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Chapter 11
I ntermolecular F orces,L iquids, and Solids
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CONTENTS11.1 Introduction: A Molecular Comparison
11.2 Types of Intermolecular Forces : 11.2.1 Ion-dipole forces
11.2.2 Dipole-dipole forces 11.2.3 Hydrogen bonding 11.2.4 London dispersion forces 11.2.5 Comparison between forces
11.3 Properties of Liquids 11.3.1 Viscosity and surface tension
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11.4 Phase Change
11.4.1 Energy changes accompanying phasechange 11.4.2 Cooling curve 11.4.3 Critical temperature and pressure
11.5 Vapour pressure
11.6 Phase diagram 11.6.1 Phase diagram of CO 2 11.6.2 Phase diagram of water
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11.7 Structure of solids 11.7.1 Crystalline and amorphous solids 11.7.2 Unit cell - PC, FCC & BCC structure
11.8 Bonding in Solids 11.8.1 Molecular solids 11.8.2 Covalent-network solids 11.8.3 Ionic and metallic solids
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Learning Outcomes
Able to differentiate the 4 main intermolecularforces (IMF)
Able to relate IMF of a compound to the boiling point, surface tension, viscosity and vapor pressureAble to use phase diagram in problem solvingAble to use knowledge on unit cell of an elementto calculate molar mass, density, radius etc.
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11.1 I ntroduction: A M olecular ComparisonSolid - L iqui d - Gas
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Solid - L iquid - GasState Gas Liquid Solid
Volume Assumes vol. &shape ofcontainer
Assumes shapeof the portion of
container but
not volume
Retain its ownshape &volume
Compressibility
Compressible Virtuallyincompressible
Virtuallyincompressible
Diffusion Occurs rapidly Occurs slowly Extremely low
Flow Readily Readily Does not flow
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11.2 I ntermolecular F orces
The attraction between molecules is anintermolecular force.Intermolecular forces are much weaker than
ionic or covalent bonds.When a substance melts or boils, intermolecularforces are broken.When a substance condenses, intermolecular
forces are formed.Boiling points/melting points indicate the strengthof intermolecular forces.
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11.2 Types of I ntermolecular F orces
Ion-dipole force
Dipole-dipole force
Hydrogen bonding
London-Dispersion force
+ +- -
+-+
A - H :B -
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11.2.1 I on-Dipole F orces
Exists between ions and the partial charge onthe end of polar molecules/dipoleImportant for solutions of ionic compounds in
polar liquidsE.g.: NaCl in waterMagnitude of attraction increases:
charge of the ion increases magnitude of dipole moment increase
Ionic bonding > Ion-dipole > Dipole-dipole
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I on-Dipole F orces
+ -
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11.2.2 Dipole-dipole F orces
Exists between neutral polar moleculesThe partially positive end of one molecule attracts thepartially negative end of another.
Weaker than ion-dipole forces
E.g.: CO---CO
C
O
C C
C
C O
OO
O
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Dipole-dipole F orces
+
- Attractionsare greater
thanrepulsion, sothe
moleculesfeel a netattraction toeach other
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Dipole-dipole F orces
For molecules of approximately equal mass and size , strength of attraction increases
with increasing polarityMass
(amu)Dipole
moment ( )Boiling point
(K)CH3CH2CH3 44 0.1 231
CH3OCH 3 46 1.3 248CH3CHO 44 3.9 294
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Dipole-dipole F orces
For molecules of comparable polarity ,those with smaller molecular volumes generally experience higher dipole-dipole attractive forces.
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11.2.3 H ydrogen Bonding
Special type of intermolecular attraction.Hydrogen bonding is a special case of dipole-dipole interactions.H-bonding requires:
H bonded to a small electronegative element (mostimportant for compounds of F, O, and N)
An unshared electron pair on a nearby smallelectronegative ion or atoms (usually F, O, or N onanother molecule)
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11.2.3 H ydrogen Bonding
E.g.:
Ion-dipole > Hydrogen bonding > Dipole-dipole
OH
H
OH
H
NH
H
OH
H
H
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H ydrogen Bonding
Hydrogenbond
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H ydrogen Bonding
Hydrogen bonding is important in :
Stabilising the structure of protein
Folding of protein moleculesSurvival of aquatic in frozen lake
Ice is less dense than water Ice floats so forms an insulating layer on top of
lakes or river. Therefore, aquatic life cansurvive in winter.
