Intermolecular Attractions & the Properties of Liquids ...facweb.northseattle.edu/jpikul/chem162/Lectures/Lecture_ch12_pikul.pdf · Intermolecular Attractions & the Properties of

Intermolecular Attractions & the Properties of Liquids & Solids

CHAPTER 12

Chemistry: The Molecular Nature of Matter, 6th edition By Jesperson, Brady, & Hyslop

CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 2

!  Understand, describe, and rank in order of strength the types of

intermolecular forces.

!  Difference between bonds and intermolecular forces

!  Changes of state: heat of vaporization, fusion, & sublimation

!  Clausius-Clapyron equation

!  Heating and cooling curves: !H, phase transition temperatures

!  Phase diagrams

!  Solids: Unit cell, stoichiometry, packing patterns, XRD, common

types and their properties



Lecture Road Map:

①  Properties of gas, liquids, solids

②  Intermolecular forces

③  Changes of state

④  Dynamic Equilibrium

⑤  Structure & Characterization of a solid



Properties of gases, liquids, &

solids


Intermolecular Forces Important differences between gases, solids, and liquids: o Gases

o Expand to fill their container o Liquids

o Retain volume, but not shape o Solids

o  Retain volume and shape

6

Intermolecular Forces o Physical state of molecule depends on

o Average kinetic energy of particles o Recall KE ! Tave

o Intermolecular Forces o Energy of Inter-particle attraction

o Physical properties of gases, liquids and solids determined by o How tightly molecules are packed together o Strength of attractions between

molecules

7

o Converting gas "# liquid or solid o Molecules must get closer together

o Cool or compress

o Converting liquid or solid "# gas o Requires molecules to move farther

apart o Heat or reduce pressure

o As T decreases, kinetic energy of molecules decreases o At certain T, molecules don’t have

enough energy to break away from one another’s attraction

Intermolecular Attractions

8

Inter vs. Intra-Molecular Forces o  Intramolecular forces

o Covalent bonds within molecule o Strong o  $Hbond (HCl) = 431 kJ/mol

o  Intermolecular forces o Attraction forces between molecules o Weak o  $Hvaporization (HCl) = 16 kJ/mol

Cl H Cl H

Covalent Bond (strong) Intermolecular attraction (weak)

9

Electronegativity Review

Electronegativity: Measure of attractive force that one atom in a covalent bond has for electrons of the bond

10

Bond Dipoles o Two atoms with different electronegativity

values share electrons unequally o Electron density is uneven

o Higher charge concentration around more electronegative atom

o Bond dipoles o Indicated with delta (") notation o Indicates partial charge has arisen

H F

!+ !"

o  11

o  Net Dipoles o Symmetrical molecules

o Even if they have polar bonds o Are non-polar because bond dipoles cancel

o Asymmetrical molecules o Are polar because bond dipoles do not cancel o These molecules have permanent, net dipoles

o Molecular dipoles o Cause molecules to interact o Decreased distance between molecules increases

amount of interaction

COVALENT BOND

IONIC BOND

POLAR COVALENT

BOND

CHCl3

TiO2

F2

CaBr2

!

!

!

!

Group Problem


Identify the overall dipole moment for CHCl3

Group Problem


Identify the overall dipole moment for these molecules:

Solubility

15

LIKE DISSOLVES LIKE polar molecules dissolve in polar solvents

nonpolar molecules dissolve in nonpolar solvents Polar Solvents

Water: H2O Methanol: CH3OH Ethanol: CH3CH2OH Acetone: (CH3)2CO Acetic Acid: CH3CO2H Ammonia: NH3 Acetonitrile: CH3CN

Nonpolar Solvents Pentane: C5H12 Hexane: C6H14 Cyclohexane: C6H12 Benzene: C6H6 Toluene: CH3C6H5 Chloroform: CHCl3 Diethylether: (CH3CH2)2O

Which molecule will dissolve in water?

16

Vitamin A

Vitamin B12

Group Problem



Intermolecular Forces

18


The forces of attraction or repulsion between neighboring particles (atoms or molecules).

