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8/8/2019 Intermolecular Forces in Liquids Only
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Copyright 1999, PRENTICE HALL Chapter 11 1
A Molecular Comparison of Liquids andA Molecular Comparison of Liquids and
SolidsSolids
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Copyright 1999, PRENTICE HALL Chapter 11 2
A Molecular Comparison of Liquids andA Molecular Comparison of Liquids and
SolidsSolids Physical properties of substances understood in terms
of kinetic molecular theory: Gases are highly compressible, assumes shape and volume
of container:
Gas molecules are far apart and do not interact much with each other.
Liquids are almost incompressible, assume the shape butnot the volume of container:
Liquids molecules are held closer together than gas molecules, but not so
rigidly that the molecules cannot slide past each other.
Solids are incompressible and have a definite shape andvolume:
Solid molecules are packed closely together. The molecules are so
rigidly packed that they cannot easily slide past each other.
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Copyright 1999, PRENTICE HALL Chapter 11 3
The 3 StatesThe 3 States
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Copyright 1999, PRENTICE HALL Chapter 11 4
A Molecular Comparison of Liquids andA Molecular Comparison of Liquids and
SolidsSolids
Converting a gas into a liquid or solid requires themolecules to get closer to each other: cool or compress.
Converting a solid into a liquid or gas requires themolecules to move further apart: heat or reduce pressure.
The forces holding solids and liquids together are
called intermolecular forces.
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Copyright 1999, PRENTICE HALL Chapter 11 5
Intermolecular ForcesIntermolecular Forces
The covalent bond holding a molecule together is anintramolecular forces.
The attraction between molecules is an intermolecularforce.
Intermolecular forces are much weaker thanintramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/molfor HCl).
When a substance melts or boils the intermolecularforces are broken (not the covalent bonds).
When a substance condenses intermolecular forcesare formed.
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Copyright 1999, PRENTICE HALL Chapter 11 6
Intermolecular ForcesIntermolecular Forces
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Copyright 1999, PRENTICE HALL Chapter 11 7
Intermolecular ForcesIntermolecular Forces
1. Ion - Dipole
2. Dipole-Dipole Forces3. Dipole - Induced Dipole
4. Instantaneous Dipole-Induced Dipole
5. Hydrogen Bonding
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Copyright 1999, PRENTICE HALL Chapter 11 8
Table of Force EnergiesTable of Force Energies
Type of Force Energy (kJ/mol)
Ionic Bond 300-600Covalent 200-400
Hydrogen Bonding 20-40
Ion-Dipole 10-20
Dipole-Dipole 1-5
Instantaneous Dipole/
Induced Dipole 0.05-2
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Copyright 1999, PRENTICE HALL Chapter 11 9
Intermolecular ForcesIntermolecular Forces
IonIon--Dipole ForcesDipole Forces Interaction between an ion (e.g. Na+) and a dipole (e.g.
water).
Strongest of all intermolecular forces:
Since Q1 is a full charge and Q2 is a partial charge, Fis
comparatively large. Fincreases as Q increases and as ddecreases:
the larger the charge and smaller the ion, the larger the ion-dipole attraction.
221
d
QQkF!
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Copyright 1999, PRENTICE HALL Chapter 11 10
Intermolecular ForcesIntermolecular Forces
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Copyright 1999, PRENTICE HALL Chapter 11 11
IonIon -- Dipole InteractionsDipole Interactions
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Copyright 1999, PRENTICE HALL Chapter 11 12
Intermolecular ForcesIntermolecular Forces
DipoleDipole--Dipole ForcesDipole Forces Interaction between an ion (e.g. Na+) and a dipole (e.g.
water).
Dipole-dipole forces exist between neutral polarmolecules.
Polar molecules need to be close together.
Weaker than ion-dipole forces:
Q1 and Q2 arepartialcharges.
2 21dQQkF!
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Copyright 1999, PRENTICE HALL Chapter 11 13
DipoleDipole--Dipole ForcesDipole Forces
dd
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Copyright 1999, PRENTICE HALL Chapter 11 14
Intermolecular ForcesIntermolecular Forces
DipoleDipole--Dipole ForcesDipole Forces There is a mix of attractive and
repulsive dipole-dipole forces as the
molecules tumble. If two molecules have about the
same mass and size, then dipole-dipole forces increase with
increasing polarity.
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Copyright 1999, PRENTICE HALL Chapter 11 15
Intermolecular ForcesIntermolecular Forces
London Dispersion ForcesLondon Dispersion Forces Weakest of all intermolecular forces.
It is possible for two adjacent neutral molecules toaffect each other.
The nucleus of one molecule (or atom) attracts theelectrons of the adjacent molecule (or atom).
For an instant, the electron clouds become distorted.
In that instant a dipole is formed (called aninstantaneous dipole).
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Copyright 1999, PRENTICE HALL Chapter 11 16
Intermolecular ForcesIntermolecular Forces
London Dispersion ForcesLondon Dispersion Forces
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Copyright 1999, PRENTICE HALL Chapter 11 17
Intermolecular ForcesIntermolecular Forces
London Dispersion ForcesLondon Dispersion Forces One instantaneous dipole can induce another
instantaneous dipole in an adjacent molecule (oratom).
The forces between instantaneous dipoles are calledLondon dispersion forces.
Polarizability is the ease with which an electron cloud
can be deformed. The larger the molecule (the greater the number of
electrons) the more polarizable.
