Chapter 12

Intermolecular Forces:

Liquids, Solids, and Phase Changes

If you are doing this lecture “online” then print the lecture notes available as a word document, go through this ppt lecture, and do all the example and practice assignments for discussion time.

Why do gases differ from liquids & solids?Why do gases differ from liquids & solids?

Gases are tiny particles far apart with no attraction for each other Gases are tiny particles far apart with no attraction for each other (ideally) and they are moving rapidly in random directions(ideally) and they are moving rapidly in random directions

Gases obey a set of laws: Ideal Gas LawsGases obey a set of laws: Ideal Gas Laws

But liquids & solids But liquids & solids don'tdon't have a set of "laws" because.... have a set of "laws" because....

Liquids: - condensed from gasesLiquids: - condensed from gases - not compressible therefore not much space- not compressible therefore not much space between moleculesbetween molecules -moving randomly but more slowly, are attracted-moving randomly but more slowly, are attracted to each other!to each other!

Solids: - ordered fixed in place particlesSolids: - ordered fixed in place particles - close together- close together - strong forces of attraction!- strong forces of attraction!

Interparticle ForcesInterparticle ForcesInterparticle forces of attraction between "particles" that

affect physical state & physical behaviorNote: particles can be ions, atoms or molecules

True chemical bonding forces are intra-particle forces: within a chemical substanceThey are atom to atom or ion to ion, not molecule to molecule: ionic bonding, metallic bonding, covalent bonding, and (new to us) network covalent bonding

Interparticle forces are between particles, not within themSpecial group of interparticle forces between molecules

and atoms:- called intermolecular forces- affect behavior of covalent compounds

Next slide is table from your packet of handouts

SUMMARY OF INTERPARTICLE FORCES INTERPARTICLE STRENGTH PREDICTED FORCE PARTICLES DESCRIPTION kJ/mol MP/BP EXAMPLES Covalent atoms sharing of e-s strong high/high H2O (0,100) Bonding between two atoms Network Covalent atoms sharing of e-s very strong very high diamond (35530,?) Bonding betw two atoms extended Metallic Bonding atoms sharing of e-s in a sea strong high iron (1555, 3000) Between all atoms Ionic Bonding cations & electrostatic attraction 400-4000 high NaCl (804, 1413) anions between opposite charges Hydrogen Bonding* polar molecules attraction between H on 15-40 medium H2O with H bonded to one molecule to F, O or N F, O or N on other molecules Dipole-Dipole polar molecules attraction between slightly 5-25 low/medium (CH3)2C=O (-95, 56) Attraction opposite charges London Dispersion all atoms, ions attraction between induced 0.05-40 low/medium I2 (114, 183) Forces and molecules dipole opposite charges Ion-Dipole ions and polar attraction between 40-600 na NaCl(aq) Attraction molecules opposite charges Dipole-Induced polar and nonpolar attraction between dipole 2-10 na O2 in H2O Dipole Attraction molecules and induced dipole *Not really bonding, just an unfortunate choice for naming this intermolecular force of attraction. Italics indicate the three Intermolecular Forces.

Network Covalent Atom-Atom like cov bond500+ Diamond

ADD:

Properties

High MP, brittle solid

Low MP, soft when solid

Range of MPs, malleable

Very high MP, very hard solid

Network Covalent BondingNetwork Covalent Bonding

Make special note of the network covalent Make special note of the network covalent solids that I added to Table 12.2: solids that I added to Table 12.2:

- covalent bonds are extended throughout - covalent bonds are extended throughout the crystal solidthe crystal solid

- diamond and SiC, also SiO- diamond and SiC, also SiO22

Diamond is fcc with 4 C's in "holes" in unit Diamond is fcc with 4 C's in "holes" in unit cell, which we see later in chaptercell, which we see later in chapter

Graphite has unique structure (see diagram) Graphite has unique structure (see diagram)

**“True” Intermolecular Forces in pure substances

**

**

**

Ion-Dipole Attraction:Ion-Dipole Attraction:

Interparticle forces involved in dissolving ionic Interparticle forces involved in dissolving ionic solids in water or other polar solvents: solids in water or other polar solvents: Ion-dipole attraction has to overcome ion-ion Ion-dipole attraction has to overcome ion-ion attraction in a solid’s attraction in a solid’s crystal latticecrystal latticeThis is why some compounds are soluble and This is why some compounds are soluble and some are notsome are not

Called Called HydrationHydration of an ion: typically endothermic of an ion: typically endothermic - takes energy to pull ions apart- takes energy to pull ions apart

