1 Liquids Liquids Section 13.5 Section 13.5 In a liquid In a liquid • • molecules are in molecules are in constant motion constant motion • • there are there are appreciable appreciable intermolec. forces intermolec. forces • • molecules close molecules close together together • • Liquids are almost Liquids are almost incompressible incompressible • • Liquids do not fill Liquids do not fill the container the container
Liquids Section 13.5. In a liquid •molecules are in constant motion •there are appreciable intermolec. forces •molecules close together •Liquids are almost incompressible •Liquids do not fill the container. LIQUID. VAPOR. Liquids. - PowerPoint PPT Presentation
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
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
2
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.
0
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
5
Distribution of Energy in a Distribution of Energy in a LiquidLiquid
Figure 13.12
6
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
7
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.
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.
12
Boiling LiquidsBoiling Liquids
Liquid boils when its vapor pressure equals atmospheric pressure.
Liquid boils when its vapor pressure equals atmospheric pressure.
13
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.
14
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.
15
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
16
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
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.
18
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.
19
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
20
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.