1 why? why is water usually a liquid and not a gas? why does liquid water boil at such a high...

11WHYWHY??

• Why is water usually a liquid and not a gas?

• Why does liquid water boil at such a high temperature for such a small molecule?

• Why does ice float on water?

• Why do snowflakes have 6 sides?

• Why is I2 a solid whereas Cl2 is a gas?

• Why are NaCl crystals little cubes?

22Liquids, Solids Liquids, Solids

& Intermolecular Forces& Intermolecular ForcesChap. 13Chap. 13

Liquids, Solids Liquids, Solids & Intermolecular Forces& Intermolecular Forces

Chap. 13Chap. 13

33

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.

44Ion-Ion ForcesIon-Ion Forcesfor comparison of for comparison of

magnitudemagnitude

Ion-Ion ForcesIon-Ion Forcesfor comparison of for comparison of

magnitudemagnitude

NaNa++—Cl—Cl-- in salt in salt

These are the These are the strongest forces.strongest forces.

Lead to solids with Lead to solids with high melting high melting temperatures.temperatures.

NaCl, mp = 800 NaCl, mp = 800 ooCC

MgO, mp = 2800 MgO, mp = 2800 ooCC

55

Attraction Between Attraction Between Ions and Permanent Ions and Permanent

DipolesDipoles


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.

HHwater dipole••O-δ+δ

66


DipolesDipoles


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.

HHwater dipole••O-δ+δ

77


DipolesDipoles


DipolesDipoles

• Many metal ions are Many metal ions are hydrated. This is the hydrated. This is the reason metal salts reason metal salts dissolve in waterdissolve in water..

88

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

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

-1922 kJ/mol-1922 kJ/mol -405 kJ/mol-405 kJ/mol -263 kJ/mol-263 kJ/mol


DipolesDipoles


DipolesDipoles

OHHδ+δ-• • •OHHδ+δ-• • •OHHδ+δ-• • •Na+Mg2+ Cs+

99

Dipole-Dipole Dipole-Dipole ForcesForces


Such forces bind molecules having Such forces bind molecules having permanent dipoles to one another.permanent dipoles to one another.

1010



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 Boil PointPoint

NN22 2828 -196 -196 ooCC

COCO 2828 -192 -192 ooCC

BrBr22 160160 59 59 ooCC

IClICl 162162 97 97 ooCC

1111Hydrogen 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 H-bonding is strongest when X strongest when X and Y are and Y are N, O, N, O,

or For F

1212

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--δδ

++δδ

--δδ

1313

H-Bonding Between H-Bonding Between Two Methanol Two Methanol

MoleculesMolecules

H-Bonding Between H-Bonding Between Two Methanol Two Methanol

MoleculesMolecules

H-bondH-bondH-bondH-bond

--δδ++δδ

--δδ

1414

H-Bonding Between H-Bonding Between Ammonia and WaterAmmonia and WaterH-Bonding Between H-Bonding Between Ammonia and WaterAmmonia and Water

H-bondH-bondH-bondH-bond

--δδ

++δδ --δδ

This H-bond leads to the formation of This H-bond leads to the formation of NHNH44

++ and OH and OH--

1515

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 very the O—H bond is very polarpolar

• 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.

1616


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: http://www.its.caltech.edu/~atomic/snowcrystals/snow3x.jpg

1717


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

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

1818

Hydrogen Bonding in HHydrogen Bonding in H22OOHydrogen Bonding in HHydrogen Bonding in H22OOH bonds ---> abnormally high specific heat capacity of water (4.184 H bonds ---> abnormally high specific heat capacity of water (4.184

g/K•mol). g/K•mol).

This is the reason water is used to put out fires, it is the reason This is the reason water is used to put out fires, it is the reason lakes/oceans control climate, and is the reason thunderstorms lakes/oceans control climate, and is the reason thunderstorms release huge energy.release huge energy.

1919Hydrogen BondingHydrogen BondingHydrogen BondingHydrogen Bonding

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

See Screen 13.7See Screen 13.7

2020Boiling Points of Boiling Points of Simple Simple

Hydrocarbon Hydrocarbon CompoundsCompounds

2222Methane Methane ClathrateClathrate

2323

Hydrogen 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

2424

Portion of a Portion of a DNA chainDNA chain

Double helix Double helix of DNAof DNA

2525

Base-Pairing through H-Base-Pairing through H-BondsBonds

2626

Double Double Helix of Helix of

DNADNA

2727



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

See Screen 13.6See Screen 13.6

2828FORCES INVOLVING FORCES INVOLVING INDUCED DIPOLESINDUCED DIPOLES

FORCES INVOLVING FORCES INVOLVING INDUCED DIPOLESINDUCED DIPOLES

How 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 a dipole in the Odipole in the O22 electric cloud. electric cloud.

Dipole-induced Dipole-induced dipoledipole

Dipole-induced Dipole-induced dipoledipole



Solubility increases with mass the gasSolubility increases with mass the gas



• Consider IConsider I22 dissolving in dissolving in alcohol, CHalcohol, CH33CHCH22OH.OH.

