intermolecular forces, liquids and solids - copy

8/13/2019 Intermolecular Forces, Liquids and Solids - Copy

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Intermolecular

Forces

Chapter 11

Intermolecular Forces,Liquids, and Solids

John D. Bookstaver

St. Charles Community College

St. Peters, MO

2006, Prentice Hall, Inc.

Chemistry, The Central Science, 10th edition

Theodore L. Brown; H. Eugene LeMay, Jr.;

and Bruce E. Bursten


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Intermolecular

Forces

States of Matter

The fundamental difference between states ofmatter is the distance between particles.


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Intermolecular

Forces

States of Matter

Because in the solid and liquid statesparticles are closer together, we refer to them

as condensed phases.


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Intermolecular

Forces

The States of Matter

The state a substance is

in at a particular

temperature and

pressure depends ontwo antagonistic entities:

The kinetic energy of the

particlesThe strength of the

attractions between the

particles


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Intermolecular

Forces

Intermolecular Forces

The attractions between molecules are not

nearly as strong as the intramolecularattractions that hold compounds together.


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Intermolecular

Forces


They are, however, strong enough to control

physical properties such as boiling andmelting points, vapor pressures, and

viscosities.


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Intermolecular

Forces


These intermolecular forces as a group are

referred to as van der Waals forces.


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Intermolecular

Forces

van der Waals Forces

Dipole-dipole interactions

Hydrogen bonding

London dispersion forces


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Intermolecular

Forces

Dipole-Dipole Interactions

Molecules that have

permanent dipoles are

attracted to each other.

The positive end of one isattracted to the negative

end of the other and vice-

versa.

These forces are only

important when the

molecules are close to

each other.


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Intermolecular

Forces

Dipole-Dipole Interactions

The more polar the molecule, the higher

is its boiling point.


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Intermolecular

Forces

London Dispersion Forces

While the electrons in the 1sorbital of helium

would repel each other (and, therefore, tendto stay far away from each other), it does

happen that they occasionally wind up on the

same side of the atom.


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Intermolecular

Forces


At that instant, then, the helium atom is polar,

with an excess of electrons on the left sideand a shortage on the right side.


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Intermolecular

Forces


Another helium nearby, then, would have a

dipole induced in it, as the electrons on theleft side of helium atom 2 repel the electrons

in the cloud on helium atom 1.


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Intermolecular

Forces


These forces are present in allmolecules,

whether they are polar or nonpolar. The tendency of an electron cloud to distort in

this way is called polarizability.


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Intermolecular

Forces

Factors Affecting London Forces

The shape of the molecule

affects the strength of dispersion

forces: long, skinny molecules

(like n-pentane tend to havestronger dispersion forces than

short, fat ones (like neopentane).

This is due to the increasedsurface area in n-pentane.


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Intermolecular

Forces

Factors Affecting London Forces

The strength of dispersion forces tends to

increase with increased molecular weight. Larger atoms have larger electron clouds,

which are easier to polarize.


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Intermolecular

Forces

Which Have a Greater Effect:Dipole-Dipole Interactions or Dispersion Forces?

If two molecules are of comparable size

and shape, dipole-dipole interactions

will likely be the dominating force. If one molecule is much larger than

another, dispersion forces will likely

determine its physical properties.


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Intermolecular

Forces

How Do We Explain This?

The nonpolar series

(SnH4to CH4) follow

the expected trend.

The polar series

follows the trend

from H2Te through

H2S, but water isquite an anomaly.


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Intermolecular

Forces

Hydrogen Bonding

The dipole-dipole interactions

experienced when H is bonded to

N, O, or F are unusually strong. We call these interactions

hydrogen bonds.


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Intermolecular

Forces

Hydrogen Bonding

Hydrogen bonding

arises in part from the

high electronegativity

of nitrogen, oxygen,and fluorine.

Also, when hydrogen is bonded to one of those

very electronegative elements, the hydrogen

nucleus is exposed.


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Intermolecular

Forces

Summarizing Intermolecular Forces


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Intermolecular

Forces

Viscosity

Resistance of a liquidto flow is called

viscosity.

It is related to the ease

with which moleculescan move past each

other.