Some energy is gained back as Heat of Hydration Some energy is gained back as Heat of Hydration - polar water molecules orient themselves and - polar water molecules orient themselves and surround the individual ionssurround the individual ions

More detail coming soon in Chapter 13More detail coming soon in Chapter 13

The Major InterMolecularThe Major InterMolecular (IM) Forces (IM) ForcesThe three major IM Forces are very weak to

moderate forces of attraction between molecules (or atoms)

- London (dispersion) forces (LDF)- Dipole-dipole attractions- Special case of enhanced dipole

attraction called Hydrogen-bonding- Dipole-induced dipole attraction,

between different types of molecules or atoms

London Dispersion Forces London Dispersion Forces (LDF)(LDF)"Instantaneous dipole" causes neighboring electron clouds "Instantaneous dipole" causes neighboring electron clouds

to also move to one side, to also move to one side, inducinginducing a dipole in them a dipole in them Leads to a Leads to a smallsmall force of attraction between slight positive force of attraction between slight positive

and slight negative ends of two different particles – these and slight negative ends of two different particles – these attractions are called London dispersion forcesattractions are called London dispersion forces

Strength of LDF depends on: (1) size of electron cloud in Strength of LDF depends on: (1) size of electron cloud in atom and/or (2) number of atoms in a molecule atom and/or (2) number of atoms in a molecule Polarizability increases down a groupPolarizability decreases left to right across a period

Look at the elemental halogens: fluorine and chlorine are Look at the elemental halogens: fluorine and chlorine are gases; bromine is liquid and iodine is solid because of gases; bromine is liquid and iodine is solid because of size of e- cloudsize of e- cloud around each molecule around each molecule

Figure 12.14 Dispersion forces among nonpolar molecules.

separated Cl2 molecules

instantaneous dipoles

An instantaneous dipole in one Cl2 molecule will induce a dipole in a nearby Cl2 molecule. The partial charges attract the molecules together. This process takes place with the particles throughout the container.

Figure 12.15

Molar mass and boiling point.

The strength of LDF increases with the number of electrons, which correlates with molar mass. Therefore, LDF increases down a group in the Periodic Table, which can be verified by the increase in boiling points.

Figure 12.16 Molecular shape and boiling point.

more points for dispersion

forces to act

fewer points for dispersion

forces to act

(It’s just Pentane.)

2,2-dimethylpropane

Spherical molecular shapes make less contact with each other than do cyllindrical shapes, so they have a lower boiling point.

I have corrected the organic names – compare to your textbook, which is using old naming rules.

Figure 12.11 Polar molecules and dipole-dipole forces.

solid

liquidIn a solid or a liquid, polar molecules are close enough for the attraction to hold. Orientation is more orderly in a solid because the average KE of the particles is lower.

Dipole-dipole AttractionDipole-dipole Attraction

Dipoles involve polar molecules which are Dipoles involve polar molecules which are attracted to each other because of the attracted to each other because of the slight positive and slight negative slight positive and slight negative "poles" to the molecules"poles" to the molecules

Compare molecules of FCompare molecules of F22, HF, HCl, HBr, HI, HF, HCl, HBr, HI

Boiling points: FBoiling points: F22<HCl<HBr<HI<HF<HCl<HBr<HI<HF

Data for IM ForcesData for IM Forces

FF22 HFHF HClHCl HBrHBr HIHITot # e-sTot # e-s 1818 1010 1818 3636 5454MMMM 3838 2020 36.536.5 8181 128128ENEN 00 1.81.8 1.01.0 0.80.8 0.50.5Dip MomDip Mom 00 1.41.4 1.11.1 0.80.8 0.40.4% Disp% Disp 100100 Low 81.4Low 81.4 94.594.5 99.599.5% Dipole% Dipole 00 HighHigh 18.618.6 5.55.5 0.50.5BP, KBP, K 85 85 291291 188188 206206 238238

HHvapvap 6.866.86 HighHigh 1616 1818 2020

Enhanced Dipole-Dipole or Enhanced Dipole-Dipole or Hydrogen-Bonding?Hydrogen-Bonding?

