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LiquidsLiquidsSection 13.5Section 13.5

LiquidsLiquidsSection 13.5Section 13.5

In a liquidIn a liquid•• molecules are in molecules are in

constant motionconstant motion

•• there are appreciable there are appreciable intermolec. forcesintermolec. forces

•• molecules close molecules close togethertogether

•• Liquids are almost Liquids are almost incompressibleincompressible

•• Liquids do not fill the Liquids do not fill the containercontainer

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LiquidsLiquids

The two key properties we need to describe The two key properties we need to describe are are EVAPORATIONEVAPORATION and its opposite— and its opposite—CONDENSATIONCONDENSATION

The two key properties we need to describe The two key properties we need to describe are are EVAPORATIONEVAPORATION and its opposite— and its opposite—CONDENSATIONCONDENSATION

break IM bonds

make IM bonds

Add energy

Remove energy

LIQUID VAPOR

<---condensation<---condensation

evaporation--->evaporation--->

3Liquids—Liquids—EvaporationEvaporation

To evaporate, molecules To evaporate, molecules must have sufficient must have sufficient energy to break IM forces.energy to break IM forces.

Breaking IM forces Breaking IM forces requires energy. The requires energy. The process of process of evaporation is evaporation is endothermicendothermic..

4Liquids—Liquids—Distribution of EnergiesDistribution of Energies

Distribution of Distribution of molecular molecular energies in a energies in a liquid.liquid.

KE is propor-KE is propor-tional to T.tional to T.

Distribution of Distribution of molecular molecular energies in a energies in a liquid.liquid.

KE is propor-KE is propor-tional to T.tional to T.

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Nu

mb

er o

f m

olec

ule

s

Molecular energy

higher Tlower T

See Figure 13.12See Figure 13.12

Minimum energy req’d to break IM forces and evaporate

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Distribution of Energy in a Distribution of Energy in a LiquidLiquid

Figure 13.12

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LiquidsLiquids At higher T a much At higher T a much larger number of larger number of molecules has high molecules has high enough energy to enough energy to break IM forces and break IM forces and move from liquid to move from liquid to vapor state.vapor state.

High E molecules carry High E molecules carry away E. You cool away E. You cool down when sweating down when sweating or after swimming.or after swimming.

.

0

Num

ber

of m

olec

ules

Molecular energy

minimum energy neededto break IM forces and evaporate

higher Tlower T

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LiquidsLiquidsWhen molecules of liquid When molecules of liquid

are in the vapor state, are in the vapor state, they exert a they exert a VAPOR VAPOR PRESSUREPRESSURE

EQUILIBRIUM EQUILIBRIUM VAPOR VAPOR PRESSUREPRESSURE is the is the pressure exerted by a pressure exerted by a vapor over a liquid in a vapor over a liquid in a closed container when closed container when the the rate of evaporation rate of evaporation = the rate of = the rate of condensation.condensation.

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Equilibrium Vapor Equilibrium Vapor PressurePressure

Liquid in flask evaporates and exerts pressure on manometer.

See Fig. 13.15See Fig. 13.15

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Vapor PressureVapor PressureCD, Screen 13.9CD, Screen 13.9

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Equilibrium Vapor PressureEquilibrium Vapor PressureFigure 13.16Figure 13.16

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LiquidsLiquidsEquilibrium Vapor PressureEquilibrium Vapor Pressure

FIGURE 13.16:FIGURE 13.16: VP as a function of T.VP as a function of T.

1. The curves show all conditions of P and 1. The curves show all conditions of P and T where LIQ and VAP are in T where LIQ and VAP are in EQUILIBRIUMEQUILIBRIUM

2. The VP rises with T.2. The VP rises with T.

3. When VP = external P, the liquid boils.3. When VP = external P, the liquid boils.

This means that BP’s of liquids change This means that BP’s of liquids change with altitude.with altitude.

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Boiling LiquidsBoiling Liquids

Liquid boils when its vapor pressure equals atmospheric pressure.

Liquid boils when its vapor pressure equals atmospheric pressure.

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Boiling Point Boiling Point at Lower Pressureat Lower Pressure

When pressure is lowered, the vapor When pressure is lowered, the vapor pressure can equal the external pressure at pressure can equal the external pressure at

a lower temperature.a lower temperature.

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Consequences of Vapor Consequences of Vapor Pressure ChangesPressure Changes

When can cools, vp of water drops. When can cools, vp of water drops. Pressure in the can is less than that of Pressure in the can is less than that of

atmosphere, so can is crushed. atmosphere, so can is crushed.

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4. If external P = 760 mm Hg, T of boiling is the 4. If external P = 760 mm Hg, T of boiling is the

NORMAL BOILING POINTNORMAL BOILING POINT

5. VP of a given molecule at a given T depends 5. VP of a given molecule at a given T depends

on IM forces. Here the VP’s are in the orderon IM forces. Here the VP’s are in the order

C2H5H5C2 HH5C2 HH

wateralcoholether

increasing strength of IM interactions

extensiveH-bondsH-bonds

dipole-dipole

OOO

LiquidsLiquidsFigure 13.16: VP versus TFigure 13.16: VP versus T

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LiquidsLiquids

HEAT OF VAPORIZATIONHEAT OF VAPORIZATION is the heat is the heat req’d (at constant P) to vaporize the liquid.req’d (at constant P) to vaporize the liquid.

LIQ + heat ---> VAPLIQ + heat ---> VAP

Compd.Compd. ∆H∆Hvapvap (kJ/mol) (kJ/mol) IM ForceIM Force

HH22OO 40.7 (100 40.7 (100 ooC)C) H-bondsH-bonds

SOSO22 26.8 (-47 26.8 (-47 ooC)C) dipoledipole

XeXe 12.6 (-107 12.6 (-107 ooC)C) induced induced dipole dipole

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LiquidsLiquidsMolecules at surface behave differently than those in the interior.Molecules at surface behave differently than those in the interior.

Molecules at surface experience net INWARD Molecules at surface experience net INWARD force of attraction. force of attraction. This leads to This leads to SURFACE TENSIONSURFACE TENSION — the energy — the energy req’d to break the surface.req’d to break the surface.

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Surface TensionSurface Tension

SURFACE TENSIONSURFACE TENSION also leads to spherical also leads to spherical liquid droplets.liquid droplets.

SURFACE TENSIONSURFACE TENSION also leads to spherical also leads to spherical liquid droplets.liquid droplets.

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LiquidsLiquidsIntermolec. forces also lead to Intermolec. forces also lead to CAPILLARYCAPILLARY

action and to the existence of a concave action and to the existence of a concave meniscus for a water column.meniscus for a water column.

concavemeniscus

H2O in

glasstube

ADHESIVE FORCESbetween waterand glass

COHESIVE FORCESbetween watermolecules

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Capillary ActionCapillary Action

Movement of water up a piece of paper Movement of water up a piece of paper depends on H-bonds between Hdepends on H-bonds between H22O and O and the OH groups of the cellulose in the the OH groups of the cellulose in the paper.paper.