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Intermolecular Attractions and the Properties of Liquids and Solids
2
Chapter 12 Intermolecular Forces• Important differences between gases,
solids, and liquids:– Gases
• Expand to fill their container – Liquids
• Retain volume, but not shape– Solids
• Retain volume and shape
At room temperature, some are solid, others are liquid, others are gaseous.
Why?
• Physical Properties of Gases, Liquids and Solids determined by – How tightly molecules are packed together– Strength of attractions between molecules
4
Inter vs. Intra-Molecular Forces• Intramolecular forces
– Covalent bonds within molecule – Strong – Hbond (HCl) = 431 kJ/mol
• Intermolecular forces – Attraction forces between molecules– Weak– Hvaporization (HCl) = 16 kJ/mol
Cl H Cl H
Covalent Bond (strong) Intermolecular attraction (weak)
• When substance melts or boils– Intermolecular forces are broken– Not covalent bonds
• Responsible for existence of condensed states of matter
• Responsible for bulk properties of matter– Boiling Points and Melting Points
6
Electronegativity Review
Electronegativity: Measure of attractive force that one atom in a covalent bond has for electrons of the bond
7
Bond Dipoles• Two atoms with different electronegativity
values share electrons unequally• Electron density is uneven
– Higher charge concentration around more electronegative atom
• Bond dipoles – Indicated with delta (δ) notation– Indicates partial charge has arisen
H F
8
Three Important Types of Intermolecular Forces
1. Dipole-dipole forces– Hydrogen bonds
2. London dispersion forces3. Ion-dipole forces
– Ion-induced dipole forces
9
I. Dipole-dipole Attractions• Occur only between polar
molecules– Possess dipole moments
• Molecules need to be close together
• Polar molecules tend to align their partial charges– + to –
• As dipole moment , intermolecular force
+ +
+ +
+ +
10
I. Dipole-dipole Attractions• Tumbling molecules
– Mixture of attractive and repulsive dipole-dipole forces
– Attractions (- -) greater than repulsions(- -)
– Get net attraction – ~ 1% of covalent bond
11
Hydrogen Bonds
• Special type of Dipole-Dipole Interaction– Very strong dipole-dipole attraction – ~40 kJ/mol
• Occurs between H and highly electronegative atom (O, N, or F)– H—F, H—O, and H—N bonds very polar
• Positive end of one can get very close to negative end of another
12
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
H
H N
H
H
H O
H
13
Effects of Hydrogen Bonding
• Boiling points of H compounds of elements of Groups IVA, VA, VIA, and VIIA.
• Boiling points of molecules with H bonding are higher than expected.
• Don’t follow rule that BP as MM (London forces )
Boili
ng P
oint
(°C)
14
Hydrogen Bonding in Water
• Responsible for expansion of water as it freezes • Hydrogen bonding produces strong attractions in liquid • Hydrogen bonding (dotted lines) between
water molecules in ice form tetrahedral configuration
15
II. London Dispersion Forces• Intermolecular forces between
nonpolar molecules• Two neutral molecules (atoms) can
affect each other– Nucleus of 1 molecule (atom) attracts
e’s of adjacent molecule (atom)– Electron cloud distorts– Temporary or instantaneous dipole
forms– One instantaneous dipole can induce
another in adjacent molecule (atom)– Results in net attractive force
e
e
2+
e
e
Electrostaticattraction
He atom 1 He atom 2
2+
16
London Dispersion Forces• Instantaneous dipole-induced dipole
attractions – London Dispersion Forces– London forces– Dispersion forces
• Decrease as 1/d6 (d = distance between molecules)
• Effect enhanced with increased particle mass• Operate between all molecules
– Neutral or net charged– Nonpolar or polar
London Forces as MM More e, less tightly held
London Forces as electron cloud volume (size)
Larger molecules have stronger London forces and thus higher boiling points.
18
2. Number of Atoms in Molecule
• London forces depend on number atoms in molecule• Boiling point of hydrocarbons demonstrates this trend
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
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III. Ion-dipole Attractions• Attractions between ion and charged end of
polar molecules– Attractions can be quite strong as ions have full
charges
(a) Negative ends of water dipoles surround cation (b) Positive ends of water dipoles surround anion
20
Ex. Ion-dipole Attractions AlCl3·6H2O
Positive charge of Al3+ ion attracts partial negative charges – on O of water molecules
Ion-dipole attractions hold water molecules to metal ion in hydrate Water molecules are found at
vertices of octahedron around aluminum ion
• Attractions between ion and polar molecules
21
Using Intermolecular Forces• Often can predict physical properties (like BP and
MP) by comparing strengths of intermolecular attractions– Ion-Dipole– Hydrogen Bonding– Dipole-Dipole– London Dispersion Forces
• Larger, longer, heavier molecules have stronger IMFs• Smaller, more compact, lighter molecules have
weaker IMFs
Weakest
Strongest
22
Phase Changes
• Changes of physical state – Deal with motion of molecules
• As temperature changes– Matter will undergo phase changes
• Liquid Gas– Evaporation– As heat H2O, forms steam or water vapor– Requires energy or source of heat to occur
23
Phase Changes • Solid Gas
– Sublimation– Ice cubes in freezer, leave in long enough disappear– Endothermic
• Gas Liquid– Cooling or Condensation– Dew is H2O vapor condensing onto cooler ground– Exothermic
24
Phase ChangesEn
ergy
of S
yste
m
Gas
Solid
Liquid
Meltingor Fusion
Vaporization Condensation
Freezing
SublimationDeposition
Exothermic, releases heat Endothermic, absorbs heat
27
Rate of Evaporation• Depends on
– Temperature– Surface area– Strength of
intermolecular attractions
• Molecules that escape from liquid have larger than average KE’s
• When they leave– Average KE of
remaining molecules is less
– T lower
28
Effect of Temperature on Evaporation Rate
• For given liquid– Rate of evaporation per
unit surface area as T • Why?
– At higher T, total fraction of molecules with KE large enough to escape is larger
– Result: rate of evaporation is larger
29
Kinetic Energy Distribution in Two Different Liquids
• Smaller IMF’s• Lower KE required to
escape liquid• A evaporates faster
• Larger IMF’s• Higher KE required to
escape liquid• B evaporates slower
A B
30
Vapor Pressure Diagram
T-t curves
Supercooling
34
Phase Diagram of Water