OH

-δ

+δ

I-I

R-δ

+δ

OH

+δ

-δ

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



The induced forces between IThe induced forces between I22 molecules are molecules are very weak, so solid Ivery weak, so solid I22 sublimes (goes from a sublimes (goes from a solid to gaseous molecules).solid to gaseous molecules).



The magnitude of the induced dipole depends The magnitude of the induced dipole depends on the tendency to be distorted. on the tendency to be distorted.

Higher molec. weight ---> larger induced Higher molec. weight ---> larger induced dipoles.dipoles.

MoleculeMolecule Boiling Point Boiling Point ((ooC)C)

CHCH44 (methane) (methane) - 161.5- 161.5

CC22HH66 (ethane) (ethane) - 88.6 - 88.6

CC33HH88 (propane) (propane) - 42.1- 42.1

CC44HH1010 (butane) (butane) - 0.5- 0.5

3434Boiling Points of Boiling Points of HydrocarbonsHydrocarbons

Note linear relation between bp and molar mass.

CHCH44

CC22HH66

CC33HH88

CC44HH1010

3535

Intermolecular Forces Intermolecular Forces SummarySummary

3636Methane Methane HydrateHydrate

http://www.gsj.go.jp/dMG/hydrate/MH.burn.gif

3737

LiquidsLiquidsSection 13.5Section 13.5

LiquidsLiquidsSection 13.5Section 13.5

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

3838

LiquidsLiquids

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

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

break IM bonds

make IM bonds

Add energy

Remove energy

LIQUID VAPOR

<---condensation<---condensation

evaporation--->evaporation--->

3939Liquids—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 process of evaporation is evaporation is endothermicendothermic..

4040Liquids—Liquids—Distribution of EnergiesDistribution of Energies

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

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

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

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

0

Nu

mb

er o

f m

olec

ule

s

Molecular energy

higher Tlower T

See Figure 13.12See Figure 13.12

Minimum energy req’d to break IM forces and evaporate

4141Distribution of Energy in a Distribution of Energy in a

LiquidLiquid

Figure 13.12

4242

LiquidsLiquids At higher T a much At higher T a much larger number of larger number of molecules has high molecules has high enough energy to enough energy to break IM forces and break IM forces and move from liquid to move from liquid to vapor state.vapor state.

High E molecules carry High E molecules carry away E. You cool away E. You cool down when sweating down when sweating or after swimming.or after swimming.

.

0 Molecular energy

minimum energy neededto break IM forces and evaporate

higher Tlower T

4343

LiquidsLiquidsWhen molecules of liquid When molecules of liquid

are in the vapor state, are in the vapor state, they exert a they exert a VAPOR VAPOR PRESSUREPRESSURE

EQUILIBRIUM EQUILIBRIUM VAPOR VAPOR PRESSUREPRESSURE is the is the pressure exerted by a pressure exerted by a vapor over a liquid in a vapor over a liquid in a closed container when closed container when the the rate of evaporation rate of evaporation = the rate of = the rate of condensation.condensation.

4444

Equilibrium Vapor Equilibrium Vapor PressurePressure

Liquid in flask evaporates and exerts pressure on manometer.

See Fig. 13.15See Fig. 13.15

4545Vapor PressureVapor PressureCD, Screen 13.9CD, Screen 13.9

4646

Equilibrium Vapor PressureEquilibrium Vapor PressureFigure 13.16Figure 13.16

4747LiquidsLiquidsEquilibrium Vapor PressureEquilibrium Vapor Pressure

FIGURE 13.16:FIGURE 13.16: VP as a function of T.VP as a function of T.

1. The curves show all conditions of P and 1. The curves show all conditions of P and T where LIQ and VAP are in T where LIQ and VAP are in EQUILIBRIUMEQUILIBRIUM

2. The VP rises with T.2. The VP rises with T.

3. When VP = external P, the liquid boils.3. When VP = external P, the liquid boils.

This means that BP’s of liquids change This means that BP’s of liquids change with altitude.with altitude.

4848

Boiling LiquidsBoiling Liquids

Liquid boils when its vapor pressure equals atmospheric pressure.

Liquid boils when its vapor pressure equals atmospheric pressure.

4949

Boiling Point Boiling Point at Lower Pressureat Lower Pressure

When pressure is lowered, the vapor When pressure is lowered, the vapor pressure can equal the external pressure at pressure can equal the external pressure at

a lower temperature.a lower temperature.

5050

Consequences of Vapor Consequences of Vapor Pressure ChangesPressure Changes

When can cools, vp of water drops. When can cools, vp of water drops. Pressure in the can is less than that of Pressure in the can is less than that of

atmosphere, so can is crushed. atmosphere, so can is crushed.