+ / - charges attract one another - / - or + / + forces repel each other

r r KE

19


o When substance melts or boils o Intermolecular forces are broken, not covalent

bonds o Responsible for non-ideal behavior of gases o Responsible for existence of condensed

states of matter o Responsible for bulk properties of matter

o Boiling points and melting points reflect strength of intermolecular forces

20

Types of Intermolecular Forces

①  London dispersion forces ②  Dipole-dipole forces ③  Hydrogen bonds ④  Ion-dipole forces

o  Ion-induced dipole forces

21

London-Dispersion Forces o When atoms near one another,

their valence electrons interact o  Repulsion causes electron clouds

in each to distort and polarize o  Instantaneous dipoles result from

this distortion o Effect enhanced with increased

volume of electron cloud size o Effect diminished by increased distance between particles and compact arrangement of atoms

22

London Dispersion Forces Affects ALL molecules, both polar & nonpolar Boiling Point (BP) is an indication of relative intermolecular force strength. Ease with which dipole moments can be induced and thus London Forces depend on ①  Distance between particles ②  Polarizability of electron cloud ③  Points of attraction

o Number atoms o Molecular shape (compact or elongated)

23

Polarizability = Ease with which the electron cloud can be distorted

Larger molecules often more polarizable o  Larger number of less tightly held

electrons o  Magnitude of resulting partial

charge is larger o  Larger electron cloud

24

Group Problem

Which is more polarizable? F2 or I2?

Table 12.1 Boiling Points of Halogens and Noble Gases

Larger molecules have stronger London forces and thus higher boiling points.

26

Number of Atoms in Molecule

o  London dispersion forces increase with the number atoms in molecule because more points of attraction

Formula BP at 1 atm, °C Formula BP at 1 atm, °C CH4 –161.5 C5H12 36.1 C2H6 –88.6 C6H14 68.7 C3H8 –42.1 : : C4H10 –0.5 C22H46 327

Hexane, C6H14 BP 68.7 °C

27

Which of the following molecules will have the highest boiling point?

Propane, C3H8 BP –42.1 °C

Group Problem

28

Molecular Shape o  Increased surface area available for contact =

increased points of contact = increase in London Dispersion forces. o More compact molecules:

Less surface area to interact with other molecules

o Less compact molecules: More surface area to interact with other molecules

29

•  Small area for interaction

•  Larger area for interaction

More compact – lower BP Less compact – higher BP

Which of the following molecules experience the strongest Dispersion forces?

30

Group Problem

31




32

Dipole-Dipole Attractions

o Occurs only between polar molecules

o Proportional to distance between molecules

o Polar molecules tend to align their partial charges: + / -

o As dipole moment increases, intermolecular force increases

+ - + -

- + - +

+ - + -

33

Dipole-Dipole Attractions Tumbling molecules

o Mixture of attractive and repulsive dipole-dipole forces

o Attractions (- -) are maintained longer than repulsions(- -)

o Get net attraction o ~1–4% of covalent bond

In the liquid state, which species has the strongest intermolecular forces, CH4, Cl2, O2 or HF?

HF The polar molecule

34

Group Problem

35




36

Hydrogen Bonds o Very strong dipole-dipole attraction: ~10% of a covalent

bond o Occurs between H and highly electronegative atom (O, N, or

F): H—F, H—O, and H—N bonds very polar o Electrons are drawn away from H giving atoms high

partial charges o H only has one electron, so %+

H presents almost bare proton

o %–X almost full –1 charge

o Element’s small size, means high charge density

37

Examples of Hydrogen Bonding H O

H

H O

H

H O

H

H N

H

H

H F H O

H

H F H N

H

H

H N

H

H

H N

H

HH N

H

H

H O

H

38

Hydrogen Bonding in Water

Hydrogen Bonds are strong! o  Responsible for the high boiling point of water o  Responsible for expansion of water as it freezes o  Hydrogen bonding (dotted lines) between

water molecules in ice form tetrahedral configuration

Hydrogen Bonding in Water

0.957 Å 1.97 Å

List all intermolecular forces for CH3CH2OH. Hydrogen-bonds, dipole-dipole attractions, London dispersion forces

40

Group Problem

41




42

Ion-Dipole Attractions o Attractions between ion and charged end of

polar molecules o Ions have full charges, increasing the attraction

(a) Negative ends of water dipoles surround cation (b) Positive ends of water dipoles surround anion