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11.2.4 L ondon Dispersion F orces
What intermolecular forces exist betweennonpolar molecules in liquid and solid state ? Dipole-dipole attractions cannot exist betweennon-polar atoms or molecules Non-polar molecules do not have permanentdipoles but all nonpolar substances can be
liquefied Therefore, there must exist some kind ofattractive interactions between the particles
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Two schematic presentations of theinstantaneous dipoles on two adjacent heliumatoms, showing the electrostatic attractionbetween them
L ondon Dispersion F orces
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L ondon Dispersion F orces
In a collection of helium atoms, the averagedistribution of electrons about each nucleus isspherically symmetrical
The atoms are nonpolar and posses nopermanent dipole moment
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L ondon Dispersion F orces
In any atom or molecule, electrons constantlymovingThe nucleus of one molecule (or atom) attractsthe electrons of the adjacent molecule (oratom).For an instant, the electron clouds become
distorted.In that instant a dipole id formed (called ani n s t an taneous d ipo le )
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L ondon Dispersion F orces
One instantaneous dipole can induce anotherinstantaneous dipole in ad adjacent molecule(or atom).
These two temporary dipoles attract eachother.The attraction is called London-dispersionforce (LDF) , or simply a dispersion force.LDF is the weakest of all intermolecular forces.LDF exist between all molecules (polar ornonpolar)
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L ondon Dispersion F orces (L DF )
What affects the strength of a dispersion force? Molecules must be very close together for
these attractive forces to occur. POLARIZABILITY is the ease with which an
electron distribution can be deformed. The larger the molecule, the more polarize it is. LDF increase as molecular weight increases. LDF depend on the shape of the molecule.
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L ondon Dispersion F orces
Factors affecting London dispersion force :1. Molecular weight
increase in mol. wt. (increase in atomic radii)results in increase number of electrons the larger the molecule, the farther its electron
from the nuclei, the greater its polarizability greater polarizability, the greater the strength of
London dispersion force
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L ondon Dispersion F orces
E.g. : Noble gases . As go down theperiodic table, mol. wt. increase
(atomic radii increase) , resultinggreater boiling point
Substance Mol. Wt (amu) Boiling point (K)He 4.0 4.6
Ne 20.2 27.3 Ar 39.9 87.5Kr 83.8 120.9Xe 131.3 166.1
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L ondon Dispersion F orces
Factors affecting London dispersion force2. Shape of molecule
greater the surface area available
for contact, greater the dispersionforce
e.g.: straight chain molecule
>branched chain molecule
Neopentane
(bp=282.7K)
N-pentane
(bp=309.4K)
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11.2.5 Comparison between F orces
LDF are found in all substances. Their strength depends on molecular shapes
and molecular weights. Dipole-dipole forces add to the effect of LDF.
They are found only in polar molecules. Ion-dipole interactions are stronger than H-
bonding.
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11.2.5 Comparison between F orces
H-bonding is a special case of dipole-dipole interactions.
It is the strongest of intermolecular forcesinvolving neutral species.
H-bonding is most important for H
compounds of N, O, and F.
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If ions are involved, ion-dipole (if a dipoleis present) and ionic bonding are
possible. Ion-dipole interactions are stronger than H-bonding.
Ordinary ionic or covalent bonds aremuch stronger than these interactions!
11.2.5 Comparison between F orces
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11.3 Proper ties of L iquids
11.3.1 VISCOSITY Viscosity - resistance of liquid to flow. Liquid flows by sliding molecules over
each other. The stronger the intermolecular
forces, the higher the viscosity. Viscosity usually decreases with an
increase in temperature.
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11.3.2 SURFACE TENSION bulk molecules (those in liquid) are equally
attracted to their neighbours
surface molecules are only attractedinwards to the bulk molecules therefore surface molecules are packed
more closely than bulk molecules
Surface tension - amount of energy required toincrease the surface area of a liquid by a unitamount.