Dipole forces are Dipole forces are decreasingdecreasing down the down the hydrohalogen group because hydrohalogen group because EN is decreasingEN is decreasing

WHY is HF so very different in boiling point?WHY is HF so very different in boiling point?HF represents a special case of dipole-dipole HF represents a special case of dipole-dipole attraction called attraction called Hydrogen-bonding Hydrogen-bonding

Occurs when H is bonded to a highly EN atom Occurs when H is bonded to a highly EN atom that is also very small: H to F, O or Nthat is also very small: H to F, O or N

Size of EN atom is important also, allows H to get Size of EN atom is important also, allows H to get very close, as seen with radius in pm below:very close, as seen with radius in pm below:

N-70 O-73 F-72 Cl-100 (no H-bond)N-70 O-73 F-72 Cl-100 (no H-bond)

SAMPLE PROBLEM 12.3 Drawing Hydrogen Bonds Between Molecules of a Substance

SOLUTION:

PROBLEM: Which of the following substances exhibits H bonding? For those that do, draw two molecules of the substance with the H bonds between them.

C2H6(a) CH3OH(b) CH3C NH2

O

(c)

PLAN: Find molecules in which H is bonded to N, O or F. Draw H bonds in the format -B: H-A-.

(a) C2H6 has no H bonding sites.

(c)(b)C O H

H

H

H

COH

H

H

H

CH3C N

O

H

H

CH3CN

O

H

H

CH3CN

O

H

H

CH3CN

O

H

H

The N-H is also attracted to the N-H.

Figure 12.13 Hydrogen bonding and boiling point.

Boiling points of the binary covalent hydrides of Groups 14 – 17 plotted agains Period digit. Shows that H2O, HF and NH3 do not follow the downward trend, as shown by the dashed line for group 16.

Dipole-Induced Dipole: Dipole-Induced Dipole: between two different between two different compounds’ particlescompounds’ particlesDipole-induced dipole forces account for Dipole-induced dipole forces account for

limited solubility of oxygen in waterlimited solubility of oxygen in water

Ability to do this = function of Ability to do this = function of polarizability of moleculepolarizability of molecule

Compare HCompare H22 to I to I22: bigger molecule : bigger molecule polarizes, soluble in water, which is polarizes, soluble in water, which is demonstrated by the much greater demonstrated by the much greater solubility of Isolubility of I22 in water in water

SAMPLE PROBLEM 12.4 Predicting the Type and Relative Strength of Intermolecular Forces

PROBLEM: For each pair of substances, identify the dominant interparticle forces affecting the physical properties of each substance, and then select the substance with the higher boiling point.

(a) MgCl2 or PCl3

(b) CH3NH2 or CH3F

(c) CH3OH or CH3CH2OH

(d) Hexane (CH3CH2CH2CH2CH2CH3)

or 2,2-dimethylbutaneCH3CCH2CH3

CH3

CH3

•Bonding forces are stronger than nonbonding(intermolecular) forces.

•Hydrogen bonding is a strong type of dipole-dipole force.

•Dispersion forces are decisive when the difference is molar mass or molecular shape.

SOLUTION:

SAMPLE PROBLEM 12.4 Predicting the Type and Relative Strength of Intermolecular Forces

continued

(a) Mg2+ and Cl- are held together by ionic bonds while PCl3 is covalently bonded and the molecules are held together by dipole-dipole interactions. Ionic bonds are stronger than dipole interactions and so MgCl2 has the higher boiling point.

(b) CH3NH2 and CH3F are both covalent compounds and have bonds which are polar. The dipole in CH3NH2 can H-bond while CH3F is just dipole-dipole. Therefore CH3NH2 has the stronger interactions and the higher boiling point.

(c) Both CH3OH and CH3CH2OH can H bond but CH3CH2OH has more CH for more London dispersion force interaction. Therefore CH3CH2OH has the higher boiling point.(d) Hexane and 2,2-dimethylbutane are both nonpolar with only London dispersion forces to hold the molecules together. Hexane has the larger surface area, thereby the greater dispersion forces and the higher boiling point.

Practice with Intermolecular Practice with Intermolecular Forces: explain the forces Forces: explain the forces behind this data:behind this data:

1. Butane (CH1. Butane (CH33CHCH22CHCH22CHCH33) melts at -138) melts at -138ooC and C and boils at 0.5boils at 0.5ooC, while acetone (CHC, while acetone (CH33C=OCHC=OCH33) melts ) melts at -95at -95ooc and boils at +56c and boils at +56ooC, yet both weigh 58 C, yet both weigh 58 g/mol. Draw Lewis structures and explain the g/mol. Draw Lewis structures and explain the differences in MPs and BPs.differences in MPs and BPs.

2. Guess BP order for CCl2. Guess BP order for CCl44, N, N22, Cl, Cl22, ClNO (chlorine-, ClNO (chlorine-nitrogen-oxygen). nitrogen-oxygen).

Figure 12.17 modified

Summary diagram for analyzing the interparticle forces in a sample.

