Intermolecular Attractions & the Properties of Liquids...
Transcript of Intermolecular Attractions & the Properties of Liquids...
Intermolecular Attractions & the Properties of Liquids & Solids
CHAPTER 12
Chemistry: The Molecular Nature of Matter, 6th edition By Jesperson, Brady, & Hyslop
CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 2
! Understand, describe, and rank in order of strength the types of
intermolecular forces.
! Difference between bonds and intermolecular forces
! Changes of state: heat of vaporization, fusion, & sublimation
! Clausius-Clapyron equation
! Heating and cooling curves: !H, phase transition temperatures
! Phase diagrams
! Solids: Unit cell, stoichiometry, packing patterns, XRD, common
types and their properties
CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 3
Lecture Road Map:
① Properties of gas, liquids, solids
② Intermolecular forces
③ Changes of state
④ Dynamic Equilibrium
⑤ Structure & Characterization of a solid
CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 4
Properties of gases, liquids, &
solids
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 5
Intermolecular Forces Important differences between gases, solids, and liquids: o Gases
o Expand to fill their container o Liquids
o Retain volume, but not shape o Solids
o Retain volume and shape
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Intermolecular Forces o Physical state of molecule depends on
o Average kinetic energy of particles o Recall KE ! Tave
o Intermolecular Forces o Energy of Inter-particle attraction
o Physical properties of gases, liquids and solids determined by o How tightly molecules are packed together o Strength of attractions between
molecules
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o Converting gas "# liquid or solid o Molecules must get closer together
o Cool or compress
o Converting liquid or solid "# gas o Requires molecules to move farther
apart o Heat or reduce pressure
o As T decreases, kinetic energy of molecules decreases o At certain T, molecules don’t have
enough energy to break away from one another’s attraction
Intermolecular Attractions
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Inter vs. Intra-Molecular Forces o Intramolecular forces
o Covalent bonds within molecule o Strong o $Hbond (HCl) = 431 kJ/mol
o Intermolecular forces o Attraction forces between molecules o Weak o $Hvaporization (HCl) = 16 kJ/mol
Cl H Cl H
Covalent Bond (strong) Intermolecular attraction (weak)
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Electronegativity Review
Electronegativity: Measure of attractive force that one atom in a covalent bond has for electrons of the bond
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Bond Dipoles o Two atoms with different electronegativity
values share electrons unequally o Electron density is uneven
o Higher charge concentration around more electronegative atom
o Bond dipoles o Indicated with delta (") notation o Indicates partial charge has arisen
H F
!+ !"
o 11
o Net Dipoles o Symmetrical molecules
o Even if they have polar bonds o Are non-polar because bond dipoles cancel
o Asymmetrical molecules o Are polar because bond dipoles do not cancel o These molecules have permanent, net dipoles
o Molecular dipoles o Cause molecules to interact o Decreased distance between molecules increases
amount of interaction
COVALENT BOND
IONIC BOND
POLAR COVALENT
BOND
CHCl3
TiO2
F2
CaBr2
!
!
!
!
Group Problem
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 13
Identify the overall dipole moment for CHCl3
Group Problem
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 14
Identify the overall dipole moment for these molecules:
Solubility
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LIKE DISSOLVES LIKE polar molecules dissolve in polar solvents
nonpolar molecules dissolve in nonpolar solvents Polar Solvents
Water: H2O Methanol: CH3OH Ethanol: CH3CH2OH Acetone: (CH3)2CO Acetic Acid: CH3CO2H Ammonia: NH3 Acetonitrile: CH3CN
Nonpolar Solvents Pentane: C5H12 Hexane: C6H14 Cyclohexane: C6H12 Benzene: C6H6 Toluene: CH3C6H5 Chloroform: CHCl3 Diethylether: (CH3CH2)2O
Which molecule will dissolve in water?
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Vitamin A
Vitamin B12
Group Problem
CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 17
Intermolecular Forces
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Intermolecular Forces
The forces of attraction or repulsion between neighboring particles (atoms or molecules).
