

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Ion-dipole: Ionization in Ion-dipole: Ionization in aqueous medium (water)aqueous medium (water)

1) stronger attraction if ion/dipole 1) stronger attraction if ion/dipole closercloser LiLi++ vs. Cs vs. Cs+ + in waterin water

2) higher ion charge, stronger 2) higher ion charge, stronger attractionattraction BeBe2+2+ vs. Li vs. Li++ in water in water

3) greater dipole, stronger attraction3) greater dipole, stronger attraction Dissolved salt has stronger attraction to Dissolved salt has stronger attraction to

water than methanolwater than methanol

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Solvation energySolvation energy

Or, enthalpy of hydration (if water) = Or, enthalpy of hydration (if water) = energy of ionization in aq. mediaenergy of ionization in aq. media

Water molecules surround both ionsWater molecules surround both ions Example:Example: Take hydration energies of G I metal ionsTake hydration energies of G I metal ions

Exothermicity decreases as you go down the Exothermicity decreases as you go down the columncolumn

Cations become largerCations become larger Easier to dissociateEasier to dissociate

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Permanent dipolesPermanent dipoles Positive end of one molecule attracted to Positive end of one molecule attracted to

negative end of othernegative end of other For ex: HClFor ex: HCl

Dipole-dipole attractionsDipole-dipole attractions

Cmpds that exhibit greater d-d attractions Cmpds that exhibit greater d-d attractions have higher b.p., and Hhave higher b.p., and Hvapvap

Polar cmpds exhibit greater d-d attractions Polar cmpds exhibit greater d-d attractions than non-polar cmpdsthan non-polar cmpds NHNH33 vs. CH vs. CH44 equivalent molar masses (g/mol): 17 vs. 16, equivalent molar masses (g/mol): 17 vs. 16,

respectivelyrespectively Boiling points: -33Boiling points: -33°C vs. -162°C, respectively°C vs. -162°C, respectively

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Hydrogen BondingHydrogen Bonding A type of “super” dipole-dipole interactionA type of “super” dipole-dipole interaction Interaction between eInteraction between e---rich atom connected to H -rich atom connected to H

entity & another H attached to eentity & another H attached to e——rich atomrich atom ee---rich atom = O, F, N-rich atom = O, F, N Density water Density water > than ice> than ice

Opposite of almost every other substanceOpposite of almost every other substance Inordinately high heat capacity of waterInordinately high heat capacity of water High surface tensionHigh surface tension

Insects walk on waterInsects walk on water Concave meniscusConcave meniscus

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Hydrogen BondingHydrogen Bonding

Boiling pts. of HBoiling pts. of H22O, HF, and NHO, HF, and NH33 muchmuch higher higher

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Surface TensionSurface Tension

Outer molecules interact with surface, Outer molecules interact with surface, while inner interact with other while inner interact with other moleculesmolecules

It has a “skin”It has a “skin” Skin toughness = Skin toughness = surface tensionsurface tension Energy required to break through Energy required to break through

surfacesurface Smaller surface area reason that Smaller surface area reason that

water drops sphericalwater drops spherical

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Capillary ActionCapillary Action When water goes up a small glass tubeWhen water goes up a small glass tube Due to polarity of Si-O bonding with Due to polarity of Si-O bonding with

waterwater Adhesive forces Adhesive forces > cohesive forces of > cohesive forces of

waterwater Creates a chain or bridgeCreates a chain or bridge Pulls water up tubePulls water up tube Limited by balancing gravity with Limited by balancing gravity with

adhesive/cohesive forcesadhesive/cohesive forces Thus, water has a concave Thus, water has a concave meniscusmeniscus

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MercuryMercury

Forms a convex meniscusForms a convex meniscus Doesn’t “climb” a glass tubeDoesn’t “climb” a glass tube Due to cohesive forces Due to cohesive forces > adhesive > adhesive

forcesforces

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ViscosityViscosity

Hydrogen-bonding increases viscosityHydrogen-bonding increases viscosity But large non-polar liquids like oil have:But large non-polar liquids like oil have: 1) large unwieldy molecules w/greater 1) large unwieldy molecules w/greater

intermolecular forcesintermolecular forces 2) greater ability to be entangled w/one 2) greater ability to be entangled w/one

anotheranother

Did you ever hear the expression, Did you ever hear the expression, “You’re as slow as molasses in January”?“You’re as slow as molasses in January”?