INTERACTING PARTICLES(atoms, molecules, ions)

INTERACTING PARTICLES(atoms, molecules, ions)

ions onlyIONIC BONDING(Section 9.2)

ions onlyIONIC BONDING(Section 9.2)

ion + polar moleculeION-DIPOLE FORCESion + polar moleculeION-DIPOLE FORCES

ions present ions not present

polar molecules onlyDIPOLE-DIPOLE

FORCES

polar molecules onlyDIPOLE-DIPOLE

FORCES

HYDROGENBONDING

HYDROGENBONDING

polar + nonpolar moleculesDIPOLE-INDUCED DIPOLE FORCES

polar + nonpolar moleculesDIPOLE-INDUCED DIPOLE FORCES

nonpolar molecules or atomsOnly, LONDONDISPERSIONFORCES

nonpolar molecules or atomsOnly, LONDONDISPERSIONFORCES

LONDON DISPERSION FORCES ALSO PRESENT IN ALL OF ABOVE.NETWORK COVALENT BONDING POSSIBLE FOR VERY FEW ATOMS.

H bonded toN, O, or F

METALLIC BONDING

Metal atoms only

PracticePractice

See handoutsSee handouts

Also chapter problems: 2, 29, 31, 33, Also chapter problems: 2, 29, 31, 33, 37, 39, 43, 45 37, 39, 43, 45

44thth ed. #119: What forces are ed. #119: What forces are overcome when the following events overcome when the following events occur: (a) NaCl dissolves in water, (b) occur: (a) NaCl dissolves in water, (b) krypton boils, (c) water boils, (d) COkrypton boils, (c) water boils, (d) CO22 sublimes?sublimes?

QUESTIONS TO ASK IN PREDICTING THE QUESTIONS TO ASK IN PREDICTING THE KINDS OF INTERPARTICLE FORCES THAT KINDS OF INTERPARTICLE FORCES THAT WILL BE PRESENT IN A SOLID OR A WILL BE PRESENT IN A SOLID OR A LIQUIDLIQUID  Start at the top with the first question, “Is it metallic?”. When you can answer yes, you are done. If the answer is no, keep going down the list. The Start at the top with the first question, “Is it metallic?”. When you can answer yes, you are done. If the answer is no, keep going down the list. The

“it” refers to whatever substance you are working with.“it” refers to whatever substance you are working with.

  

QuestionQuestion If yes, this force is present* MP & BPIf yes, this force is present* MP & BP Examples (MP, BP)Examples (MP, BP)

  Is it metallic? (ONLY metal present) Is it metallic? (ONLY metal present) Metallic bondingMetallic bonding High High Iron (1555, 3000)Iron (1555, 3000)

   

Is it ionic? (cation & anion present) Is it ionic? (cation & anion present) Ionic bondingIonic bonding High High NaCl(804, not defined)NaCl(804, not defined)

   

Is it network covalent compound?Is it network covalent compound? Network covalent bondingNetwork covalent bonding High High Diamond (3550, not def), SiC, SiODiamond (3550, not def), SiC, SiO22

Is the substance molecular?Is the substance molecular? (covalent bonds present) (covalent bonds present)

  

In the molecule, is H attached by a covalent Hydrogen bondingIn the molecule, is H attached by a covalent Hydrogen bonding Medium Medium Water (0.100)Water (0.100)

bond to F, O or N?bond to F, O or N?

  

Does the molecule have a dipole moment? Dipole-dipole attractionDoes the molecule have a dipole moment? Dipole-dipole attraction Low Low HCl (-114, -85)HCl (-114, -85)

  

Is it a molecule with no dipole moment? Is it a molecule with no dipole moment? Only London ForcesOnly London Forces Very low Very low Hydrogen (-257, -253)Hydrogen (-257, -253)

Iodine (114, 183)Iodine (114, 183)

  

Does the substance consist of atoms with no Only London ForcesDoes the substance consist of atoms with no Only London Forces Extremely lowExtremely low Neon (-249, -246) Neon (-249, -246)

covalent bonds between them?covalent bonds between them?

    

*Remember – London forces are present in *Remember – London forces are present in allall liquids and solids. liquids and solids.

    

Practice with these, supposing all to be in liquid or solid phase: methane, ethanol, sucrose (look up structure), NaOH, SiC, FPractice with these, supposing all to be in liquid or solid phase: methane, ethanol, sucrose (look up structure), NaOH, SiC, F22O, ClO, Cl22O, octane, radon, O, octane, radon, uranium, hydrobromic acid.uranium, hydrobromic acid.