43

AlCl3!6H2O

o  Positive charge of Al3+ ion attracts partial negative charges %– on O of water molecules

o  Ion-dipole attractions hold water molecules to metal ion in hydrate o  Water molecules are found

at vertices of octahedron around aluminum ion

Attractions between ion and polar molecules

44

Ion-Induced Dipole Attractions o Attractions between ion and dipole it induces on

neighboring molecules o Depends on

o Ion charge and o Polarizability of its neighbor

o Attractions can be quite strong as ion charge is constant, unlike instantaneous dipoles of London-dispersion forces

45

Group Problem

How many water molecules would be attracted to this molecule by Ion-Dipole interactions?

46

Group Problem

List the intermolecular forces and rank in order of strength for the liquids of each molecule.

o Ion-Dipole o Hydrogen Bonding o Dipole-Dipole o London Forces

• Larger, longer, and therefore heavier molecules often have stronger intermolecular forces

• Smaller, more compact, lighter molecules have generally weaker intermolecular forces

Weakest

Strongest

Group Problem

Intermolecular Forces and Temperature

Decrease with increasing temperature o Increasing kinetic energy overcomes attractive

forces o If allowed to expand, increasing temperature

increases distance between gas particles and decreases attractive forces

48

49

Group Problem

GROUP PROBLEM SET 12.1



More properties of gases,

liquids, & solids

Compressibility Surface Tension

Diffusion

Retention of Volume & shape

Wetting Viscosity

Melting Point

Boiling Point

51

Melting & Boiling Point Often can predict physical properties by comparing strengths of intermolecular attractions:

Boiling Point increases when intermolecular forces increase

Melting Point increases when intermolecular forces increase

52

Compressibility

Measure of the ability of a substance to be forced into smaller volume

o Determined by strength of intermolecular forces o Gases highly compressible

o Molecules far apart o Weak intermolecular forces

o Solids and liquids nearly incompressible o Molecules very close together o Stronger intermolecular forces

53

Retention of volume and shape

o Solids retain both volume and shape o Strongest intermolecular attractions o Molecules closest

o Liquids retain volume, but not shape o Attractions intermediate

o Gases, expand to fill their containers o Weakest intermolecular attractions o Molecules farthest apart

54

Diffusion In Gases

o  Molecules travel long distances between collisions

o  Diffusion rapid In Liquids

o  Molecules closer o  Encounter more collisions o  Takes a long time to move

from place to place In Solids

o  Diffusion close to zero at room temperature

o  Will increase at high temperature

55

Surface Tension

Inside body of liquid o  Intermolecular forces are

the same in all directions Molecules at surface

o Potential energy increases when removing neighbors

o Molecules move together to reduce surface area and potential energy " sphere

Why does H2O bead up on a freshly waxed car instead of forming a layer?

56

Surface Tension

Liquids containing molecules with strong intermolecular forces have high surface tension

Allows us to fill glass above rim o Gives surface rounded

appearance o Surface resists expansion and

pushes back

o  Surface tension increases as intermolecular forces increase

o  Surface tension decreases as temperature increases

57

Wetting o  Ability of liquid to spread

across surface to form thin film

o  Greater similarity in attractive forces between liquid and surface, yields greater wetting effect

o  Occurs only if intermolecular attractive force between surface and liquid about as strong as within liquid itself

58

Wetting: Surfactants (Detergents) o  Detergents added to water to lower surface tension so water can

spread on greasy glass o  Substances that have both polar and non-polar characteristics o  Long chain hydrocarbons with polar tail

OS

O

O! Na+O

O

O! Na+

o  Nonpolar end dissolves in nonpolar grease o  Polar end dissolves in polar H2O o  Thus increasing solubility of grease in water

59

Viscosity o Resistance to flow o Measure of fluid’s

resistance to flow or changing form

o Decreases as Temp increases

o  Not just a property of liquids: o Gas: respond to instantly

to form changing force o  Amorphous solids, like

glass

60

Viscosity

Acetone Polar molecule

o Dipole-dipole and o  London forces

Ethylene glycol Polar molecule

o Hydrogen-bonding o Dipole-dipole and o  London forces

Which is more viscous?

