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Liquids, Solids andLiquids, Solids andChanges of StateChanges of State

Kinetic-Molecular View of Liquids and SolidsKinetic-Molecular View of Liquids and SolidsIntermolecular AttractionsIntermolecular Attractions

Properties of LiquidsProperties of LiquidsVapor Pressure and Boiling PointVapor Pressure and Boiling Point

Melting Points and FreezingMelting Points and FreezingHeating and Cooling CurvesHeating and Cooling Curves

Phase DiagramsPhase DiagramsCrystalsCrystals

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Kinetic-molecular viewKinetic-molecular viewof liquids and solidsof liquids and solids

All real gases can be condensed to liquids by lowering the temperature and increasing the pressure.

• This decreases the average speed of the molecules.

• When moving slow enough, they will be attracted to each other and form a liquid.

IncreasedP and T

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Kinetic-molecular viewKinetic-molecular viewof liquids and solidsof liquids and solids

If the temperature is further decreased:• Molecules can no longer move about

freely.• Motion is limited to vibration.

Rapid temperature decrease results in a disorderly arrangement - amorphousamorphous.

A slow temperature decrease allows molecules to form a crystallinecrystalline solid.

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Intermolecular forcesIntermolecular forcesFor molecules to form liquids and solids, there

must be attractions between the them.Intermolecular attractive forces

•dipole-dipole attraction including dipole-dipole attraction including hydrogen bondinghydrogen bonding

•London (dispersion) forcesLondon (dispersion) forces

Relative strengthRelative strengthhydrogen bonding > dipole-dipole > London

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Dipole-dipole attractionsDipole-dipole attractions- When electrons that make up a bond are

not equally shared because of a difference in electronegativity.

+ and - ends are attracted to each other.

HCl+-

HCl+-

HCl+-

HCl+-

HCl+-

HCl+-

HCl+-

HCl+-

HCl+- HCl

+-

HCl+-

solid liquid

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Hydrogen bondingHydrogen bonding

An unusually strong dipole-dipole attraction.

• Occurs when hydrogen is bound to fluorine, oxygen and nitrogen -- the most electronegative elements.

• The small sizes of the elements involved and the large electronegativity differences result in large + and - values.

• Hydrogen bonds are usually represented using a dashed line.

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Hydrogen bondingHydrogen bonding

The hydrogens of one water moleculeinteract with theoxygen on otherwater molecules.

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London forcesLondon forcesTemporary dipole attractions that exist

between molecules - also called the dispersivedispersive.

Results from random electron motion.Relatively weak force.

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Properties of liquidsProperties of liquids

DiffusionDiffusionThis takes place in both liquids and gases. It is the spontaneous mixing of materials that results from the random motion of molecules.

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ViscosityViscosityResistance to flow.This increases with increased intermolecular

attractions.

Also, liquids composed of long, flexible molecules can entwine, resulting in increased viscosity - motor oil.

Properties of liquidsProperties of liquids

CH3CH2CH2

OHCH3CH CH2

OHOHCH2CH CH2

OHOHOH

Increasing viscosity

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Properties of liquidsProperties of liquidsSurface TensionSurface Tension

Force in the surface of a liquid that makes the area of the surface as small as possible.

Molecules at thesurface interactonly with neighborsinside the liquid.

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Properties of liquidsProperties of liquids

Capillary actionCapillary actionIt is the competition between two forces.

Cohesive forcesCohesive forcesThe attractions between molecules of a substance.

Adhesive forcesAdhesive forcesAttractions between molecules of different substances.

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Properties of liquidsProperties of liquidsCapillary action.Capillary action.

MercuryCohesive is largerthan adhesive.

WaterAdhesive is largerthan cohesive.

Capillary tube

meniscus

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Properties of liquidsProperties of liquidsVaporizationVaporization

The formation of a gas from a liquid.At any temperature, at least a few of the molecules in the liquid are moving fast enough to escape.

initially equilibrium

after sometime

molecules leave andrenter liquid atthe same rate

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EquilibriumEquilibriumA state where the forward and reverse

conditions occur at the same rate.

DynamicEquilibrium

I’m in staticequilibrium.

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EquilibriumEquilibrium

A point is ultimatelyreached where therates of the forwardand reverse changesare the same.

At this point, equilibrium is reached.

Rate

Time

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Chemical equilibriumChemical equilibrium

A dynamic process on the molecular level achieved when concentration of reactants and products remain constant over time.

- for a physical process:

H2O(l) H2O(s)

(reactant) (product)

- the equilibrium process is indicated with an equilibrium arrows.

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EquilibriumEquilibriumCo

ncen

tratio

n

Time

Kinetic EquilibriumRegion Region

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Le Chatelier’s principleLe Chatelier’s principle

Any stress placed on an equilibrium system Any stress placed on an equilibrium system will cause the system to shift to minimize will cause the system to shift to minimize the effect of the stress.the effect of the stress.

You can put stress on a system by adding or removing something from one side of a reaction.

