Ch 11: Intermolecular Forces and Types of Solids Brown, LeMay AP Chemistry Monta Vista High School.

Ch 11: Intermolecular Forces and Types of Solids

Brown, LeMayAP ChemistryMonta Vista High School

Inter-Inter-moleculamolecular Forcesr Forces

Inter-Inter-moleculamolecular Forcesr Forces

Have studied Have studied INTRAINTRAmolecular molecular forces—the forces holding forces—the forces holding atoms together to form atoms together to form molecules.molecules.

Now turn to forces between Now turn to forces between molecules —molecules —

INTERINTERmolecular forces. molecular forces.

Forces between molecules, Forces between molecules, between ions, or between between ions, or between molecules and ions.molecules and ions.

311.1: Intermolecular Forces (IMF)

IMF < intramolecular forces (covalent, metallic, ionic bonds)

IMF strength: solids > liquids > gases Boiling points and melting points are good

indicators of relative IMF strength.

Summary of Intermolecular Summary of Intermolecular ForcesForcesSummary of Intermolecular Summary of Intermolecular ForcesForces

• Ion-Ion forcesIon-Ion forces• Ion-dipole forcesIon-dipole forces•Dipole-dipole forcesDipole-dipole forces

– Special dipole-dipole force: Special dipole-dipole force: hydrogen bonds (sometimes hydrogen bonds (sometimes treated as a separate IMF)treated as a separate IMF)

•Forces involving non polar Forces involving non polar molecules: molecules: induced forces induced forces (LDFs)(LDFs)

Intermolecular Forces SummaryIntermolecular Forces Summary

611.2: Types of IMF1. Electrostatic forces: act over larger

distances in accordance with Coulomb’s law

a. Ion-ion forces: strongest; found in ionic crystals (i.e. lattice energy)

2d

QQF

http://chemmovies.unl.edu/ChemAnime/IONSIZED/IONSIZED.html Ion size and LE

7b. Ion-dipole: between an ion and a dipole (a neutral, polar molecule/has separated partial charges) Increase with increasing polarity of

molecule and increasing ion charge.

2d

QQF

Cl-

S2-<

Ex: Compare IMF in Cl- (aq) and S2- (aq).

http://chemmovies.unl.edu/ChemAnime/NACL1D/NACL1D.htmlNaCl dissolving in Water

Attraction Between Attraction Between Ions and Permanent Ions and Permanent DipolesDipoles


Water is highly polar Water is highly polar and can interact and can interact with positive ions to with positive ions to

give give hydratedhydrated ions in water.ions in water.

HH

water dipole

••

••

O-

+

Attraction between ions and dipole depends Attraction between ions and dipole depends on on ion chargeion charge and and ion-dipole distanceion-dipole distance..

Measured by ∆H for MMeasured by ∆H for Mn+n+ + H + H22O O ff [M(H [M(H22O)O)xx]]n+n+



10c. Dipole-dipole: weakest electrostatic force (Not all IMFs, LDFs weaker than dipole-dipole); exist between neutral polar molecules Increase with increasing

polarity (dipole moment) of molecule

Ex: What IMF exist in NaCl (aq)?

Dipole-Dipole ForcesDipole-Dipole ForcesDipole-Dipole ForcesDipole-Dipole Forces

Influence of dipole-dipole forces is seen in Influence of dipole-dipole forces is seen in the boiling points of simple molecules.the boiling points of simple molecules.

CompdCompd Mol. Wt.Mol. Wt. Boil PointBoil Point

NN22 2828 -196 -196 ooCC

COCO 2828 -192 -192 ooCC

BrBr22 160160 59 59 ooCC

IClICl 162162 97 97 ooCC

Partner Activity

• Discuss with your partner the difference between ion-dipole and dipole-dipole interactions, in terms of the following:

- How they are formed- Strength- Examples

12

13

d. Hydrogen bonds (or H-bonds): H is unique among the elements

because it has a single e- that is also a valence e-.