Group Problem

61

Group Problem

For each pair given, which is has more viscosity?

CH3CH2CH2CH2OH, CH3CH2CH2CHO C6H14, C12H26 NH3(l ), PH3(l )

62

Group Problem




Changes of State

Heating/Cooling Curves "H

Phase Diagrams

64

Group Problem

65

Group Problem




Dynamic Equilibria

67

Group Problem

68

Group Problem




Solid Structures

70

Group Problem

71

Group Problem


72

Phase Changes

•  Changes of physical state – Deal with motion of molecules

•  As temperature changes – Matter will undergo phase changes

•  Liquid # Gas – Evaporation, vaporization – As heat is added, H2O, forms steam or water

vapor – Requires energy or source of heat

73

Phase Changes •  Solid # Gas

– Sublimation – Ice cubes in freezer, leave in long enough disappear – Endothermic

•  Gas # Liquid – Condensation – Dew is H2O vapor condensing onto cooler ground – Exothermic – Often limits lower night time temperature

74

Rate of Evaporation •  Depends on

– Temperature –  Surface area –  Strength of

intermolecular attractions

•  Molecules that escape from liquid have larger than minimum escape KE

•  When they leave – Average KE of

remaining molecules is less and so T lower

75

Effect of Temperature on Evaporation Rate

•  For given liquid – Rate of evaporation per

unit surface area increases as T increases

•  Why? – At higher T, total

fraction of molecules with KE large enough to escape is larger

– Result: rate of evaporation is larger

76

Kinetic Energy Distribution in Two Different Liquids

•  Smaller intermolecular forces

•  Lower KE required to escape liquid

•  A evaporates faster

•  Larger intermolecular forces

•  Higher KE required to escape liquid

•  B evaporates slower

A B

77

Changes Of State Involve Equilibria •  Fraction of molecules in condensed state is

higher when intermolecular attractions are higher

•  Intermolecular attractions must be overcome to separate the particles, while separated particles are simultaneously attracted to one another condensedphase

separatedphase

78

Before System Reaches Equilibrium

•  Liquid is placed in empty, closed, container – Begins to evaporate

•  Once in gas phase – Molecules can condense

by – Striking surface of liquid

and giving up some kinetic energy

79

System At Equilibrium •  Rate of evaporation =

rate of condensation •  Occurs in closed

systems where molecules cannot escape

80

Similar Equilibria Reached in Melting

Melting Point (mp) – Solid begins to change

into liquid as heat added •  Dynamic equilibria

exists between solid and liquid states –  Melting (red arrows) and

freezing (black arrows) occur at same rate

–  As long as no heat added or removed from equilibrium mixture

81

Equilibria Reached in Sublimation

At equilibrium •  Molecules sublime

from solid at same rate as molecules condense from vapor

82

Phase Changes

Ene

rgy

of S

yste

m

Gas

Solid

Liquid

Melting or Fusion

Vaporization Condensation

Freezing

Sublimation Deposition

& Exothermic, releases heat ' Endothermic, absorbs heat

83

Energy Changes Accompanying Phase Changes

•  All phase changes are possible under the right conditions

•  Following sequence is endothermic

heat solid # melt # heat liquid # boil # heat gas

•  Following sequence is exothermic

cool gas # condense # cool liquid # freeze # cool solid

84

Enthalpy Of Phase Changes Endothermic Phase Changes

1.  Must add heat 2.  Energy entering system (+)

Sublimation: $Hsub > 0 Vaporization: $Hvap > 0 Melting or Fusion: $Hfus > 0

Exothermic Phase Changes 1.  Must give off heat 2.  Energy leaving system (–)

Deposition: $H < 0 = –$Hsub Condensation: $H < 0 = –$Hvap Freezing: $H < 0 = –$Hfus

85

Phase Changes

•  As T changes, matter undergoes phase changes

•  Phase Change – Transformation from one phase to another

•  Liquid-Vapor Equilibrium – Molecules in liquid

• Not in rigid lattice •  In constant motion • Denser than gas, so more collisions • Some have enough kinetic energy to

escape, some don’t

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