PROPERTIES OF LIQUIDS & INTERPARTICLE FORCES: PROPERTIES OF LIQUIDS & INTERPARTICLE FORCES:

Why would a metal object with higher density Why would a metal object with higher density than water float on water? than water float on water?

Why can we fill a glass of water above its rim? Why can we fill a glass of water above its rim?

Surface tensionSurface tension is related to strength of is related to strength of attractive forces in liquid: the stronger the attractive forces in liquid: the stronger the attractive forces the greater the surface tensionattractive forces the greater the surface tension

Surface tension is the energy required to increase Surface tension is the energy required to increase surface area by a unit amount; units are J/msurface area by a unit amount; units are J/m22

Figure 12.18 The molecular basis of surface tension.

hydrogen bondingoccurs in three

dimensions

hydrogen bondingoccurs across the

surfaceand below the surface

the net vectorfor attractive

forces is downward

Molecules in the interior of a liquid experience IM forces in all directions. Molecules at the surface experience a net attraction downward, causing the liquid to minimize the number of molecules at the surface, ergo surface tension.

Table 12.3 Surface Tension and Forces Between Particles

Substance FormulaSurface Tension

(J/m2) at 200C Major Force(s)

diethyl ether

ethanol

1-butanol

water

mercury

dipole-dipole; dispersion

H bonding

H bonding; dispersion

H bonding

metallic bonding

1.7x10-2

2.3x10-2

2.5x10-2

7.3x10-2

48x10-2

CH3CH2OCH2CH3

CH3CH2OH

CH3CH2CH2CH2OH

H2O

Hg

Figure 12.19 Shape of water or mercury meniscus in glass.

adhesive forcesstronger

cohesive forces

H2O

capillarity

Hg

See note in box below or look in textbook.

Properties of LiquidsProperties of Liquids

Capillary action: rising of a liquid Capillary action: rising of a liquid through a narrow space against the through a narrow space against the force of gravityforce of gravity

Viscosity: resistance to flow, units in Viscosity: resistance to flow, units in Newton-seconds/mNewton-seconds/m22

Table 12.4 Viscosity of Water at Several Temperatures

Temperature(0C)Viscosity (N*s/m2)*

20

40

60

80

1.00x10-3

0.65x10-3

0.47x10-3

0.35x10-3

*The units of viscosity are Newton-seconds per square meter.

viscosity - resistance to flow

Why water is special:Why water is special:Water molecules are 80% H-bonded at normal Water molecules are 80% H-bonded at normal

conditionsconditionsMolecules are so close together that you cannot tell Molecules are so close together that you cannot tell

which H's belong to which O in each moleculewhich H's belong to which O in each moleculeThis is important to life on earth (& possibly elsewhere)This is important to life on earth (& possibly elsewhere)Ice floats on liquid water because the solid (ice) is less Ice floats on liquid water because the solid (ice) is less

dense than the liquid (good for fishies)dense than the liquid (good for fishies)Ice forms a crystal structure in tetrahedral arrangementIce forms a crystal structure in tetrahedral arrangementHydrogen-bonding also accounts for other physical Hydrogen-bonding also accounts for other physical

properties:properties:Lower weight alcohols are very soluble in water Lower weight alcohols are very soluble in water because of the -OH functional groupbecause of the -OH functional groupGreat solvent properties because water is so polarGreat solvent properties because water is so polarVery high specific heat as noted back in Chapter 6Very high specific heat as noted back in Chapter 6High surface tensionHigh surface tension

Figure 12.20 The H-bonding ability of the water molecule.

hydrogen bond donor

hydrogen bond acceptor

Because it has two O-H bonds and two lone pairs, one water molecule can engage in as many as four hydrogen-bonding attractions to surrounding water molecules, which are arranged tetrahedrally.

Figure 12.21 The hexagonal structure of ice.

A. The geometric arrangement of the hydrogen-bonding in water leads to open, hexagonally shaped crystal structure of ice. Thus, when water freezes, the volume increases.

B. The delicate six-pointed beauty of snowflakes reflects the hexagonal crystal structure of ice.

SOLIDS & CRYSTAL STRUCTURESSOLIDS & CRYSTAL STRUCTURES

SOLIDS: fixed particles that cannot move with SOLIDS: fixed particles that cannot move with velocity, but do vibrate and rotate in position, so velocity, but do vibrate and rotate in position, so they do have KEthey do have KE

Generally have long-range order - Generally have long-range order - crystalscrystals have have well-defined regular shapes, or if short-range well-defined regular shapes, or if short-range order they are order they are amorphousamorphous - no regular shape, - no regular shape, like asphalt, wax, glasslike asphalt, wax, glass