– When this e- is “hogged” by a highly EN atom (a very polar covalent bond), the H nucleus is partially exposed and becomes attracted to an e--rich atom nearby.

http://www.youtube.com/watch?v=LGwyBeuVjhU

14

H-bonds form with H-X•••X', where X and X' have high EN and X' possesses a lone pair of e-

X = F, O, N (since most EN elements) on two molecules:

F-H

O-H

N-H

:F

:O

:N

Hydrogen BondingHydrogen BondingHydrogen BondingHydrogen Bonding

A special form of dipole-dipole attraction, A special form of dipole-dipole attraction, which enhances dipole-dipole attractions.which enhances dipole-dipole attractions.

H-bonding is strongest when X and Y are N, O, or FH-bonding is strongest when X and Y are N, O, or F

16 H-bonds explain why ice is less dense than water.

http://en.wikipedia.org/wiki/Water_%28molecule%29#Density_of_water_and_ice

17

Ex: Boiling points of nonmetal hydrides

Boili

ng P

oin

ts (

ºC)

Conclusions:

Polar molecules have higher BP than nonpolar molecules

∴ Polar molecules have stronger IMF

BP increases with increasing MW

∴ Heavier molecules have stronger IMF

NH3, H2O, and HF have unusually high BP.

∴ H-bonds are stronger than dipole-dipole IMF

H-Bonding Between H-Bonding Between Methanol and WaterMethanol and WaterH-Bonding Between H-Bonding Between Methanol and WaterMethanol and Water

H-bondH-bondH-bondH-bond--

++

--

H-Bonding Between Two H-Bonding Between Two Methanol MoleculesMethanol MoleculesH-Bonding Between Two H-Bonding Between Two Methanol MoleculesMethanol Molecules

H-bondH-bondH-bondH-bond

--++

--

Hydrogen Bonding in HHydrogen Bonding in H22OOHydrogen Bonding in HHydrogen Bonding in H22OO

H-bonding is especially H-bonding is especially strong in water strong in water becausebecause

• the O—H bond is the O—H bond is very polarvery polar

• there are 2 lone pairs there are 2 lone pairs on the O atomon the O atom

Accounts for many of Accounts for many of water’s unique water’s unique properties.properties.

http://www.visionlearning.com/library/flash_viewer.php?oid=1435&mid=120Animation of Ice


Ice has open Ice has open lattice-like lattice-like structure.structure.

Ice density is Ice density is < liquid.< liquid.

And so solid And so solid floats on floats on water.water.

Snow flake: www.snowcrystals.com


Ice has open lattice-like structure.Ice has open lattice-like structure.

Ice density is < liquid and so solid floats on Ice density is < liquid and so solid floats on water.water.

One of the VERY few One of the VERY few substances where solid is substances where solid is LESS DENSE than the LESS DENSE than the liquid.liquid.

http://www.visionlearning.com/library/flash_viewer.php?oid=1380&mid=57H bonding in Water

Hydrogen BondingHydrogen BondingHydrogen BondingHydrogen Bonding

H bonds leads to H bonds leads to abnormally high abnormally high boiling point of water.boiling point of water.

See Screen 13.7See Screen 13.7

PLAY MOVIE

Boiling Points of Boiling Points of Simple Hydrogen-Simple Hydrogen-Containing Containing CompoundsCompounds

See Active Figure 12.8See Active Figure 12.8

Methane Methane HydrateHydrateMethane Methane HydrateHydrate

Hydrogen Bonding in Hydrogen Bonding in BiologyBiologyHydrogen Bonding in Hydrogen Bonding in BiologyBiology

H-bonding is especially strong in biological H-bonding is especially strong in biological systems — such as DNA. systems — such as DNA.

DNA — helical chains of phosphate groups DNA — helical chains of phosphate groups and sugar molecules. Chains are helical and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, because of tetrahedral geometry of P, C, and O.and O.

Chains bind to one another by specific Chains bind to one another by specific hydrogen bonding between pairs of Lewis hydrogen bonding between pairs of Lewis bases.bases.

——adenine with thymineadenine with thymine

——guanine with cytosineguanine with cytosine

Portion of a Portion of a DNA chainDNA chain

Double helix Double helix of DNAof DNA

Base-Pairing through H-BondsBase-Pairing through H-BondsBase-Pairing through H-BondsBase-Pairing through H-Bonds

Hydrogen Bonding in Hydrogen Bonding in BiologyBiologyHydrogen Bonding in Hydrogen Bonding in BiologyBiology

Hydrogen bonding and base pairing in DNA.Hydrogen bonding and base pairing in DNA.