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Dipole/Induced Dipole Dipole/Induced Dipole ForcesForces

Polar entities induce dipole in nonpolar Polar entities induce dipole in nonpolar species like Ospecies like O22

OO22 can now dissolve in water can now dissolve in water If not, fishes in trouble!If not, fishes in trouble!

Process called “polarization”Process called “polarization” Generally, higher molar mass, greater Generally, higher molar mass, greater

polarizability of moleculepolarizability of molecule Why?Why? (larger the species, more likely e(larger the species, more likely e-- held held

further away further away easier to polarize) easier to polarize)

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Polarizability Polarizability

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Induced dipole/induced Induced dipole/induced dipole forcesdipole forces

Non-polar entities can cause temporary dipoles between Non-polar entities can cause temporary dipoles between other non-polar entitiesother non-polar entities

causing intermolecular attractionscausing intermolecular attractions Pentane, hexane, etc.Pentane, hexane, etc.

The higher the molar mass, the greater the intermolecular The higher the molar mass, the greater the intermolecular attractionsattractions

N-pentane has greater interactions than neo-pentaneN-pentane has greater interactions than neo-pentane Latter’s smaller area for interactionsLatter’s smaller area for interactions

II22 has a higher has a higher HHvapvap & b.p. than other halogens & b.p. than other halogens cause nonpolar substances to condense to liquids cause nonpolar substances to condense to liquids and to freeze into solids and to freeze into solids (when the temperature is lowered sufficiently)(when the temperature is lowered sufficiently) Also called: London Dispersion Forces Also called: London Dispersion Forces

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Intermolecular Bonding Intermolecular Bonding ComparedCompared

StrengthStrength Strongest: Ion-dipoleStrongest: Ion-dipole Strong: Dipole-dipole (incl. H-bonding)Strong: Dipole-dipole (incl. H-bonding) Less strong: dipole/induced-dipoleLess strong: dipole/induced-dipole Least strong: induced-dipole/induced-Least strong: induced-dipole/induced-

dipole (London dispersion forces)dipole (London dispersion forces)

Keep in mind Keep in mind a compound can have a compound can have more than one of the above!more than one of the above!

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Problem Problem

Rank the following in order of increasing boiling point and explain why:

NH3, CH4, and CO2

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Properties of LiquidsProperties of Liquids

(l) (l) (g) (g) Vaporization Vaporization

= = endothermicendothermic

Condensation Condensation = exothermic= exothermic

BoilingBoiling Why do we Why do we

have have bubbles?bubbles?

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Vapor PressureVapor Pressure Leave a bottle of water Leave a bottle of water

open…. open…. Will evaporateWill evaporate

Keep the lid on….Keep the lid on…. can have equilibrium can have equilibrium

between liquid and gasbetween liquid and gas Equilibrium vapor Equilibrium vapor

pressurepressure//vapor vapor pressurepressure Measure of tendency of Measure of tendency of

molecules to vaporize at molecules to vaporize at given temp.given temp.

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What does this graph tell What does this graph tell us?us?

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VolatilityVolatility

Ability of liquid to Ability of liquid to evaporateevaporate

Higher the vapor Higher the vapor pressure, greater pressure, greater the volatilitythe volatility

Are polar cmpds or Are polar cmpds or non-polar cmpds of non-polar cmpds of equal molecular equal molecular mass more mass more volatile?volatile?

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Clausius-Clapeyron Clausius-Clapeyron EquationEquation

Calculates ∆HCalculates ∆Hvapvap

What is this an What is this an equation for?equation for?

What are the What are the variables?variables?

