12-1 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 12 Intermolecular Forces: แรงระหว่างโมเลกุล

12-1

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

Chapter 12

Intermolecular Forces:

แรงระหว่�างโมเลกุ�ล :

Liquids, Solids, and Phase Changes

ของเหลว่ ของแข�ง และกุารเปล��ยนว่�ฏภาค

12-2


Intermolecular Forces: Liquids, Solids, and Phase Changes

12.1 An Overview of Physical States and Phase Changes

12.2 Quantitative Aspects of Phase Changes

12.3 Types of Intermolecular Forces

12.4 Properties of the Liquid State

12.5 The Uniqueness of Water

12.6 The Solid State: Structure, Properties, and Bonding

12.7 Advanced Materials

12-3


ATTRACTIVE FORCES

electrostatic in nature

Intramolecular forces bonding forces

These forces exist withinwithin each molecule.They influence the chemicalchemical properties of the substance.

Intermolecular forces nonbonding forces

These forces exist betweenbetween molecules.They influence the physicalphysical properties of the substance.

12-4


Phase Changes

solid liquid gas

melting

freezing

vaporizing

condensing

sublimination

endothermic

exothermic

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Table 12.1

A Macroscopic Comparison of Gases, Liquids, and Solids

State Shape and Volume Compressibility Ability to Flow

Gas Conforms to shape and volume of container

high high

Liquid Conforms to shape of container; volume limited by surface

very low moderate

Solid Maintains its own shape and volume

almost none

almost none

12-6


Figure 12.1

Heats of vaporization and fusion for several common substances.

12-7


Figure 12.2 Phase changes and their enthalpy changes.

12-8


Figure 12.3 A cooling curve for the conversion of gaseous water to ice.

12-9


Within a phase, a change in heat is accompanied by a change in temperature which is associated with a change in average Ek as the most probable speed of the molecules changes.

Quantitative Aspects of Phase Changes

During a phase change, a change in heat occurs at a constant temperature, which is associated with a change in Ep, as the average distance between molecules changes.

q = (amount)(molar heat capacity)(T)

q = (amount)(enthalpy of phase change)

12-10


Figure 12.4

Liquid-gas equilibrium.

12-11


Figure 12.5The effect of temperature on the distribution of

molecular speed in a liquid.

12-12


Figure 12.6 Figure 12.7

Vapor pressure as a function of temperature and intermolecular forces.

A linear plot of vapor pressure- temperature

relationship.

12-13


The Clausius-Clapeyron Equation

ln P =

-HvapR

1T

C

ln P2P1

= -Hvap

R1T2

1T1

12-14


SAMPLE PROBLEM 12.1 Using the Clausius-Clapeyron Equation

SOLUTION:

PROBLEM: The vapor pressure of ethanol is 115 torr at 34.90C. If Hvap of ethanol is 40.5 kJ/mol, calculate the temperature (in 0C) when the vapor pressure is 760 torr.

PLAN: We are given 4 of the 5 variables in the Clausius-Clapeyron equation. Substitute and solve for T2.

ln

P2P1

= -Hvap

R1

T2

1T1

34.90C = 308.0K

ln760 torr115 torr

=-40.5 x103 J/mol8.314 J/mol*K

1T2

1308K

-

T2 = 350K = 770C

12-15


Figure 12.8 Iodine subliming.

iodine solid

iodine vapor

iodine solid

test tube with ice

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Figure 12.9 Phase diagrams for CO2 and H2O.

CO2 H2O

12-17


bond lengthcovalent radius

van der Waal’s distance

van der Waal’s radius

Figure 12.10 Covalent and van der Waals radii.

12-18


Figure 12.11

Periodic trends in covalent and van der Waals radii (in pm).

12-19


12-20


12-21


Figure 12.12 Polar molecules and dipole-dipole forces.

solid

liquid

12-22


THE HYDROGEN BOND

a dipole-dipole intermolecular force

The elements which are so electronegative are N, O, and F.

A hydrogen bond may occur when an H atom in a molecule, bound to small highly electronegative atom with lone pairs of electrons, is attracted to the lone pairs in another molecule.

..F..

.. ..H O..

N.. FH

..

..

..

O.. ..

..NH

hydrogen bonddonor

hydrogen bondacceptor


hydrogen bonddonor

hydrogen bonddonor


12-23


Figure 12.13 Dipole moment and boiling point.

12-24


SAMPLE PROBLEM 12.2 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

12-25


Figure 12.14 Hydrogen bonding and boiling point.

12-26


Polarizability and Charged-Induced Dipole Forces

distortion of an electron cloud

•Polarizability increases down a group

size increases and the larger electron clouds are furtherfrom the nucleus

•Polarizability decreases left to right across a period

increasing Zeff shrinks atomic size and holds the electronsmore tightly

•Cations are less polarizable than their parent atom because they are smaller.

•Anions are more polarizable than their parent atom because they are larger.

12-27


Figure 12.15 Dispersion forces among nonpolar molecules.

separated Cl2

molecules

instantaneous dipoles

12-28


Figure 12.16

Molar mass and boiling point.

12-29


Figure 12.17 Molecular shape and boiling point.

more points for dispersion forces to act

fewer points for dispersion forces to act

12-30


PROBLEM: For each pair of substances, identify the dominant intermolecular forces in each substance, and 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

CH3PLAN:

•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.

SAMPLE PROBLEM 12.3 Predicting the Type and Relative Strength of Intermolecular Forces

12-31


SOLUTION:

SAMPLE PROBLEM 12.3 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 that in CH3F cannot. 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 dispersion force interaction. Therefore CH3CH2OH has the higher boiling point.(d) Hexane and 2,2-dimethylbutane are both nonpolar with only dispersion forces to hold the molecules together. Hexane has the larger surface area, thereby the greater dispersion forces and the higher boiling point.

12-32


Figure 12.18

Summary diagram for analyzing the intermolecular 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 onlyDISPERSIONFORCES only

nonpolar molecules onlyDISPERSIONFORCES only

Dispersion forces also present

H bonded toN, O, or F

12-33


Figure 12.19 The molecular basis of surface tension.

hydrogen bondingoccurs in three

dimensions

hydrogen bondingoccurs across the surface

and below the surfacethe net vectorfor attractive

forces is downward

12-34


Table 12.3 Surface Tension and Forces Between Particles

Substance FormulaSurface Tension

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

diethyl ether

ethanol

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

12-35


Figure 12.20 Shape of water or mercury meniscus in glass.

adhesive forcesstronger

cohesive forces

H2O

capillarity

Hg

12-36


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

12-37


Figure 12.21 The H-bonding ability of the water molecule.

hydrogen bond donor

hydrogen bond acceptor

12-38


The Unique Nature of Water

•great solvent properties due to polarity andhydrogen bonding ability

•exceptional high specific heat capacity

•high surface tension and capillarity

•density differences of liquid and solid states

12-39


Figure 12.22 The hexagonal structure of ice.

12-40


Figure 12.23 The expansion and contraction of water.

12-41


Figure 12.24 The macroscopic properties of water and their atomic and molecular “roots”.

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12-1 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 12 Intermolecular Forces: แรงระหว่างโมเลกุล