H Bonding Activity

With your elbow partner, draw the following on the same sheet of paper taking turns:

1.Water Molecule2.Dipole of this water molecule3.Another water molecule4.Hydrogen Bonding Between these

molecules5.Structure of Ice6.Reflect on your beautiful drawings and

give each other high fives.

30

31 * There is no strict cutoff for the ability to form H-bonds (S forms a biologically important hydrogen bond in proteins).

* Hold DNA strands together in double-helix

Nucleotide pairs form H-bonds

DNA double helix

322. Inductive forces: Arise from distortion of the e- cloud

induced by the electrical field produced by another particle or molecule nearby.

London dispersion: between polar or nonpolar molecules or atoms– * Proposed by Fritz London in 1930– Must exist because nonpolar molecules

form liquids

Fritz London(1900-1954)

33How they form:1. Motion of e- creates an instantaneous

dipole moment, making it “temporarily polar”.

2. Instantaneous dipole moment induces a dipole in an adjacent atom• * Persist for about 10-14 or 10-15 second

Ex: two He atoms

FORCES INVOLVING FORCES INVOLVING INDUCED DIPOLESINDUCED DIPOLESFORCES INVOLVING FORCES INVOLVING INDUCED DIPOLESINDUCED DIPOLESHow can non-polar molecules such as OHow can non-polar molecules such as O2 2 and Iand I22 dissolve in water? dissolve in water?

The water dipole The water dipole INDUCESINDUCES a dipole in a dipole in the Othe O22 electric cloud. electric cloud.

Dipole-induced Dipole-induced dipoledipole

Dipole-induced Dipole-induced dipoledipole

http://antoine.frostburg.edu/chem/senese/101/liquids/faq/h-bonding-vs-london-forces.shtmlDipole-Dipole and LDFs

FORCES INVOLVING FORCES INVOLVING INDUCED DIPOLESINDUCED DIPOLESFORCES INVOLVING FORCES INVOLVING INDUCED DIPOLESINDUCED DIPOLES

Solubility increases with mass the gasSolubility increases with mass the gas


• Process of inducing Process of inducing a dipole is a dipole is polarizationpolarization

• Degree to which Degree to which electron cloud of an electron cloud of an atom or molecule atom or molecule can be distorted in can be distorted in its its polarizabilitypolarizability..

IM FORCES — INDUCED DIPOLESIM FORCES — INDUCED DIPOLESIM FORCES — INDUCED DIPOLESIM FORCES — INDUCED DIPOLES

Consider IConsider I22 dissolving dissolving in ethanol, in ethanol, CHCH33CHCH22OH.OH.

OH

-

+

I-I

R-

+

OH

+

-

I-I

R

The alcohol The alcohol temporarily temporarily creates or creates or INDUCESINDUCES a a dipole in Idipole in I22..


Formation of a dipole in two nonpolar IFormation of a dipole in two nonpolar I22 molecules. molecules.

Induced dipole-Induced dipole-induced dipoleinduced dipoleInduced dipole-Induced dipole-induced dipoleinduced dipole

http://chemmovies.unl.edu/ChemAnime/LONDOND/LONDOND.htmlLDFs


The induced forces between IThe induced forces between I22 molecules are molecules are

very weak, so solid Ivery weak, so solid I22 sublimessublimes (goes (goes from a solid to gaseous molecules).from a solid to gaseous molecules).

PLAY MOVIE

Intermolecular ForcesIntermolecular Forces

See Figure 12.12See Figure 12.12

LiquidsLiquidsSection 12.4Section 12.4

LiquidsLiquidsSection 12.4Section 12.4

In a liquidIn a liquid•• molecules are in molecules are in

constant motionconstant motion

•• there are appreciable there are appreciable intermolec. forcesintermolec. forces

•• molecules close molecules close togethertogether

•• Liquids are almost Liquids are almost incompressibleincompressible

•• Liquids do not fill the Liquids do not fill the containercontainer

PLAY MOVIE

LiquidsLiquidsThe two key properties we need to The two key properties we need to

describe are describe are EVAPORATIONEVAPORATION and its and its opposite—opposite—CONDENSATIONCONDENSATION

The two key properties we need to The two key properties we need to describe are describe are EVAPORATIONEVAPORATION and its and its opposite—opposite—CONDENSATIONCONDENSATION

break IM bonds

make IM bonds

Add energy

Remove energy

LIQUID VAPOR

r r condensationcondensation

Evaporation Evaporation ff

Liquids—Liquids—EvaporationEvaporation

To evaporate, molecules To evaporate, molecules must have sufficient must have sufficient energy to break IM forces.energy to break IM forces.

