lecture 7.2 - liquids & phase changes

8/14/2019 Lecture 7.2 - Liquids & Phase Changes

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States OfStates Of

Matter II:Matter II:

Liquid Properties & Phase ChangesLiquid Properties & Phase Changes


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q p gq p g

Liquid PropertiesLiquid Properties

Cohesive ForcesCohesive Forces the intermolecular

attraction between like molecules Adhesive ForcesAdhesive Forces the attraction

between unlike molecules

Water (red) hasstronger adhesive

forces. Mercury hasstronger cohesiveforces.

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3

H2O vs Hg

Adhesion

Cohesion


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Properties of LiquidsProperties of Liquids

1.Surface Tension

2.Capillary Action

3.Viscosity

4.Vapor Pressure

4


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Surface TensionSurface Tension

Surface tension is the amount of energy required to

stretch or increase the surface of a liquid by a unitarea.

Strong intermolecular forces

= High surface tension

- tendency to minimizesurface area


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Surface Tension Fun

6
http://flickr.com/photos/72553346@N00/237875014/


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In Orbit (Space Shuttle), waterdroplets are spherical

7http://spaceflightsystems.grc.nasa.gov/WaterBalloon/#AIRPLANE


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Water Boiling in ZeroGravity

http://science.nasa.gov/headlines/images/boiling/bubble0g.mpg

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http://www.unoriginal.co.uk/nuvideos1234/balloon%20-

%20UNORIGINAL.CO.UK.wmv

next


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Capillary ActionCapillary Action The ability of a liquid to flow against gravity up a narrow

tube.

Attraction of water to glass walls draws water up tubes 10


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ViscosityViscosity

measure of a fluids resistance to flow.

If a liquid has strong intermolecular

interactions then particles will not flow pasteach other easily and viscosity will be high.

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Viscosity of Several Hydrocarbons

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Viscosity of Some Common Liquids


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15Greatest Order

Least Order

Phase ChangesPhase Changes

Endo

thermic

Exoth

erm

ic


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Some Molecules in an open

beaker have enoughkinetic energy to vaporizefrom the surface of theliquid.

Vaporization

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A system at equilibrium is dynamic (changing) but

has no net macroscopic changes


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Vapor PressureVapor Pressure

The pressure exerted by a vapor in

equilibrium with its liquid phase.

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As IMF strength decreases, vapor pressure increases


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Volatile Liquids Liquids with high vapor pressures

They evaporate rapidly in an open dish

They have weak intermolecular forces

- more of its molecules can break free andvaporize

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As temperature increases, vaporpressure increases

- More molecules will have enoughKE to break away fromintermolecular forces

Vapor Pressure & TemperatureVapor Pressure & Temperature

T1 < T2 21


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Vapor Pressure Vs Temperature for Three Liquids

M l E th l f V i ti


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Hvap = energy required to vaporize 1 mole of a liquidat its boiling point (in kJ/mol)

Determined by the Clausius-Clapeyron Equation

Molar Enthalpy of Vaporization

( Hvap )

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Clausius-Clapeyron Equation

(equilibrium) vapor pressure= temperature (K)

gas constant (8.314 J/Kmol) 24


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or

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Alternate Forms of the Clausius-Clapeyron Equation

At two temperatures:

or


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Example Problem

Dry ice sublimes at 78oC andhas a H

subof 25.2 kJ/mol.

Calculate the vapor pressureof CO

2at 100oC.

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atmP

P

atm

P

atm

KK

KK

molKJ

molJ

P

atm

TT

TT

R

H

P

P

vap

vap

vap

vap

vap

vap

vap

138.0

23.710

00.1

859.000.1

log

)173)(195(

173195

)/31.8)(303.2(

/2520000.1log

303.2log

1

859.0

1

1

1

21

12

1

2

=

==

=

=

=

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A liquid boils when thermal energy is high enough to cause

molecules in the interior of the liquid to become gaseous,forming bubbles that rise to the surface.29


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The boiling pointis the temperature at which the

(equilibrium) vapor pressure of a liquid is equal to

the external pressure. (Pvap

= Patm

)

The normal boiling pointis the temperature at

which a liquid boils when the external pressure is 1

atm.


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H2O (s) H2O (l)

The melting pointof asolid or the freezing point

of a liquid is the

temperature at which the

solid and liquid phasescoexist in equilibrium

Solid-LiquidSolid-Liquid

EquilibriumEquilibrium


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Molar Enthalpy of Fusion ( Hfus )

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Hfus =is the energy (in kJ/mol) required to melt 1mole of a solid substance at its freezing point.


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HeatingHeating

CurveCurve

Energy input increases

Length of plateau depends

on Hvap

Length of plateau depends

on Hfus

Slope decreases as specific heat capacity of a phase increases


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AP Exam Diagrams

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H2O (s) H2O (g)

Molar heat ofsublimation ( Hsub )is the energy requiredto sublime 1 mole of a

solid. Hsub = Hfus + Hvap

( Hesss Law)

Solid-GasSolid-Gas

EquilibriumEquilibrium


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Aphase diagram summarizes the conditions at

which a substance exists as a solid, liquid, or gas.

Phase Diagramof Water


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Effect of Increase in Pressure on the Melting Point

of Ice and the Boiling Point of Water


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Phase diagram for CO2

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Triple point

A three-way intersection representing the

unique temp, pressure where all threephases exist simultaneously in equilibrium.

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Critical Point (Tc)

The temperature (critical temperature) where a gascannot be liquefied no matter what the pressure.


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The Critical Phenomenon of SF6

T < Tc T > Tc T ~ Tc T < Tc

Th iti l t t (T ) i th t t b hi h th


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The critical temperature (Tc) is the temperature above which the gas

cannot be made to liquefy, no matter how great the applied pressure.

The critical pressure(Pc) is the minimum

pressure that must be

applied to bring aboutliquefaction at the criticaltemperature.


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Supercritical fluid

Neither a liquid nor a gas. The liquidand gas forms becomeindistinguishable at this point.

http://www.nottingham.ac.uk/supercritical/scintro.

html

Ph di f H O


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Phase diagram for H2O


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Density of PhasesDensity of Phases

+ slope: dsolid > dliquid slope: dsolid < dliquid

most substances H2O

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AP Exam Diagram

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3 D Structure of Water

W t i U i S b t


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3-D Structure of Water

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Density of Water

Maximum

Density40C

Ice is less dense than water

Water is a Unique Substance


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Water

colorless, odorless, tasteless, liquid at ordinarytemperatures

only inorganic compound occurring naturally asa liquid

Composes ~65% of mass of living organisms excellent solvent for many things

abnormally high boiling and melting point

ice is less dense than water (it floats)

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Water purification

Hard water -- Contains Ca+2 , Mg+2 , Fe+3 and other minerals.

Soft water -- Doesnt contain Ca+2 , Mg+2 ,Fe+3 ions.

Softened water -- metal cations in hardwater are replaced by Na+.

Deionized water -- cations are replaced

by H+ and anions are replaced by OH-

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Chemistry In Action: Ice Skating


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Chemistry In Action: Ice Skating


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HomeworkHomework

p. 501 # 40c-f, 41, 42p. 501 # 40c-f, 41, 42p. 504 # 81, 82p. 504 # 81, 82 (use 250.0 g of substance X)(use 250.0 g of substance X)

p. 505 # 86 - 89p. 505 # 86 - 89

lecture 7.2 - liquids & phase changes

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