Crystal structure includes the Crystal structure includes the four types of solidsfour types of solidsionic (all cation-anion units)ionic (all cation-anion units)metallic (Cu, Zn, U, etc.)metallic (Cu, Zn, U, etc.)molecular/atomic (ice, Imolecular/atomic (ice, I22, etc.), etc.)network covalent (diamond, SiC, SiOnetwork covalent (diamond, SiC, SiO22))

General Properties of the General Properties of the Four Types of Crystalline Four Types of Crystalline SolidsSolids1. Ionic (KNO3, MgO): high MP/BP; some water-

solb, brittle, conduct only when molten or aqueous

2. Molecular (C10H8, I2): low MP/BP; more solb in nonpolar; nonconductors

3. Network Covalent (Cdiamond, SiC, SiO2): very high MP/BP; insolb, brittle, non- or semi-conductor

4. Metallic (Cu, Fe, U): wide range of MPs; insolb, malleable, ductile, elec conductor

Figure 12.22 The striking beauty of crystalline solids.

celestite pyrite amethyst halite

Figure 12.22 in current 2nd edition has wulfanite, barite, calcite, quartz as amethyst, and beryl (emerald)

Crystal StructuresCrystal StructuresCrystals have a crystal lattice arrangement of which Crystals have a crystal lattice arrangement of which

smallest pieces are unit cell in 3-D, containing smallest pieces are unit cell in 3-D, containing >> one one formula unitformula unit

Seven basic types: cubic, tetragonal, orthorhombic, Seven basic types: cubic, tetragonal, orthorhombic, monoclinic, hexagonal, rhombohedral, triclinicmonoclinic, hexagonal, rhombohedral, triclinic

See packet of handouts and Dry Lab VI(?) in Lab Manual See packet of handouts and Dry Lab VI(?) in Lab Manual for “Crystal Structures and Characteristics”for “Crystal Structures and Characteristics”

Simplest are the cubic, of which there are three typesSimplest are the cubic, of which there are three types

Simple cubic (sc): metals and ionic cmpdsSimple cubic (sc): metals and ionic cmpds

Body-centered cubic (bcc): metalsBody-centered cubic (bcc): metals

Face-centered cubic (fcc): metals and ionic cmpdsFace-centered cubic (fcc): metals and ionic cmpds

portion of a 3-D lattice

Figure 12.23 The crystal lattice and the unit cell.

lattice point

unit cell

portion of a 2-D lattice

unit cell

A. The lattice is an array of points that defines the positions of the particles in a crystal structure. It is shown here as points connected by lines. One unit cell is highlighted.

A checkerboard is a two-dimensional analogy for a lattice.

Figure 12.24 (1 of 3) The three cubic unit cells.

Simple Cubic

coordination number = 6

Atoms/unit cell = 1/8 * 8 = 1

Cell length = 2r

1/8 atom at 8 corners

See notes box below slide.

Atoms touch along edge of cube

Figure 12.24 (2 of 3) The three cubic unit cells.

Body-centered Cubic

coordination number = 8

1/8 atom at 8 corners

1 atom at center

Atoms/unit cell = (1/8*8) + 1 = 2

Cell length = 4r/(3)1/2

See notes box below slide.

Atoms touch along main diagonal.

Figure 12.24 (3 of 3) The three cubic unit cells.

Face-centered Cubic

coordination number = 12Atoms/unit cell = (1/8*8)+(1/2*6) = 4

Cell length = 4r/(2)1/2

1/8 atom at 8 corners

1/2 atom at 6 faces

See notes box below slide.

Atoms touch along face diagonal.

Cell length and cell volumeCell length and cell volume

See figure 12.28 for derivation of cell length See figure 12.28 for derivation of cell length based on which cubic structure makes up the based on which cubic structure makes up the unit cell. For metals cell length is: sc length unit cell. For metals cell length is: sc length = 2r; bcc length = 4r/(3)= 2r; bcc length = 4r/(3)1/21/2; fcc length = ; fcc length = 4r/(2)4r/(2)1/21/2

Cell length determination is different for ionic Cell length determination is different for ionic compounds, which are simple cubic or face-compounds, which are simple cubic or face-centered cubic: sc length = 2(r+R)/(3)centered cubic: sc length = 2(r+R)/(3)1/21/2; fcc ; fcc length = 2(r+R)length = 2(r+R)

Volume of any cube = (length)Volume of any cube = (length)33

Crystal Structures PracticeCrystal Structures Practice

See handouts and practice problemsSee handouts and practice problems

Also chapter problems: 61, 64, 67, 73, 75 Also chapter problems: 61, 64, 67, 73, 75

4th ed. #98: Polonium is a rare 4th ed. #98: Polonium is a rare radioactive metal that is the only element radioactive metal that is the only element with a crystal structure based on the with a crystal structure based on the simple cubic unit cell. If its density is simple cubic unit cell. If its density is 9.142 g/cm9.142 g/cm33, calculate an atomic radius for , calculate an atomic radius for a polonium atom.a polonium atom.

