Intermolecular Forces and Solids and Liquids

8/4/2019 Intermolecular Forces and Solids and Liquids

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Intermolecular Forces and

Liquids and Solids

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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A phaseis a homogeneous part of the system incontact with other parts of the system butseparated from them by a well-defined boundary.

2 Phases

Solid phase - ice

Liquid phase - water

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Intermolecular Forces

11.2

Intermolecular forcesare attractive forces between molecules.

Intramolecular forceshold atoms together in a molecule.

Intermolecular vs Intramolecular

41 kJ to vaporize 1 mole of water (inter)

930 kJ to break all O-H bonds in 1 mole of water (intra)

Generally,

intermolecularforces are muchweaker thanintramolecularforces.

Measure of intermolecular force

boiling point

melting point

DHvap

DHfus

DHsub


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Dipole-Dipole Forces

Attractive forces between polar molecules

Orientation of Polar Molecules in a Solid

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Ion-Dipole Forces

Attractive forces between an ion and a polar molecule

11.2

Ion-Dipol

e Interaction


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Dispersion Forces

Attractive forces that arise as a result of temporarydipoles induced in atoms or molecules

11.2

ion-induced dipole interaction

dipole-induced dipole interaction


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Dispersion Forces Continued

11.2

Polarizabilityis the ease with which the electron distributionin the atom or molecule can be distorted.

Polarizability increases with:

greater number of electrons

more diffuse electron cloud

Dispersionforces usuallyincrease withmolar mass.


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S

What type(s) of intermolecular forces exist betweeneach of the following molecules?

HBrHBr is a polar molecule: dipole-dipole forces. There are

also dispersion forces between HBr molecules.

CH4

CH4 is nonpolar: dispersion forces.

SO2SO2 is a polar molecule: dipole-dipole forces. There arealso dispersion forces between SO2 molecules.

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Hydrogen Bond

11.2

The hydrogen bondis a special dipole-dipole interactionbetween they hydrogen atom in a polar N-H, O-H, or F-H bondand an electronegative O, N, or F atom.

A HB A HAor

A & B are N, O, or F


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Why is the hydrogen bond considered aspecial dipole-dipole interaction?

Decreasing molar massDecreasing boiling point

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Properties of Liquids

Surface tensionis the amount of energy required to stretch

or increase the surface of a liquid by a unit area.

Strong

intermolecularforces

Highsurfacetension

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Cohesionis the intermolecular attraction between like molecules

11.3

Adhesionis an attraction between unlike molecules

Adhesion

Cohesion


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Viscosityis a measure of a fluids resistance to flow.

11.3

Strongintermolecular

forces

High

viscosity


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Maximum Density40C

Ice is less dense than water

Density of Water

11.3

Water is a Unique Substance


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A crystalline solidpossesses rigid and long-range order. In acrystalline solid, atoms, molecules or ions occupy specific(predictable) positions.

An amorphoussoliddoes not possess a well-definedarrangement and long-range molecular order.

A unit cellis the basic repeating structural unit of a crystallinesolid.

Unit Cell

latticepoint

Unit cells in 3 dimensions 11.4

At lattice points:

Atoms

Molecules

Ions


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11.4


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11.4


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11.4


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11.4


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Shared by 8unit cells

Shared by 2unit cells

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11.4

1 atom/unit cell

(8 x 1/8 = 1)

2 atoms/unit cell

(8 x 1/8 + 1 = 2)

4 atoms/unit cell

(8 x 1/8 + 6 x 1/2 = 4)


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11.4


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When silver crystallizes, it forms face-centered cubiccells. The unit cell edge length is 409 pm. Calculatethe density of silver.

d=mV

V= a3= (409 pm)3 = 6.83 x 10-23 cm3

4 atoms/unit cell in a face-centered cubic cell

m= 4 Ag atoms107.9 g

mole Agx

1 mole Ag

6.022 x 1023 atomsx = 7.17 x 10-22 g

d=mV

7.17 x 10-22 g

6.83 x 10-23 cm3= = 10.5 g/cm3

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11.5


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Types of Crystals

Ionic Crystals

Lattice points occupied by cations and anions

Held together by electrostatic attraction Hard, brittle, high melting point

Poor conductor of heat and electricity

CsCl ZnS CaF2

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Types of Crystals

Covalent Crystals

Lattice points occupied by atoms

Held together by covalent bonds Hard, high melting point


11.6diamond graphite

carbonatoms


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Types of Crystals

Molecular Crystals

Lattice points occupied by molecules

Held together by intermolecular forces Soft, low melting point


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Types of Crystals

Metallic Crystals

Lattice points occupied by metal atoms

Held together by metallic bonds Soft to hard, low to high melting point

Good conductors of heat and electricity

11.6

Cross Section of a Metallic Crystalnucleus &

inner shell e-

mobile seaof e-

T f C l


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Types of Crystals

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Evaporatio

n

Greatest

Order

LeastOrder

11.8

Condensat

ion

T2 > T1


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The equilibrium vapor pressureis the vapor pressuremeasured when a dynamic equilibrium exists betweencondensation and evaporation

H2O (l) H2O (g)

Rate ofcondensation

Rate ofevaporation

=

Dynamic Equilibrium

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BeforeEvaporation

AtEquilibrium

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The boiling pointis the temperature at which the(equilibrium) vapor pressure of a liquid is equal to theexternal pressure.

The normal boiling pointis the temperature at which a liquidboils when the external pressure is 1 atm.

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The critical temperature(Tc) is the temperature above whichthe gas cannot be made to liquefy, no matter how great theapplied pressure.

The critical pressure(Pc) is the minimum

pressure that must beapplied to bring aboutliquefaction at thecritical temperature.

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Melting

11.8

Free

zing

H2O (s) H2O (l)

The melting pointof a solidor the freezing pointof aliquid is the temperature atwhich the solid and liquidphases coexist in equilibrium


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11.8


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A phase diagramsummarizes the conditions at which asubstance exists as a solid, liquid, or gas.

Phase Diagram of Water

11.8


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Intermolecular Forces and Solids and Liquids

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