Proper ties of L iquids
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Explaining Surface Tension
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Surface
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SURFACE TENSION (CONT..)
Strong intermolecular forces cause higher
surface tension. Water has a high surface tension (H-
bonding)
Hg(l) has even higher surface tension (thereare very strong metallic bonds between Hgatoms)
Proper ties of L iquids
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SURFACE TENSION (CONT..)
Cohesive forces Intermolecular forces that bind molecules to
one another Adhesive forces
Intermolecular forces that bind molecules toa surface
Proper ties of L iquids
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Proper ties of L iquids
To illustrate this:Meniscus - the shape of the liquid surface
if adhesive forces > than cohesive forces - the liquid is attracted to its container, meniscus is U-shaped (e.g. water in glass)
if cohesive forces > than adhesive forces - meniscus curved downwards (e.g. Hg in glass)
Capillary action the rise of liquids up very narrowtubes.The liquid climbs until adhesive and cohesive forcesare balanced by gravity.
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Liquid Levels in Capillaries: Capillary Rise
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11_19a
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11.4 Phase Changes
A phase change occurs when the matter istransformed from one physical state to another
During phase change temperature remains constant Sublimation : Solid Gas Vaporization : Liquid Gas Melting or Fusion : Solid Liquid Deposition : Gas Solid Condensation : Gas Liquid Freezing : Liquid Solid
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During phase change, the energy change of thesystem are as given below
Sublimation DHSUB > 0 (Endothermic) Vaporisation DHVAP > 0 (Endothermic) Melting OR Fusion DHFUS > 0 (Endothermic)
Deposition DHDEP < 0 (Exothermic) Condensation DHCON < 0 (Exothermic) Freezing DHFREEZ < 0 (Exothermic)
11.4.1 Energy changes accompanying phase changes
Phase Changes
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The amount of energy required to cause a phasechange increases as the strength of the intermolecularforces increases
Generally heat of fusion is less than heat ofvaporisation
Less energy is needed to allow particles to movepast one another than to separate them totally
Steam can cause severe burns, when it comes in
contact with skin because condensation (exothermic)
11.4.1 Energy changesaccompanying phase changes
11 4 2 Cooling curve
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11.4.2 Cooling curve
Cooling curve for the conversion of gaseous water to ice.
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11.4.2 Cooling curve
Supercooling:When a liquid iscooled below itsfreezing pointand it stillremains a liquid.
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11.4.3 Cri tical Temperatur e andPressure
Gas can be liquefied by compressing them at a suitable T As temperature increases, gas become more difficult to liquefy
because of the increasing kinetic energy
For every substance, there exist a temperature above which the
gas cannot be liquefied, regardless of the pressure
CRITICAL TEMPERATURE (T C) - the highest temperature atwhich a substance can exist as a liquid (using pressure)
CRITICAL PRESSURE (P C) - the pressure required to bringabout liquefaction at T C
The greater the intermolecular forces, the easier it is to liquefy asubstance, thus the higher the T C of the substance.
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11.5 Vapour Pressure
What is vapor pressure ? We place a quantity of ethanol ( b.p. 78.3C @ 1 atm)
in an evacuated container some of the molecules on the surface of the liquid have
enough energy to escape the attraction of the bulkliquid
these molecules moves into the gaseous phase as the number of molecules in the gas phase
increases, some of the gas phase molecules struck thesurface and returns to the liquid
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Measurement of the Vapour Pressure of Water
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It will reach a condition where the rate at which moleculesreturn to the liquid is equal to the rate they escape from theliquid.
At this condition ; the number of molecules in the gas phase reaches a
steady state pressure of the vapor remains constant dynamic equilibrium is attained between the rate of
condensation and rate of evaporation pressure of the vapor remains constant (vapor pressure)
Vapour Pressure
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Vapour pressure of a liquid isdefined as the pressure exerted by its
vapor when the liquid and the vaporstate are in dynamic equilibrium(Dynamic equilibrium is a condition inwhich two opposing processes occursimultaneously at equal rates .)