Viscosity increases

with stronger

intermolecular forces

and decreases with

higher temperature.


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Intermolecular

Forces

Phase Changes


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Intermolecular

Forces

Energy Changes Associated

with Changes of State

Heat of Vaporization: Energy required tochange a liquid at its boiling point to a gas.


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Intermolecular

Forces

Energy Changes Associated

with Changes of State The heat added to the

system at the melting and

boiling points goes into

pulling the moleculesfarther apart from each

other.

The temperature of the

substance does not riseduring the phase change.


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Intermolecular

Forces

Vapor Pressure

At any temperature, some molecules in aliquid have enough energy to escape.

As the temperature rises, the fraction ofmolecules that have enough energy toescape increases.


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Intermolecular

Forces

Vapor Pressure

The liquid and vapor

reach a state of

dynamic equilibrium:liquid molecules

evaporate and vapor

molecules condense

at the same rate.


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Intermolecular

Forces

Vapor Pressure

The boiling point of aliquid is thetemperature at whichits vapor pressure

equals atmosphericpressure.

The normal boilingpoint is thetemperature at whichits vapor pressure is760 torr.


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Intermolecular

Forces

Phase Diagrams

Phase diagrams display the state of asubstance at various pressures and

temperatures and the places where equilibria

exist between phases.


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Intermolecular

Forces

Phase Diagrams

TheABline is the liquid-vapor interface.

It starts at the triple point (A), the point at

which all three states are in equilibrium.


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Intermolecular

Forces

Phase Diagrams

It ends at the critical point (B); above thiscritical temperature and critical pressure the

liquid and vapor are indistinguishable from

each other.


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Intermolecular

Forces

Phase Diagrams

TheADline is the interface between liquidand solid.

The melting point at each pressure can be

found along this line.


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Intermolecular

Forces

Phase Diagrams

BelowAthe substance cannot exist in theliquid state.

Along theACline the solid and gas phasesare in equilibrium; the sublimation point at

each pressure is along this line.


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Intermolecular

Forces

Phase Diagram of Water

Note the high critical

temperature and critical

pressure:These are due to the

strong van der Waals

forces between water

molecules.


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Intermolecular

Forces

Phase Diagram of Carbon Dioxide

Carbon dioxide

cannot exist in the

liquid state atpressures below

5.11 atm; CO2

sublimes at normal

pressures.


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Intermolecular

Forces

Phase Diagram of Carbon Dioxide

The low critical

temperature and

critical pressure forCO2make

supercritical CO2a

good solvent for

extracting nonpolarsubstances (such as

caffeine).


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Intermolecular

Forces

Solids

We can think of

solids as falling into

two groups:

Crystallineparticles

are in highly ordered

arrangement.


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Intermolecular

Forces

Attractions in Ionic Crystals

In ionic crystals, ionspack themselves so as

to maximize the

attractions and

minimize repulsionsbetween the ions.


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Intermolecular

Forces

Crystalline SolidsBecause of the

order in a crystal,

we can focus on the

repeating pattern of

arrangement calledthe unit cell.


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Intermolecular

Forces

Crystalline Solids

There are several types of basic

arrangements in crystals, such as the onesshown above.


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Intermolecular

Forces

Crystalline Solids

We can determine

the empirical

formula of an ionic

solid by determining

how many ions of

each element fall

within the unit cell.


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Intermolecular

Forces

Ionic Solids

What are the empirical formulas for thesecompounds?

(a) Green: chlorine; Gray: cesium

(b) Yellow: sulfur; Gray: zinc

(c) Green: calcium; Gray: fluorine

CsCl ZnS CaF2

(a) (b) (c)

Types of Bonding in


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Intermolecular

Forces

Types of Bonding in

Crystalline Solids

Covalent Network and


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Intermolecular

Forces

Covalent-Network and

Molecular Solids

Diamonds are an example of a covalent-

network solid in which atoms are covalentlybonded to each other.

They tend to be hard and have high melting

points.

Covalent Network and


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Intermolecular

Forces

Covalent-Network and

Molecular Solids

Graphite is an example of a molecular solid in

which atoms are held together with van derWaals forces.

They tend to be softer and have lower melting

points.


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intermolecular forces, liquids and solids - copy

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