Crystal Structures PracticeCrystal Structures Practice

44thth ed. #101: Tantalum, with D = ed. #101: Tantalum, with D = 16.634 g/cm16.634 g/cm33, has a bcc structure , has a bcc structure with an edge length of 3.3058 with an edge length of 3.3058 Angstroms. Use its molar mass and Angstroms. Use its molar mass and this data to prove Avogadro’s this data to prove Avogadro’s number.number.

Phase Changes

solid liquid gas

melting

freezing

vaporizing

condensing

sublimination

endothermic

exothermic

deposition

Vapor PressureVapor PressureEvaporation/vaporization: small fraction of Evaporation/vaporization: small fraction of

molecules have high enough velocity to escape molecules have high enough velocity to escape force of attraction at surfaceforce of attraction at surface

RATE OF EVAPORATION: will increase with RATE OF EVAPORATION: will increase with increasing T, since fraction of molecules with increasing T, since fraction of molecules with escape vel will increaseescape vel will increase

In a closed system, a dynamic equilibrium will be In a closed system, a dynamic equilibrium will be reached: reached: Rate of evaporation = rate of condensationRate of evaporation = rate of condensationVapor pressure: vapor molecules exert a Vapor pressure: vapor molecules exert a partial pressure called vapor pressurepartial pressure called vapor pressure

Figure 12.4 Liquid-gas equilibrium.

A. In a closed flask at const T, with air removed, Pi = 0. As molecules escape surface to become vapor, P increases. B. At equilibrium, # of molecules escaping liquid = # of molecules condensing, P is constant. C. Plot of P vs. time shows P becomes constant.

Figure 12.5The effect of temperature on the distribution of

molecular speed in a liquid.

With T1 lower than T2, most probable molecular speed, u1, is less than u2. Fraction of molecules with “escape velocity” is greater at the higher temperature. At higher T, equilibrium is reached with more molecules in the vapor phase, therefore at a higher P.

Vapor Pressure PracticeVapor Pressure PracticeIf 1.00 L of water is placed in 2.30x10If 1.00 L of water is placed in 2.30x1044 L closed L closed

room, will all the water evaporate? Given D = room, will all the water evaporate? Given D = 0.997 g/mL, Vapor Pressure = 23.8 torr at 0.997 g/mL, Vapor Pressure = 23.8 torr at 25.025.0ooC.C.

Water will evap till room is at 23.8 torr partial Water will evap till room is at 23.8 torr partial pressure of water vapor.pressure of water vapor.

See how many moles at that point: See how many moles at that point:

n = PV/RT = 29.4 moln = PV/RT = 29.4 mol

How many moles in 1.0 L beaker? About 55 How many moles in 1.0 L beaker? About 55 moles - won't all evaporatemoles - won't all evaporate

Vapor Pressure vs. Vapor Pressure vs. TemperatureTemperatureBoiling point: occurs when you see bubbles Boiling point: occurs when you see bubbles

of gas forming in the liquid and coming to of gas forming in the liquid and coming to surfacesurface

Any pure liquid remains at constant T while Any pure liquid remains at constant T while boiling, since this is a change of stateboiling, since this is a change of state

Definition:Definition:BP is the Temperature at which VP = BP is the Temperature at which VP = barometric P barometric P

Why does water boil at 100Why does water boil at 100ooC in Fairfield and C in Fairfield and at 95at 95ooC in Denver? C in Denver?

Figures 12.6 and 12.7 Figure 12.7

Vapor pressure as a function of temperature and intermolecular forces.

A linear plot of vapor pressure- temperature

relationship.

The Clausius-Clapeyron equation comes from this graph: y = mx + b

You practice drawing and You practice drawing and labelling a generic VP curvelabelling a generic VP curve

Vapor pressure curves:Vapor pressure curves:

Initial "phase diagrams" incorporated Initial "phase diagrams" incorporated into P/T diagrams that will include into P/T diagrams that will include solid phase later solid phase later

Why can NHWhy can NH33 be condensed from gas be condensed from gas to liquid at Room T by compression, to liquid at Room T by compression, but Nbut N22 can't? can't?