Breaking IM forces Breaking IM forces requires energy. The requires energy. The process of evaporation process of evaporation is is endothermicendothermic..

Liquids—Liquids—Distribution of EnergiesDistribution of Energies

Distribution of Distribution of molecular molecular energies in energies in a liquid.a liquid.

KE is propor-KE is propor-tional to T.tional to T.

Distribution of Distribution of molecular molecular energies in energies in a liquid.a liquid.

KE is propor-KE is propor-tional to T.tional to T.

0

Num

ber

of m

olec

ules

Molecular energy

higher Tlower T

See Figure 12.13See Figure 12.13

Minimum energy req’d to break IM forces and evaporate

Vapor PressureVapor Pressure

Equilibrium Vapor PressureEquilibrium Vapor Pressure

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/vaporv3.swfVapor Pressure

In Class Activity

• Your teacher will explain

47

LiquidsLiquidsHEAT OF VAPORIZATIONHEAT OF VAPORIZATION is the heat is the heat

req’d (at constant P) to vaporize the liquid.req’d (at constant P) to vaporize the liquid.

LIQ + heat LIQ + heat ff VAP VAP

Compd.Compd. ∆∆vapvapH (kJ/mol) H (kJ/mol) IM ForceIM Force

HH22OO 40.7 (100 40.7 (100 ooC)C) H-bondsH-bonds

SOSO22 26.8 (-47 26.8 (-47 ooC)C) dipoledipole

XeXe 12.6 (-107 12.6 (-107 ooC)C) induced induced dipole dipole

Equilibrium Vapor Pressure & Equilibrium Vapor Pressure & the Clausius-Clapeyron Equationthe Clausius-Clapeyron Equation

• Clausius-Clapeyron equation — Clausius-Clapeyron equation —

used to find ∆used to find ∆vapvapH˚.H˚.

• The logarithm of the vapor The logarithm of the vapor

pressure P is proportional to pressure P is proportional to

∆∆vapvapH and to 1/T.H and to 1/T.

• ln P = –(∆ln P = –(∆vapvapH˚/RT) + CH˚/RT) + C

ln P2

P1

= ĘvapH

R

1

T1

- 1

T2

Surface TensionSurface Tension

SURFACE TENSIONSURFACE TENSION also leads to spherical also leads to spherical liquid droplets.liquid droplets.

SURFACE TENSIONSURFACE TENSION also leads to spherical also leads to spherical liquid droplets.liquid droplets.

5111.3: Properties resulting from IMF

1. Viscosity: resistance of a liquid to flow

2. Surface tension: energy required to increase the surface area of a liquid

LiquidsLiquidsIntermolec. forces also lead to Intermolec. forces also lead to CAPILLARYCAPILLARY

action and to the existence of a concave action and to the existence of a concave meniscus for a water column.meniscus for a water column.

concavemeniscus

H2O in

glasstube

ADHESIVE FORCESbetween waterand glass

COHESIVE FORCESbetween watermolecules

Capillary ActionCapillary Action

Movement of water up a piece of Movement of water up a piece of paper depends on H-bonds between paper depends on H-bonds between HH22O and the OH groups of the O and the OH groups of the cellulose in the paper.cellulose in the paper.

PLAY MOVIE

543. Cohesion: attraction of molecules for other molecules of the same compound

4. Adhesion: attraction of molecules for a surface

55

5. Meniscus: curved upper surface of a liquid in a container; a relative measure of adhesive and cohesive forcesEx:

Hg H2O(cohesion rules) (adhesion rules)

56* Geckos!

Geckos’ feet make use of London dispersion forces to climb almost anything. A gecko can hang on a glass

surface using only one toe.