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Copyright 1999, PRENTICE HALL Chapter 11 18
London Dispersion ForcesLondon Dispersion Forces
-Temporary Induced Dipole-Dipole interactions
Very Weak always present in the condensed
phase
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Copyright 1999, PRENTICE HALL Chapter 11 19
LondonLondon
DispersionDispersion
ForcesForces
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Copyright 1999, PRENTICE HALL Chapter 11 20
Intermolecular ForcesIntermolecular Forces
London Dispersion ForcesLondon Dispersion Forces London dispersion forces increase as molecular
weight increases.
London dispersion forces exist between all molecules.
London dispersion forces depend on the shape of themolecule.
The greater the surface area available for contact, the
greater the dispersion forces. London dispersion forces between spherical molecules
are lower than between sausage-like molecules.
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Copyright 1999, PRENTICE HALL Chapter 11 21
Intermolecular ForcesIntermolecular Forces
London Dispersion ForcesLondon Dispersion Forces
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Copyright 1999, PRENTICE HALL Chapter 11 22
Intermolecular ForcesIntermolecular ForcesHydrogen BondingHydrogen Bonding
Special case of dipole-dipole forces. By experiments: boiling points of compounds with H-
F, H-O, and H-N bonds are abnormally high.
Intermolecular forces are abnormally strong.
H-bonding requires H bonded to an electronegativeelement (most important for compounds of F, O, andN). Electrons in the H-X (X = electronegative element) lie much
closer to X than H.
H has only one electron, so in the H-X bond, the H+ Hpresents an almost bare proton to the H- X.
Therefore, H-bonds are strong.
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Copyright 1999, PRENTICE HALL Chapter 11 23
HH--BondingBonding
Occurs when Hydrogen is attached to ahighly electronegative atom.
N-H N- O-H N- F-H N-
N-H O- O-H O- F-H O-
N-H
F- O-H
F- F-H
F-H+ H-
Requires Unshared Electron Pairs of Highly
Electronegative Elements
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Copyright 1999, PRENTICE HALL Chapter 11 24
Hydrogen
Bonding
in Water
Molecules
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Copyright 1999, PRENTICE HALL Chapter 11 25
Structure of Ice
Observe the orientation of theHydrogen Bonds
HB-ice
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Copyright 1999, PRENTICE HALL Chapter 11 26
Intermolecular ForcesIntermolecular ForcesHydrogen BondingHydrogen Bonding
Hydrogen bonds are responsible for: Ice Floating
Solids are usually more closely packed than liquids;
therefore, solids are more dense than liquids.
Ice is ordered with an open structure to optimize H-bonding. Therefore, ice is less dense than water.
In water the H-O bond length is 1.0 .
TheOH hydrogen bond length is 1.8 .
Ice has waters arranged in an open, regular hexagon.
Each H+ H points towards a lone pair on O. Ice floats, so it forms an insulating layer on top of lakes, rivers, etc.
Therefore, aquatic life can survive in winter.
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Copyright 1999, PRENTICE HALL Chapter 11 27
Why Does Ice Float?Why Does Ice Float?
D2O(s)H2O(s)
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Copyright 1999, PRENTICE HALL Chapter 11 28
The Boiling Points of the CovalentHydrides of theThe Boiling Points of the CovalentHydrides of the
Elements in Groups 4A, 5A, 6A, and 7AElements in Groups 4A, 5A, 6A, and 7A
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Copyright 1999, PRENTICE HALL Chapter 11 29
Intermolecular ForcesIntermolecular ForcesHydrogen BondingHydrogen Bonding
Hydrogen bonds are responsible for: Protein Structure
Protein folding is a consequence of H-bonding.
DNATransport of Genetic Information
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Copyright 1999, PRENTICE HALL Chapter 11 30
Intermolecular ForcesIntermolecular ForcesComparing Intermolecular ForcesComparing Intermolecular Forces
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Copyright 1999, PRENTICE HALL Chapter 11 31
Some Properties of LiquidsSome Properties of Liquids
ViscosityViscosity Viscosity is the resistance of a liquid to flow. A liquid flows by sliding molecules over each other.
The stronger the intermolecular forces, the higher the
viscosity.
Surface TensionSurface Tension
Bulk molecules (those in the liquid) are equallyattracted to their neighbors.
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Copyright 1999, PRENTICE HALL Chapter 11 32
Some Properties of LiquidsSome Properties of Liquids
Surface TensionSurface Tension
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Copyright 1999, PRENTICE HALL Chapter 11 33
Some Properties of LiquidsSome Properties of Liquids
Surface TensionSurface Tension Surface molecules are only attracted inwards towards
the bulk molecules. Therefore, surface molecules are packed more closely than
bulk molecules.
Surface tension is the amount of energy required toincrease the surface area of a liquid.
Cohesive forces bind molecules to each other.
Adhesive forces bind molecules to a surface.
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Copyright 1999, PRENTICE HALL Chapter 11 34
Some Properties of LiquidsSome Properties of Liquids
Surface TensionSurface Tension
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Copyright 1999, PRENTICE HALL Chapter 11 35
Some Properties of LiquidsSome Properties of Liquids
Surface TensionSurface Tension Meniscus is the shape of the liquid surface.
If adhesive forces are greater than cohesive forces, theliquid surface is attracted to its container more than thebulk molecules. Therefore, the meniscus is U-shaped (e.g.
water in glass). If cohesive forces are greater than adhesive forces, the
meniscus is curved downwards.
CapillaryAction: When a narrow glass tube is placed
in water, the meniscus pulls the water up the tube.