Relative HumidityRelative Humidity

NOT IN TEXT: NOT IN TEXT:

Relative humidity as reported by Relative humidity as reported by weather forecasters: weather forecasters:

%water evap=actual partial P/equil vapor P * %water evap=actual partial P/equil vapor P * 100100

If actual is 12.8 and VP for given T is If actual is 12.8 and VP for given T is 21.1, relative humidity is 61%21.1, relative humidity is 61%

VP and VP and HHvapvap

HHvapvap is related to VP and T thru the is related to VP and T thru the Clausius-Clapeyron equationClausius-Clapeyron equation

ln P = (-ln P = (-HHvapvap/RT) + C /RT) + C (where R = (where R = 8.314 J/mol-K, T in K)8.314 J/mol-K, T in K)

If plotted on a graph, the slope is:If plotted on a graph, the slope is:

==(ln p(ln p22 – ln p – ln p11)/(1/T)/(1/T22 – 1/T – 1/T11)= -)= -HHvapvap/R/RRearranges to Clausius-Clapeyron Rearranges to Clausius-Clapeyron Equation (next slide)Equation (next slide)

The Clausius-Clapeyron Equation

ln P =

-HvapR

1T

C

ln P2P1

= -Hvap

R1T2

1T1

MEMORIZE!

Alternately, if you don’t want to use the negative sign: ln (P2/P1) = Hvap/R(1/T1-1/T2)

Clausius-Clapeyron equation Clausius-Clapeyron equation examplesexamples

Look at Sample Problem 12.2 in text.Look at Sample Problem 12.2 in text.

My Example: hexane has My Example: hexane has HHvapvap = 30.1 kJ/mol = 30.1 kJ/mol and at 25.0and at 25.0ooC, VP = 148 torr. What will VP be C, VP = 148 torr. What will VP be at 50.0at 50.0ooC? C?

ln (Pln (P22/148) = (-30.1x10/148) = (-30.1x1033J/8.314 J/mol-K)(1/323.15 – 1/298.15)J/8.314 J/mol-K)(1/323.15 – 1/298.15)

ln (Pln (P22/148) = 0.9394 (take antilog of both /148) = 0.9394 (take antilog of both sides)sides)

PP22/148 = e/148 = e0.93940.9394 = 2.55 = 2.55

PP22 = 379 torr = 379 torr

Practice:Practice:

Chapter problems: 17 & 18Chapter problems: 17 & 18

17: A liquid has 17: A liquid has HHoovapvap of 35.5 kJ/mol of 35.5 kJ/mol

and a BP of 122and a BP of 122ooC at 1.00 atm. What C at 1.00 atm. What is its VP at 113is its VP at 113ooC?C?

18: What is the 18: What is the HHoovapvap of a liquid that of a liquid that

has a VP of 641 torr at 85.2has a VP of 641 torr at 85.2ooC and a C and a BP of 95.6BP of 95.6ooC at 1.00 atm?C at 1.00 atm?

Figure 12.8 4th ed., not in principles

Iodine subliming.

iodine solid

iodine vapor

iodine solid

test tube with ice

Simple Phase DiagramsSimple Phase DiagramsAre P vs. T diagrams showing three phases for Are P vs. T diagrams showing three phases for

pure elements or compounds, incorporates the pure elements or compounds, incorporates the VP curveVP curve

Critical PointCritical Point is where liquid and gas cannot be is where liquid and gas cannot be distinguished from each otherdistinguished from each other

Triple PointTriple Point is where solid, liquid and gas phases is where solid, liquid and gas phases meet and all three are presentmeet and all three are present

Example: For water the Triple Point is 0.01Example: For water the Triple Point is 0.01ooC and C and 4.58 torr4.58 torr

For COFor CO22 Triple Point is at -56.7 Triple Point is at -56.7ooC and 5.1 atmC and 5.1 atm

Figure 12.8 Phase diagrams for CO2 and H2O.

CO2H2O

Each region depicts the T & P under which the phase is stable. Lines between regions show conditions at which two phases exist in equilibrium. The Critical Point shows conditions beyond which liquid and gas cannot be distinguished from each other. At the triple point, three phases exist is equilibrium. CO2 phase diagram is typical with forward sloping solid-liquid line; solid is more dense than liquid. H2O phase diagram is sloping backward; solid is less dense than liquid.

Phase DiagramsPhase Diagrams

You must draw and label phase You must draw and label phase diagrams based on data given to you diagrams based on data given to you and determine the physical state of a and determine the physical state of a substance from its placement on a substance from its placement on a phase diagram.phase diagram.

Work on problems 20 & 22Work on problems 20 & 22