C = constant C = constant unique to cmpdunique to cmpd

R = ideal gas R = ideal gas constantconstant 8.314472 J/mol8.314472 J/molKK

CT

1

R

H-PLn vap

vap

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Clausius-Clapeyron Clausius-Clapeyron EquationEquation

Or, if given two pts.:Or, if given two pts.:

)T

1

T

1(

R

H)

P

Pln(

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vap

1

2

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Clausius-Clapeyron Clausius-Clapeyron ProblemProblem

Methanol has a normal boiling point of Methanol has a normal boiling point of 64.6°C and a heat of vaporization of 35.2 64.6°C and a heat of vaporization of 35.2 kJ/mol. What is the vapor pressure of kJ/mol. What is the vapor pressure of methanol at 12.0°C?methanol at 12.0°C?

Does the answer make sense?Does the answer make sense? Would water have a higher heat of Would water have a higher heat of

vaporization?vaporization? Why?Why? Heat of vaporization of water = 40.65 kJ/molHeat of vaporization of water = 40.65 kJ/mol

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Boiling PointBoiling Point

Bp Bp temp. at which vapor pressure temp. at which vapor pressure = external (atmospheric pressure)= external (atmospheric pressure)

At higher elevations atmospheric At higher elevations atmospheric pressure is lowerpressure is lower Thus, water boils at less than 100 Thus, water boils at less than 100 °C°C

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Critical Temperature and Critical Temperature and PressurePressure

As temp. rises so does vapor pressure, but not As temp. rises so does vapor pressure, but not infinitelyinfinitely

At the At the critical point critical point liq/gas interface disappearsliq/gas interface disappears Critical temp/pressureCritical temp/pressure

TTcc/T/Tpp Gives Gives supercritical fluidsupercritical fluid

Density of a liqDensity of a liq Viscosity of gasViscosity of gas

HH22O: O: TTcc = 374 = 374 °C°C TTpp = 217.7 atm! = 217.7 atm!

Normal earth pressure Normal earth pressure 1 atm 1 atm

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Supercritical fluidSupercritical fluid

COCO22 used in used in decaffeinating decaffeinating coffeecoffee

Read about it on Read about it on page 614page 614

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Phase diagramPhase diagram Gives info on phase states of a Gives info on phase states of a

substance at varying pressures and substance at varying pressures and temperaturestemperatures

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Deciphering a phase Deciphering a phase diagramdiagram

Triple pointTriple point Where all 3 states Where all 3 states

coexistcoexist Curves denote Curves denote

existence of two existence of two statesstates Fusion (solid & liq)Fusion (solid & liq) Vaporization (liq & Vaporization (liq &

gas)gas) Sublimation (solid Sublimation (solid

& gas)& gas) Off the linesOff the lines

Single stateSingle state

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Water’s phase diagramWater’s phase diagram Graph explains why Graph explains why

water boils at lower water boils at lower temps at higher temps at higher altitudes (next slide)altitudes (next slide)

If you apply increasing If you apply increasing pressure (const. T of pressure (const. T of 0°C) to ice will it convert 0°C) to ice will it convert to water? to water?

Solid-liquid line has Solid-liquid line has negative slopenegative slope It’s the opposite of It’s the opposite of

most speciesmost species Why?Why?

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SublimationSublimation Going from solid to Going from solid to

gas without going gas without going through the liquid through the liquid statestate

Enthalpy of Enthalpy of sublimationsublimation HHsublimationsublimation

Iodine & dry ice Iodine & dry ice (solid CO(solid CO22) ) sublimatesublimate

Opposite of Opposite of sublimationsublimation Deposition (gDeposition (gs)s)

Iodine demoIodine demo

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COCO22’s Phase Diagram’s Phase Diagram

Explains Explains sublimationsublimation How?How?

Why is it Why is it called “dry called “dry ice”?ice”?

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Iodine’s Phase Diagram: Iodine’s Phase Diagram: But does it really But does it really

sublimate?sublimate?

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Problem Problem The normal melting and boiling points of xenon

are -112°C and -107°C, respectively. Its triple point is a -121°C and 0.371 atm and its

critical point is at 16.6°C and 57.6 atm.

a) Sketch the phase diagram for Xe, showing the axes, the four points given above, and indicating the area in which each phase is stable.

b) If Xe gas is cooled under an external pressure of 0.131 atm, will it undergo condensation or deposition?