+ / - charges attract one another - / - or + / + forces repel each other
r r KE
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Intermolecular Forces
o When substance melts or boils o Intermolecular forces are broken, not covalent
bonds o Responsible for non-ideal behavior of gases o Responsible for existence of condensed
states of matter o Responsible for bulk properties of matter
o Boiling points and melting points reflect strength of intermolecular forces
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Types of Intermolecular Forces
① London dispersion forces ② Dipole-dipole forces ③ Hydrogen bonds ④ Ion-dipole forces
o Ion-induced dipole forces
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London-Dispersion Forces o When atoms near one another,
their valence electrons interact o Repulsion causes electron clouds
in each to distort and polarize o Instantaneous dipoles result from
this distortion o Effect enhanced with increased
volume of electron cloud size o Effect diminished by increased distance between particles and compact arrangement of atoms
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London Dispersion Forces Affects ALL molecules, both polar & nonpolar Boiling Point (BP) is an indication of relative intermolecular force strength. Ease with which dipole moments can be induced and thus London Forces depend on ① Distance between particles ② Polarizability of electron cloud ③ Points of attraction
o Number atoms o Molecular shape (compact or elongated)
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Polarizability = Ease with which the electron cloud can be distorted
Larger molecules often more polarizable o Larger number of less tightly held
electrons o Magnitude of resulting partial
charge is larger o Larger electron cloud
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Group Problem
Which is more polarizable? F2 or I2?
Table 12.1 Boiling Points of Halogens and Noble Gases
Larger molecules have stronger London forces and thus higher boiling points.
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Number of Atoms in Molecule
o London dispersion forces increase with the number atoms in molecule because more points of attraction
Formula BP at 1 atm, °C Formula BP at 1 atm, °C CH4 –161.5 C5H12 36.1 C2H6 –88.6 C6H14 68.7 C3H8 –42.1 : : C4H10 –0.5 C22H46 327
Hexane, C6H14 BP 68.7 °C
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Which of the following molecules will have the highest boiling point?
Propane, C3H8 BP –42.1 °C
Group Problem
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Molecular Shape o Increased surface area available for contact =
increased points of contact = increase in London Dispersion forces. o More compact molecules:
Less surface area to interact with other molecules
o Less compact molecules: More surface area to interact with other molecules
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• Small area for interaction
• Larger area for interaction
More compact – lower BP Less compact – higher BP
Which of the following molecules experience the strongest Dispersion forces?
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Group Problem
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Types of Intermolecular Forces
① London dispersion forces ② Dipole-dipole forces ③ Hydrogen bonds ④ Ion-dipole forces
o Ion-induced dipole forces
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Dipole-Dipole Attractions
o Occurs only between polar molecules
o Proportional to distance between molecules
o Polar molecules tend to align their partial charges: + / -
o As dipole moment increases, intermolecular force increases
+ - + -
- + - +
+ - + -
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Dipole-Dipole Attractions Tumbling molecules
o Mixture of attractive and repulsive dipole-dipole forces
o Attractions (- -) are maintained longer than repulsions(- -)
o Get net attraction o ~1–4% of covalent bond
In the liquid state, which species has the strongest intermolecular forces, CH4, Cl2, O2 or HF?
HF The polar molecule
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Group Problem
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Types of Intermolecular Forces
① London dispersion forces ② Dipole-dipole forces ③ Hydrogen bonds ④ Ion-dipole forces
o Ion-induced dipole forces
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Hydrogen Bonds o Very strong dipole-dipole attraction: ~10% of a covalent
bond o Occurs between H and highly electronegative atom (O, N, or
F): H—F, H—O, and H—N bonds very polar o Electrons are drawn away from H giving atoms high
partial charges o H only has one electron, so %+
H presents almost bare proton
o %–X almost full –1 charge
o Element’s small size, means high charge density
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Examples of Hydrogen Bonding H O
H
H O
H
H O
H
H N
H
H
H F H O
H
H F H N
H
H
H N
H
H
H N
H
HH N
H
H
H O
H
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Hydrogen Bonding in Water
Hydrogen Bonds are strong! o Responsible for the high boiling point of water o Responsible for expansion of water as it freezes o Hydrogen bonding (dotted lines) between
water molecules in ice form tetrahedral configuration
Hydrogen Bonding in Water
0.957 Å 1.97 Å
List all intermolecular forces for CH3CH2OH. Hydrogen-bonds, dipole-dipole attractions, London dispersion forces
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Group Problem
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Types of Intermolecular Forces
① London dispersion forces ② Dipole-dipole forces ③ Hydrogen bonds ④ Ion-dipole forces
o Ion-induced dipole forces
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Ion-Dipole Attractions o Attractions between ion and charged end of
polar molecules o Ions have full charges, increasing the attraction
(a) Negative ends of water dipoles surround cation (b) Positive ends of water dipoles surround anion
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AlCl3!6H2O
o Positive charge of Al3+ ion attracts partial negative charges %– on O of water molecules
o Ion-dipole attractions hold water molecules to metal ion in hydrate o Water molecules are found
at vertices of octahedron around aluminum ion
Attractions between ion and polar molecules
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Ion-Induced Dipole Attractions o Attractions between ion and dipole it induces on
neighboring molecules o Depends on
o Ion charge and o Polarizability of its neighbor
o Attractions can be quite strong as ion charge is constant, unlike instantaneous dipoles of London-dispersion forces
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Group Problem
How many water molecules would be attracted to this molecule by Ion-Dipole interactions?