5151

4. If external P = 760 mm Hg, T of boiling is the 4. If external P = 760 mm Hg, T of boiling is the

NORMAL BOILING POINTNORMAL BOILING POINT

5. VP of a given molecule at a given T depends 5. VP of a given molecule at a given T depends

on IM forces. Here the VP’s are in the orderon IM forces. Here the VP’s are in the order

C2H5H5C2 HH5C2 HH

wateralcoholether

increasing strength of IM interactions

extensiveH-bondsH-bonds

dipole-dipole

OOO

LiquidsLiquidsFigure 13.16: VP versus TFigure 13.16: VP versus T

5252

LiquidsLiquids

HEAT 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 ---> VAPLIQ + heat ---> VAP

Compd.Compd. ∆H∆Hvapvap (kJ/mol) (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

5353LiquidsLiquidsMolecules at surface behave differently than those in the interior.Molecules at surface behave differently than those in the interior.

Molecules at surface experience net INWARD Molecules at surface experience net INWARD force of attraction. force of attraction. This leads to This leads to SURFACE TENSIONSURFACE TENSION — the energy — the energy req’d to break the surface.req’d to break the surface.

5454

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.

5555

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

5656

Capillary ActionCapillary Action

Movement of water up a piece of paper Movement of water up a piece of paper depends on H-bonds between Hdepends on H-bonds between H22O and O and the OH groups of the cellulose in the the OH groups of the cellulose in the paper.paper.

5757

Finding the Lattice TypeFinding the Lattice TypePROBLEMPROBLEM Al has density = 2.699 g/cm Al has density = 2.699 g/cm33 and Al radius = 143 pm. and Al radius = 143 pm.

Verify that Al is FCC.Verify that Al is FCC.

SOLUTIONSOLUTION

1. Calc. unit cell volume1. Calc. unit cell volume

V = (cell edge)V = (cell edge)33

Edge distance comes from face diagonal.Edge distance comes from face diagonal.

Diagonal distance = √2 • edgeDiagonal distance = √2 • edge

5858

(Diagonal)2 = 2 (edge)2

Diag = √2 • ( )edge

,Therefore

(Diagonal)2 = 2 (edge)2

Diag = √2 • ( )edge

,Therefore



SOLUTIONSOLUTION

V = (cell edge)V = (cell edge)33 and face diagonal = √2 • and face diagonal = √2 • edgeedge

5959



SOLUTIONSOLUTION

Here diagonal = 4 • radius of Al = 572 pmHere diagonal = 4 • radius of Al = 572 pm

Therefore, edge = 572 pm / √2 = 404 pmTherefore, edge = 572 pm / √2 = 404 pm

In centimeters, In centimeters, edge = 4.04 x 10edge = 4.04 x 10-8-8 cm cm

So, V of unit cell = (4.04 x 10So, V of unit cell = (4.04 x 10-8-8 cm) cm)33

V = 6.62 x 10V = 6.62 x 10-23-23 cm cm33

6060



SOLUTIONSOLUTION

2.2. Use V and density to calc. mass of unit cell from Use V and density to calc. mass of unit cell from DENS = MASS / VOLDENS = MASS / VOL

Mass = density • volume Mass = density • volume

= (6.62 x 10= (6.62 x 10-23-23 cm cm33)(2.699 g/cm)(2.699 g/cm33))

= 1.79 x 10= 1.79 x 10-22-22 g/unit cell g/unit cell

6161

Finding the Lattice TypeFinding the Lattice TypePROBLEMPROBLEM Al has density = 2.699 g/cm Al has density = 2.699 g/cm33 and Al radius = 143 pm. Verify that Al and Al radius = 143 pm. Verify that Al

is FCC.is FCC.

SOLUTIONSOLUTION

3. Calculate number of Al per unit cell from mass of unit cell.3. Calculate number of Al per unit cell from mass of unit cell.

Mass 1 Al atom = 26.98 g

mol •

1 mol

6.022 x 1023 atoms Mass 1 Al atom =

26.98 gmol

• 1 mol

6.022 x 1023 atoms

1.79 x 10-22 g

unit cell •

1 atom

4.480 x 10-23 g = 3.99 Al atoms/unit cell

1.79 x 10-22 gunit cell

• 1 atom

4.480 x 10-23 g = 3.99 Al atoms/unit cell

1 atom = 4.480 x 101 atom = 4.480 x 10-23-23 g, so g, so

6262Number of Atoms per Unit Number of Atoms per Unit

CellCellHow can there be 4 atoms in a How can there be 4 atoms in a

unit cell?unit cell?

1. Each corner Al is 1/8 1. Each corner Al is 1/8 insideinside the the unit cell.unit cell.

8 corners (1/8 Al per corner) = 8 corners (1/8 Al per corner) = 1 net Al1 net Al

2. Each face Al is 1/2 inside the 2. Each face Al is 1/2 inside the cellcell

6 faces (1/2 per face) = 3 net 6 faces (1/2 per face) = 3 net Al’sAl’s

1 why? why is water usually a liquid and not a gas? why does liquid water boil at such a high...

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