Transcript Ch 11 States of Matter and Intermolecular Forces

Ch 11 States of Matter and
Intermolecular Forces
Chapter 11 Preview
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Intramolecular forces (bonds) govern
molecular properties.
Intermolecular forces are between
molecules and determine the macroscopic
physical properties of liquids and solids.
This chapter:
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describes changes from one state of matter to
another.
explores the types of intermolecular forces that
underlie these and other physical properties of
substances.
Review:
Intramolecular Forces
Ionic
 Covalent
 Metallic
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Ionic bond- electron donated
and accepted
Covalent- sharing electrons
Metallic-sea of electrons
Ionic Bonds as
“Intermolecular” Forces
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There are no molecules in an ionic solid, and
therefore there can’t be any intermolecular
forces.
These forces increase:
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as the charges on the ions increase.
as the ionic radii (size) decrease.
Interionic Forces of Attraction
Melting point of NaCl
is about 801 oC.
Mg2+ and O2– have much stronger forces of
attraction for one another than do Na+ and Cl–
. Melting point of MgO is about 2800 oC.
Molecular Forces Compared
States of Matter Compared
Intermolecular
forces are of little
significance; why?
Intermolecular
forces must be
considered.
Intermolecular
forces are very
important.
Intermolecular Forces
Hydrogen bonding
 Dipole-dipole
 London Forces
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Cohesion
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Attraction for each other
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Water
Mercury
Boiling point varies based on
cohesion
Adhesion
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A liquids attraction for solid particles
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Water’s attraction for glass etc.
WATER
MERCURY
Meniscus Formation
What conclusion
can we draw about
the cohesive forces
in mercury?
Water wets the
glass (adhesive
forces) and its
attraction for glass
forms a concaveup surface.
Plant root- capillary action
Surface Tension
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The forces present in liquids
A molecule inside the liquid experiences
cohesive forces with other molecules in all
directions.
A molecule at the surface experiences only
downward cohesive forces
Surface Tension
There is no force above the
molecule on the top of the
surface.
Pool ball floating on mercury
Adhesive and Cohesive
Forces
The liquid spreads,
because adhesive forces
are comparable in
strength to cohesive
forces.
The liquid “beads up.”
Which forces are
stronger, adhesive or
cohesive?
Cohesive vs. Adhesive
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Water on plastic
Water on metal
Water on glass
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Intermolecular Forces
Hydrogen bonding
 Dipole-dipole
 London Forces
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Hydrogen bonding
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A hydrogen is attracted to a highly
electronegative atom like O, N, or Cl.
Hydrogen Bonding in Water
Hydrogen Bonding in Ice
Hydrogen bonding arranges
the water molecules into an
open hexagonal pattern.
“Hexagonal” is reflected in the
crystal structure. “Open”
means reduced density of the
solid (vs. liquid).
Hydrogen Bonding in Acetic
Acid
Hydrogen bonding
occurs between
molecules.
Intermolecular Hydrogen
Bonds
Intermolecular hydrogen bonds give
proteins their secondary shape, forcing
the protein molecules into particular
orientations, like a folded sheet …
Intramolecular Hydrogen
Bonds
… while
intramolecular
hydrogen bonds can
cause proteins to
take a helical shape.
In which of these substances is hydrogen bonding an
important intermolecular force: N2, HI, HF, CH3CHO, and
CH3OH? Explain.
In which of these substances is hydrogen bonding an
important intermolecular force: N2, HI, CH3CHO, and
CH3OH? Explain.
CH3CHO and CH3OH because of the attraction between
the H of one molecule and the O of another.
HI would not have hydrogen bonding b/c iodine is not
highly electronegative.
Dipole–Dipole Forces
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A polar molecule has a positively charged
“end” (δ+) and a negatively charged “end”
(δ–).
When molecules come close to one another,
repulsions occur between like-charged
regions of dipoles. Opposite charges tend to
attract one another.
Dipole Forces
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The more polar a molecule, the more
pronounced is the effect of dipole–
dipole forces on physical properties.
Dipole–Dipole Interactions
Opposites
attract!
London Forcesaka Dispersion Forces
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At first no dipole, like Argon.
But the electrons are mobile, and at any one
instant they might find themselves towards
one end of the molecule, making that end -.
The other end will be temporarily short of
electrons and so becomes +.
An instant later the electrons may have
moved up to the other end, reversing the
polarity of the molecule.
Induced London Forces
What would happen if we mixed HCl with the
element argon, which has no dipole?
 The electrons on an argon atom are distributed
homogeneously around the nucleus of the
atom. But these electrons are in constant
motion. When an argon atom comes close to a
polar HCl molecule, the electrons can shift to
one side of the nucleus to produce a very small
dipole moment that lasts for only an instant.
Dispersion Forces Illustrated (1)
At a given instant, electron
density, even in a nonpolar
molecule like this one, is
not perfectly uniform.
Dispersion Forces Illustrated (2)
The region of
(momentary) higher
electron density attains
a small (–) charge …
… the other end
of the molecule is
slightly (+).
When another
nonpolar molecule
approaches …
Dispersion Forces Illustrated (3)
… this molecule
induces a tiny
dipole moment …
… in this
molecule.
Opposite charges ________.
Types of forces
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Click here for an Animation of the
forces
Molecular Shape and Polarizability
Long skinny
molecule …
… higher
boiling point.
… giving weaker
dispersion forces and
a lower boiling point.
… can have greater separation of
charge along its length. Stronger
forces of attraction, meaning …
In the compact isomer, less
possible separation of charge …
Arrange the following substances in the expected order
of increasing boiling point:
Carbon tetrabromide, CBr4;
Butane, CH3CH2CH2CH3;
Fluorine, F2;
Acetaldehyde, CH3CHO.
Arrange the following substances in the expected order
of increasing boiling point:
Carbon tetrabromide, CBr4;
Butane, CH3CH2CH2CH3;
Fluorine, F2;
Acetaldehyde, CH3CHO.
Answer: F2, CBr4, CH3CH2CH2CH3, CH3CHO
Vapor Pressure
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The vapor pressure of a liquid is the partial
pressure exerted by the vapor when it is in
dynamic equilibrium with the liquid at a
constant temperature.
vaporization
Liquid
Vapor
condensation
Liquid–Vapor Equilibrium
More vapor forms; rate
of condensation of that
vapor increases …
… until equilibrium
is attained.
Phase Diagrams
A phase diagram is a
graphical
representation of the
conditions of
temperature and
pressure under which a
substance exists as a
solid, liquid, a gas, or
some combination of
A—B, solid-vapor
these in equilibrium.
equilibrium.
A—D, solid-liquid
equilibrium.
A—C, liquid-vapor
equilibrium.
Triple point
Phase Diagram for CO2
Note that at 1 atm,
only the solid and
vapor phases of CO2
exist.
Phase Diagram for H2O
Supercritical Fluid
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Above the critical temperature and
pressure, only one phase exists…a
combination of liquid and gas.
Properties are in between those of
liquids and gases.
They act as solvents and dissolve well.
They diffuse well like gases.
CO2 and H2O- environmentally friendly
The Critical Point
At room temperature there
is relatively little vapor, and
its density is low.
At higher temperature,
there is more vapor, and its
density increases …
… while the density of the
liquid decreases; molecular
motion increases.
At Tc, the densities
of liquid and vapor
are equal; a single
phase.