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Group Problem
List the intermolecular forces and rank in order of strength for the liquids of each molecule.
o Ion-Dipole o Hydrogen Bonding o Dipole-Dipole o London Forces
• Larger, longer, and therefore heavier molecules often have stronger intermolecular forces
• Smaller, more compact, lighter molecules have generally weaker intermolecular forces
Weakest
Strongest
Group Problem
Intermolecular Forces and Temperature
Decrease with increasing temperature o Increasing kinetic energy overcomes attractive
forces o If allowed to expand, increasing temperature
increases distance between gas particles and decreases attractive forces
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Group Problem
GROUP PROBLEM SET 12.1
CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 50
More properties of gases,
liquids, & solids
Compressibility Surface Tension
Diffusion
Retention of Volume & shape
Wetting Viscosity
Melting Point
Boiling Point
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Melting & Boiling Point Often can predict physical properties by comparing strengths of intermolecular attractions:
Boiling Point increases when intermolecular forces increase
Melting Point increases when intermolecular forces increase
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Compressibility
Measure of the ability of a substance to be forced into smaller volume
o Determined by strength of intermolecular forces o Gases highly compressible
o Molecules far apart o Weak intermolecular forces
o Solids and liquids nearly incompressible o Molecules very close together o Stronger intermolecular forces
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Retention of volume and shape
o Solids retain both volume and shape o Strongest intermolecular attractions o Molecules closest
o Liquids retain volume, but not shape o Attractions intermediate
o Gases, expand to fill their containers o Weakest intermolecular attractions o Molecules farthest apart
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Diffusion In Gases
o Molecules travel long distances between collisions
o Diffusion rapid In Liquids
o Molecules closer o Encounter more collisions o Takes a long time to move
from place to place In Solids
o Diffusion close to zero at room temperature
o Will increase at high temperature
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Surface Tension
Inside body of liquid o Intermolecular forces are
the same in all directions Molecules at surface
o Potential energy increases when removing neighbors
o Molecules move together to reduce surface area and potential energy " sphere
Why does H2O bead up on a freshly waxed car instead of forming a layer?
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Surface Tension
Liquids containing molecules with strong intermolecular forces have high surface tension
Allows us to fill glass above rim o Gives surface rounded
appearance o Surface resists expansion and
pushes back
o Surface tension increases as intermolecular forces increase
o Surface tension decreases as temperature increases
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Wetting o Ability of liquid to spread
across surface to form thin film
o Greater similarity in attractive forces between liquid and surface, yields greater wetting effect
o Occurs only if intermolecular attractive force between surface and liquid about as strong as within liquid itself
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Wetting: Surfactants (Detergents) o Detergents added to water to lower surface tension so water can
spread on greasy glass o Substances that have both polar and non-polar characteristics o Long chain hydrocarbons with polar tail
OS
O
O! Na+O
O
O! Na+
o Nonpolar end dissolves in nonpolar grease o Polar end dissolves in polar H2O o Thus increasing solubility of grease in water
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Viscosity o Resistance to flow o Measure of fluid’s
resistance to flow or changing form
o Decreases as Temp increases
o Not just a property of liquids: o Gas: respond to instantly
to form changing force o Amorphous solids, like
glass
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Viscosity
Acetone Polar molecule
o Dipole-dipole and o London forces
Ethylene glycol Polar molecule
o Hydrogen-bonding o Dipole-dipole and o London forces
Which is more viscous?