Researchers at Stanford University recently developed a gecko-like robot which uses synthetic setae to climb walls

http://www.visionlearning.com/library/module_viewer.php?mid=57Jesus Lizard

http://en.wikipedia.org/wiki/Van_der_Waals%27_force

57London dispersion forces (induced dipole-induced dipole) increase with:

Increasing MW, # of e-, and # of atoms (increasing # of e- orbitals to be distorted)Boiling points:

Effect of MW: Effect of # atoms:pentane 36ºC Ne –246°C hexane 69ºC CH4 –162°Cheptane 98ºC

??? effect:H2O 100°C

D2O 101.4°C

“Longer” shapes (more likely to interact with other molecules)

C5H12 isomers: 2,2-dimethylpropane 10°C pentane

36°C

Summary of IMF

Van der Waals forces

59Ex: Identify all IMF present in a pure sample of each substance, then explain the boiling points.

BP(⁰C)

IMF Explanation

HF 20

HCl -85

HBr -67

HI -35

Lowest MW/weakest London, but most

polar/strongest dipole-dipole and has H-bonds

Low MW/weak London, moderate polarity/dipole-

dipole and no H-bonds

Medium MW/medium London, moderate

polarity/dipole-dipole and no H-bonds

Highest MW/strongest London, but least polar bond/weakest dipole-dipole and no H-bonds

London, dipole-dipole, H-bonds

London, dipole-dipole



6011.4: Phase ChangesProcesses: Endothermic: melting,

vaporization, sublimation Exothermic:

condensation, freezing, deposition

I2 (s) and (g)

Microchip

61Water: Enthalpy diagram or heating curve

J/g) 334(mQ

TmQ )CJ/g 4.18(

TmQ )CJ/g 87.1(

TmQ )CJ/g 06.2(

J/g) 2602(mQ

TmcQ mHQ

6211.5: Vapor pressure

A liquid will boil when the vapor pressure equals the atmospheric pressure, at any T above the triple point.

Pressure cooker ≈ 2 atm

Normal BP = 1 atm

10,000’ elev ≈ 0.7 atm

29,029’ elev (Mt. Everest) ≈ 0.3 atm

6311.6: Phase diagrams: CO2

Lines: 2 phases exist in equilibrium

Triple point: all 3 phases exist together in equilibrium (X on graph)

Critical point, or critical temperature & pressure: highest T and P at which a liquid can exist (Z on graph)

For most substances, inc P will cause a gas to condense (or deposit), a liquid to freeze, and a solid to become more dense (to a limit.)

Temp (ºC)

64Phase diagrams: H2O

• For H2O, inc P will cause ice to melt.

65

*

66

*

Group Activity 67

Get in groups of four.Two people will need to draw and two will need to explainChoose roles.First drawer draws a phase diagram of water.Second drawer draws a phase diagram of CO2Third member explains the water’s phase diagramFourth member explains CO2’s phase diagram.

6811.7-8: Structures of solids

Amorphous: without orderly structureEx: rubber, glass

Crystalline: repeating structure; have many different stacking patterns based on chemical formula, atomic or ionic sizes, and bonding

Types of crystalline solids (Table 11.6)

Type Particles ForcesNotable

propertiesExample

s

Atomic AtomsLondon

dispersion

Poor conductors

Very low MP

Ar (s),Kr (s)

Molecular

Molecules

(polar or non-

polar)

London dispersion, dipole-

dipole, H-bonds

Poor conductors

Low to moderate MP

CO2 (s),

C12H22O11,

H2O (s)

SucroseCarbon dioxide (dry ice)

Ice

Ionic

Anions and

cations

Electrostatic attractions

Hard & brittle

High MPPoor conductors

Some solubility in H2O

NaCl,Ca(NO3)2

Covalent (a.k.a.

covalent network)

Atoms bonded

in a covalent network

Covalent bonds

Very hardVery high MP

Generally insoluble

Variable conductivity

C (diamond

& graphite)

SiO2

(quartz)

Ge, Si, SiC, BN

DiamondGraphite SiO2

Metallic

Metal cations in a diffuse, delocalized e- cloud

Metallic bonds

Excellent conductors

MalleableDuctileHigh but wide range of MP

Cu, Al, Fe

Ch 11: Intermolecular Forces and Types of Solids Brown, LeMay AP Chemistry Monta Vista High School.

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