Vapour Pressure
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Liquids boil when the external pressure at theliquid surface equals the vapour pressure.
The normal boiling point is the boiling point at 760mmHg (1 atm).The temperature of the boiling point increases asthe external pressure increases.Two ways to get a liquid to boil:
Increase temperature or decrease pressure.
Vapour Pressure and BoilingPoint
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Under appropriate condition, equilibrium can existbetween:
liquid - gas solid - liquid solid - gas
We can summarise these conditions as phase
diagram
Phase diagram - a plot of pressure vs. temperaturesummarizing all equilibria between phases.
11.6 Phase Diagram
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(Explanation for Figure ) A-B = vapor pressure curve, represent equilibriumbetween liquid and gas phaseVapor pressure curve ends at the critical point
A = triple point (temp. & pressure at which all threephases are in equilibrium
A-D = change in melting point with increasing pressureThe solid is denser than liquid. An increase inpressure will result in more compact solid phase i.e.higher temperature are required to melt a solid in highpressure
Phase Diagram
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11.6.1 Phase Diagrams for CarbonDioxide
Notice that the solid-liquid equilibrium(melting point) line ofCO 2 follows the normal
behaviour i.e. its meltingpoint increases withincreasing temperature.
For solid CO 2 to exist asliquid the pressure mustexceed 5.11 atm
CO 2 does not melt, butsublimes at 1 atm
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11.6.2 Phase Diagram for Water
The melting point ofwater decreases with
increasing pressure Liquid form is more
compact than its solidform
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The melting point and hardness depends onarrangement of particlesattraction forces between them
We can classify solids according to types offorces (4 types)
11.8 Bonding in Solids
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Bonding in Solids
Molecular (formed from atoms or molecules) usually soft with low melting point and conductivity
Covalent network (formed from atoms) very hard with very high melting point and poor conductor
Ionic (formed from ions) hard, brittle, high melting point and poor conductivity
Metallic (formed from metal atoms) soft or hard with high melting point, good conductivity
and ductile)
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Intermolecular forces
Dipole-dipole, London dispersion and H-bonds.
Weak intermolecular forces give rise to lowmelting point.Efficient packing of molecules is important(since they are not regular spheres)
E.g. : Benzene (m.p 5C & b.p 80C)Toluene (m.p -95C & b.p 111C)
11.8.1 Molecular Solids
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E.g. : Benzene VS Toluene
Benzene
m.p 5C, b.p 80CSymmetrical planarmolecule packedefficiently in solidform - high meltingpointIMF in liquid statelower than toluene
Toluene
m.p -95C, b.p 111CLower symmetry,prevents from packingefficientlyIntermolecular forcesdepend on closecontact - not effectiveIMF in liquid state largerthan benzene
Molecular Solids
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Atoms are held together in large networks or chain bycovalent bondsCovalent bonds are stronger than intermolecular forcesE.g. Diamond & Graphite
Diamond Each carbon atom is bonded to 4 other carbon atom
Graphite The carbon atoms are arranged in layers of inter-
connected hexagonal rings The layers are held by dispersion forces Readily slide past one another
11.8.2 Covalent-NetworkSolids
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Structures of Diamond and Graphite
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Ions are held together by ionic bondsThe strength of ionic solids depends on the charges of theions
NaCl ---> Na + Cl - ---> has m.p. of 801C MgO ----> Mg 2+ O 2- ----> has m.p. of 2852C
The structure adopted by ionic solids depends largely onthe charges and relative sizes of the ions
E.g.: NaCl (Na + has CN of 6 and surrounded by 6Cl) whereas CsCl (Cs is surrounded by 8 Cl - atoms)
increase in CN results in increase in structure
11.8.3 Ionic Solids
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M etall ic Solids
Hardness and melting point depends on strength ofbonding
Bonding strength increases with increase inbonding electrons
e.g. Na (one valence electron- m.pt= 97.5C) Cr (6 valence electrons - m.pt = 1890c)
Mobility of electron explains why electrons aregood conductor of heat and electricity
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