N2(g) + 3H2 (g) 2NH3 (g)

What effect will there be if you added moreammonia? How about more nitrogen?

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Vapor Pressure and boiling pointVapor Pressure and boiling point

Equilibrium vapor pressureEquilibrium vapor pressureThe pressure of a vapor in equilibrium with a liquid.

It depends on:It depends on:• the intermolecular forces in the liquid.• temperature.

It is independent of:It is independent of:• the volume of the liquid or vapor• the surface area of the liquid

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Boiling PointsBoiling Points

Boiling pointBoiling point - temperature where the vapor pressure equals atmospheric pressure.

Boiling point of water at various elevationsCity Elevation BP (oC) San Francisco Sea level 100.0 Salt Lake City 4,390 95.6 Denver 5,280 95.0 La Paz, Bolivia 12,795 91.4 Mount Everest 20,028 76.5

This is the reason that cake mixes includehigh altitude baking instructions.

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Boiling pointBoiling pointBoiling points are dependent on pressure.

NormalNormalboiling pointboiling pointThe boilingpoint at standardatmosphericpressure(760 mmHg) 0 50 100

Temperature, oC

Vapo

r pre

ssur

em

mHg

1000

500

0 Norm

al B

P

Standard atmospheric pressure

Vapor pressure of H2O

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Melting pointMelting point

Normal melting pointNormal melting pointTemperature at which a solid changes to a liquid at atmospheric pressure.

Freezing point.Freezing point.The temperature at which a liquid changes to a solid.

For the same substance, these will both be at the same temperature.

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Changes in stateChanges in stateA substance can usually be converted to

different states by adding or removing energy from a system.If energy must be added, the change is- endothermicendothermic

If energy is given off, the change is- exothermicexothermic

The same concept can also be applied to chemical reactions.

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EndothermicEndothermicchanges of statechanges of state

SublimationSublimationThe direct conversion of a solid to a gas.Example - dry ice (solid CO2)

Melting or fusionMelting or fusionThe conversion of a solid to a liquid.Example - melting of ice

Evaporization or vaporizationEvaporization or vaporizationConverting a liquid to a gas.Example - boiling water

Most materials first melt then vaporize as you raise the temperature.

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EndothermicEndothermicchanges of statechanges of state

Gas

Solid Liquid

subli

mation

evaporation or

vaporization

melting or fusion

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ExothermicExothermicchanges of statechanges of state

Condensation or liquifactionCondensation or liquifactionThe conversion of a gas to a liquid or solid.Example - steam becoming water

Freezing or crystallizationFreezing or crystallizationWhen a liquid becomes a solid.Examples - formation of ice from water

Substances usually first condense to liquids and then become solids.

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ExothermicExothermicchanges of statechanges of state

Gas

Solid Liquid

depo

sition

liquification or

condensation

freezing orcrystallization

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Changes in state andChanges in state andattractive forcesattractive forces

As the attractive forces between molecules become larger, more energy is needed to separate them.

Vapor pressures become smaller, boiling points and melting points become larger.

Chemical MW Polarity Mp Bp N2 28 Nonpolar -210 -196 O2 32 Nonpolar -219 -183 NH3 17 Polar -78 -33 H2O 18 Polar 0 100NaCl 58 Ionic 801 1465

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Heating and CoolingHeating and Cooling

Changes in state involve several steps.

Example.Example.Producing 150 Producing 150 ooC steam from -20 C steam from -20 ooC C

ice.ice.1. Heat ice up to 0 oC.2. Convert the ice to water.3. Heat the water from 0 oC to 100

oC.4. Convert the water to steam.5. Heat the steam to 150 oC.

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Heating and CoolingHeating and Cooling

Heat of fusion, Heat of fusion, HHfusfusThe amount of thermal energy necessary to melt one mole of a substance at its melting point.

Heat of vaporization, Heat of vaporization, HHvapvapThe amount of thermal energy necessary to boil one mole of a substance at its boiling point.

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Heating and CoolingHeating and Coolingmp Hfus bp Hvap

Substance oC kJ/mol oC kJ/mol

Br2 -7.3 10.5759.2 29.5

CH3CH2OH -117.0 4.6079.0 43.5

CH3(CH2)6CH3 -56.8 20.65 125.7 38.6

H2O 0.0 6.01 100.0 40.7

Na 97.8 2.60 883 98.0

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Specific heatsSpecific heatsEach substance requires a different amount of

energy to increase its temperature.Specific heatSpecific heat - amount of energy needed to

increase a substance’s temperature by 1oC.It also depends on the state of the substance.

Substance J/g Substance J/gAluminum, solid 1.0 Ice 2.1Copper, solid 0.4 Water 4.2Hydrogen, gas 14.2 Steam 2.0Mercury, liquid 0.1

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Phase DiagramsPhase DiagramsGraphs that show the states of a substance as

a function of both pressure and temperature.

solidliquid

gas

temperature

pres

sure

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Phase DiagramsPhase DiagramsPartial phase diagram for water.Partial phase diagram for water.

ice

water

steam

-20 0 20Temperature, oC

Pres

sure

, mm

Hg

30

20

10

0

Triple point

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Phase DiagramsPhase Diagrams

Triple pointTriple pointAll three phases are in equilibrium. Temperature and pressure are fixed.