Group Problem
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Group Problem
For each pair given, which is has more viscosity?
CH3CH2CH2CH2OH, CH3CH2CH2CHO C6H14, C12H26 NH3(l ), PH3(l )
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Group Problem
GROUP PROBLEM SET 12.2
CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 63
Changes of State
Heating/Cooling Curves "H
Phase Diagrams
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Group Problem
65
Group Problem
GROUP PROBLEM SET 12.3
CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 66
Dynamic Equilibria
CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 69
Solid Structures
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Group Problem
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Group Problem
GROUP PROBLEM SET 12.5
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Phase Changes
• Changes of physical state – Deal with motion of molecules
• As temperature changes – Matter will undergo phase changes
• Liquid # Gas – Evaporation, vaporization – As heat is added, H2O, forms steam or water
vapor – Requires energy or source of heat
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Phase Changes • Solid # Gas
– Sublimation – Ice cubes in freezer, leave in long enough disappear – Endothermic
• Gas # Liquid – Condensation – Dew is H2O vapor condensing onto cooler ground – Exothermic – Often limits lower night time temperature
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Rate of Evaporation • Depends on
– Temperature – Surface area – Strength of
intermolecular attractions
• Molecules that escape from liquid have larger than minimum escape KE
• When they leave – Average KE of
remaining molecules is less and so T lower
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Effect of Temperature on Evaporation Rate
• For given liquid – Rate of evaporation per
unit surface area increases as T increases
• Why? – At higher T, total
fraction of molecules with KE large enough to escape is larger
– Result: rate of evaporation is larger
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Kinetic Energy Distribution in Two Different Liquids
• Smaller intermolecular forces
• Lower KE required to escape liquid
• A evaporates faster
• Larger intermolecular forces
• Higher KE required to escape liquid
• B evaporates slower
A B
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Changes Of State Involve Equilibria • Fraction of molecules in condensed state is
higher when intermolecular attractions are higher
• Intermolecular attractions must be overcome to separate the particles, while separated particles are simultaneously attracted to one another condensedphase
separatedphase
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Before System Reaches Equilibrium
• Liquid is placed in empty, closed, container – Begins to evaporate
• Once in gas phase – Molecules can condense
by – Striking surface of liquid
and giving up some kinetic energy
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System At Equilibrium • Rate of evaporation =
rate of condensation • Occurs in closed
systems where molecules cannot escape
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Similar Equilibria Reached in Melting
Melting Point (mp) – Solid begins to change
into liquid as heat added • Dynamic equilibria
exists between solid and liquid states – Melting (red arrows) and
freezing (black arrows) occur at same rate
– As long as no heat added or removed from equilibrium mixture
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Equilibria Reached in Sublimation
At equilibrium • Molecules sublime
from solid at same rate as molecules condense from vapor
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Phase Changes
Ene
rgy
of S
yste
m
Gas
Solid
Liquid
Melting or Fusion
Vaporization Condensation
Freezing
Sublimation Deposition
& Exothermic, releases heat ' Endothermic, absorbs heat
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Energy Changes Accompanying Phase Changes
• All phase changes are possible under the right conditions
• Following sequence is endothermic
heat solid # melt # heat liquid # boil # heat gas
• Following sequence is exothermic
cool gas # condense # cool liquid # freeze # cool solid
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Enthalpy Of Phase Changes Endothermic Phase Changes
1. Must add heat 2. Energy entering system (+)
Sublimation: $Hsub > 0 Vaporization: $Hvap > 0 Melting or Fusion: $Hfus > 0
Exothermic Phase Changes 1. Must give off heat 2. Energy leaving system (–)
Deposition: $H < 0 = –$Hsub Condensation: $H < 0 = –$Hvap Freezing: $H < 0 = –$Hfus
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Phase Changes
• As T changes, matter undergoes phase changes
• Phase Change – Transformation from one phase to another
• Liquid-Vapor Equilibrium – Molecules in liquid
• Not in rigid lattice • In constant motion • Denser than gas, so more collisions • Some have enough kinetic energy to
escape, some don’t