Download Intermolecular Forces

The Ties That Bind
Chemical Bonding and
Interactions
Chemical Bonding and Interactions
1.
2.
Stable Electron Configurations
Electron-Dot (Lewis) Structures
1.
2.
3.
3.
Ionic Bonding
1.
2.
4.
Naming ionic compounds
Drawing
Covalent Bonding
1.
2.
3.
5.
6.
Drawing, Rules for Drawing
The Octet Rule
Some Exceptions to the Rule
Naming covalent compounds
Drawing
Electronegativity and Polar Covalent Compounds
Molecular Shapes and the VSEPR Theory
Intermolecular Forces of Attraction
1.
H-bonds, Dipole-Dipole, Ion-Dipole, London Dispersion
Forces
INTERMOLECULAR FORCES OF
ATTRACTION
A phase is a homogeneous part of the system in
contact with other parts of the system but
separated from them by a well-defined boundary.
2 Phases
Solid phase - ice
Liquid phase - water
11.1
Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
Intermolecular vs Intramolecular
•
41 kJ to vaporize 1 mole of water (inter)
•
930 kJ to break all O-H bonds in 1 mole of water (intra)
“Measure” of intermolecular force
Generally,
intermolecular
forces are much
weaker than
intramolecular
forces.
boiling point
melting point
DHvap
DHfus
DHsub
11.2
Intermolecular Forces
Intermolecular Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
11.2
Intermolecular Forces
11.2
Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2
Intermolecular Forces
Dipole-Dipole 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.

Intermolecular Forces
London Dispersion Forces
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Weakest of all intermolecular forces.
It is possible for two adjacent neutral molecules to
affect each other.
The nucleus of one molecule (or atom) attracts the
electrons of the adjacent molecule (or atom).
For an instant, the electron clouds become
distorted.
In that instant a dipole is formed (called an
instantaneous dipole).
Intermolecular Forces
London Dispersion Forces
Intermolecular Forces
London Dispersion Forces
One instantaneous dipole can induce another
instantaneous dipole in an adjacent molecule
(or atom).
 Instantaneous dipoles are called London
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.

Intermolecular Forces
London Dispersion Forces
Intermolecular Forces
London Dispersion Forces

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London dispersion forces increase as molecular
weight increases.
London dispersion forces exist between all
molecules.
London dispersion forces depend on the shape of
the molecule.
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.
Intermolecular Forces
Hydrogen Bond
The hydrogen bond is a special dipole-dipole interaction
between they hydrogen atom in a polar N-H, O-H, or F-H bond
and an electronegative O, N, or F atom.
A
H…B
or
A
H…A
A & B are N, O, or F
11.2
Hydrogen Bond
11.2
Why is the hydrogen bond considered a
“special” dipole-dipole interaction?
Decreasing molar mass
Decreasing boiling point
11.2
Intermolecular Forces
Hydrogen Bonding

Hydrogen bonds are responsible for:
 Ice


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

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 Å.
The O…H hydrogen bond length is 1.8 Å.
Ice has waters arranged in an open, regular hexagon.
Each + 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.
Water is a Unique Substance
Maximum Density
40C
Density of Water
Ice is less dense than water
11.3
Intermolecular Forces
Hydrogen Bonding

Hydrogen bonds are
responsible for:
 Protein
Structure
Protein
folding
is
a
consequence of H-bonding.
 DNA Transport of Genetic
Information

Intermolecular Forces
Comparing Intermolecular Forces
Properties of Liquids
Surface tension is the amount of energy required to stretch
or increase the surface of a liquid by a unit area.
Strong
intermolecular
forces
High
surface
tension
11.3
Some Properties of Liquids
Surface Tension
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
11.3
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong
intermolecular
forces
High
viscosity
11.3
Ultrahydrophobic surfaces
PHASE CHANGES

We can use the concepts of intermolecular
forces of attraction to explain the physical
phase changes
Phase Changes
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Surface molecules are only attracted inwards
towards the bulk molecules.
Sublimation: solid  gas.
Vaporization: liquid  gas.
Melting or fusion: solid  liquid.
Deposition: gas  solid.
Condensation: gas  liquid.
Freezing: liquid  solid.
Energy Changes Accompanying Phase
Changes

Energy change of the system for the above
processes are:
Phase Changes
Energy Changes Accompanying Phase
Changes
All phase changes are possible under the right
conditions (e.g. water sublimes when snow
disappears without forming puddles).
 The sequence
heat solid  melt  heat liquid  boil  heat gas
is endothermic.
 The sequence
cool gas  condense  cool liquid  freeze  cool
solid
is exothermic.

Phase Changes
Energy Changes Accompanying Phase
Changes
Phase Changes
Heating Curves
Test yourself
1.
2.
Which has a higher boiling point, ethane (C2H6) or
dodecane (C12H26)?
What kind of IFA will be present in the following
combinations/mixtures?
1.
2.
3.
4.
5.
6.
3.
4.
Water and ammonia
Octane and water
CCl4 and CHCl3
Hydrofluoric acid (HF) and water?
Acetic acid and cysteine? (see board for structures)
Water and NaCl
Which has a higher boiling point, neopentane or npentane? (See board for structures)
Which will have a higher boiling point:
1.
2.
Ne or Xe
N2 or Kr