The triple point for water is at 0.01 oC and 4.58 mmHg.

The triple point for water, 273.16 K is used to define the Kelvin temperature scale.

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Phase DiagramsPhase Diagramssupercriticalfluid region

Tc

solidliquid

gas

Pc

temperature

pres

sure Critical

point

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Phase DiagramsPhase DiagramsCritical pointCritical point

The end of the vapor pressure curve.

Critical temperature, Critical temperature, TTccThe temperature at the critical point.

Critical pressure, Critical pressure, PPccThe pressure at the critical point.

At temperatures above Tc, liquefying a gas is impossible, no matter what the pressure.

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Phase DiagramsPhase DiagramsAt pressures and temperatures above the

critical point, a supercritical fluidsupercritical fluid is formed.

A supercritical fluid:• is a gas.• has a density similar to a liquid.• has a viscosity similar to a gas.

Supercritical fluids have a number of uses. One example is their use for extractions - removal of caffeine from coffee.

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The solid stateThe solid state

At room temperature, solids:•are not compressible•commonly have regular repeating units

Two types are observedCrystallineCrystalline solids have a definite melting

point. - ionic- ionic - covalent- covalent - molecular- molecular - metallic- metallicAmorphousAmorphous solids do not have a definite

melting point or regular repeating units.

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Ionic solidsIonic solids

Ions make up the repeating units.

NaClNaCl

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Covalent solidsCovalent solids

Repeating units of covalently bound atoms.

GraphiteGraphite

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Molecular solidsMolecular solidsRepeating units are made up of molecules.

IceIce

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Metallic solidsMetallic solidsRepeating units are made up of metal atoms,Valence electrons are free to jump from one

atom to another,

++ + +++ + +

++ + +++ + +

++ + +

++ + +++ + +++ + +

++ + +

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Arrangement of units in crystalsArrangement of units in crystals

MetalsMetals• All atoms are spherical.• In a crystal, they are packed to minimize

the space they occupy.Coordination numberCoordination numberThe number of nearest neighbors that surround an atom in a crystalUnit cellUnit cellThe smallest three dimensional unit that describes the arrangement of the atoms

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Simple cubic crystalsSimple cubic crystalsThis is one of the simplest arrangements to visualize.• Each atom has a coordination number of six.• Only 52% of the space is occupied.

Single layer Expanded model

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Close PackingClose PackingThe crystals of most metals are of this type.• Each atom is surrounded by 6 neighbors in its

layer and a total of 12 in three dimensions..• This results in a high percentage of the space

being occupied.

Singlelayer

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Close PackingClose Packing

The atoms in a layer mustrest in holes of the twolayers that touch it.

Two types of crystalscan result.

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Cubic close packingCubic close packing

Face-centered cubic cellFace-centered cubic cell

Each face has fiveatoms the maximumamount of space isoccupied by theatoms - 74%.

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Body-centered cubic unit cellBody-centered cubic unit cell

This unit cell is observed for all metals that do not crystallize in one of the two close-packed arrangements.

The exception is polonium.

The coordinationis eight.

68 % of the space isoccupied.

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Body-centered cubic, GaAsBody-centered cubic, GaAs

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Crystal structuresCrystal structuresCoordination % of space

Name number occupied Example

Face-centered 12 74 Alcubic

Body-centered 8 68 Nacubic

Simplecubic 6 52 Po

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Ionic compoundsIonic compoundsCrystal structures for these compounds are

complicated by the following:• Two or more kinds of particles are involved.• The particles are usually differ in size and

often in charge.• Not all ions are spherical.

The major attractive force is electrostatic and crystals should allow the largest number of oppositely charged particles to touch.

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Ionic compoundsIonic compoundsMany ionic compounds will

assume a close-packed arrangement of anions.

Small cations are placed in the holes.

Because each is surrounded by four spheres, the smaller holes are called tetrahedral holes.

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Ionic compoundsIonic compounds

Many compounds will have this type of structure including LiCl, NaCl, NaBr, MgO, NiO, and NH4I.

NaClNaCl

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Ionic compoundsIonic compounds

CsClCsCl

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Ionic compoundsIonic compounds

AlAl22OO33

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X-ray diffractionX-ray diffraction

This method is used to find the dimensions and shape of a crystal unit.

It provides a ‘fingerprint’ of a material which can be used:

•To deduce the structure of a material•To identify a substance•To tell structure of a polymer•For elemental analysis

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X-ray diffractionX-ray diffraction

Film can be used fordetection of the patternsIt is now more commonto rotate the crystaland detect the x-rayswith a fixed positiondetector.This way, you havedata that can be processed by a computer