Intermolecular Forces
States of matter
●Van der Waals forces
●Hydrogen bonding
●Physical effects of intermolecular forces
●Example problems
●
Intermolecular Forces
States of Matter
That there are intermolecular forces is
evidenced by the fact that every substance
known has a melting point and a boiling point.
What could account for this other than
intermolecular forces that must be overcome in
order to change phases?
Intermolecular Forces
States of Matter
The physical properties of melting point,
boiling point, vapor pressure, evaporation,
viscosity, surface tension, and solubility are
related to the strength of attractive forces
between molecules.
The amount of "stick togetherness" is important
in the interpretation of the various properties
listed above.
Intermolecular Forces
States of Matter
the properties of solids, liquids, and gases
can be explained based on the kinetic energy of
the molecules and the attractive forces between
molecules
●
kinetic energy tries to give molecules freedom
of motion
● degrees of freedom = translational,
rotational, vibrational
●
Intermolecular Forces
Phase Changes
Intermolecular Forces
Why Do Molecules Attract Each Other?
intermolecular attractions are due to
attractive forces between opposite charges
● + ion to - ion
● + end of polar molecule to - end of polar
molecule
● H-bonding especially strong
● even nonpolar molecules will have temporary
charges
●
Intermolecular Forces
Why Do Molecules Attract Each Other?
larger charge = stronger attraction
●
longer distance = weaker attraction
●
however, these attractive forces are small
relative to the bonding forces between atoms
● generally smaller charges
● generally over much larger distances
●
Intermolecular Forces
Van der Waals Forces
Molecules can attract each other at moderate
distances and repel each other at close range.
The attractive forces are collectively called
"van der Waals forces".
Van der Waals forces are much weaker than
chemical bonds, and random thermal motion
around room temperature can usually overcome or
disrupt them.
Intermolecular Forces
Van der Waals Forces
Only 16 kJ/mol of energy is
overcome the intermolecular
HCl molecules in the liquid
energy required to vaporize
required to
attraction between
state (i.e. the
the sample)
However, 431 kJ/mol of energy is required to
break the covalent bond between the H and Cl
atoms in the HCl molecule
Intermolecular Forces
Van der Waals Forces
When a molecular substance changes states the
atoms within the molecule are unchanged.
The temperature at which a liquid boils
reflects the kinetic energy needed to overcome
the attractive intermolecular forces (likewise,
the temperature at which a solid melts).
Thus, the strength of the intermolecular forces
determines the physical properties of the
substance.
Intermolecular Forces
Ion-Dipole Force
Involves an interaction between a charged ion
and a polar molecule (i.e. a molecule with a
dipole)
Cations are attracted to the negative end of a
dipole
Anions are attracted to the positive end of a
dipole
Intermolecular Forces
Ion-Dipole Force
The magnitude of the interaction energy depends
upon the charge of the ion (Q), the dipole
moment of the molecule (m) and the distance (d)
from the center of the ion to the midpoint of
the dipole.
Ion-dipole forces are important in solutions of
ionic substances in polar solvents (e.g. a salt
in aqueous solvent).
Intermolecular Forces
Ion-Dipole Force
© 2013 K. Brown
Intermolecular Forces
Dipole-Dipole Force
Polar molecules attract one another when the
partial positive charge on one molecule is near
the partial negative charge on the other
molecule
The polar molecules must be in close proximity
for the dipole-dipole forces to be significant
Intermolecular Forces
Dipole-Dipole Force
Dipole-dipole forces are characteristically
weaker than ion-dipole forces
Dipole-dipole forces increase with an increase
in the polarity of the molecule
© 2013 K. Brown
Dipole-Dipole Force
Intermolecular Forces
London Dispersion Force
Nonpolar molecules would not seem to have any
basis for attractive interactions, however:
gases of nonpolar molecules can be liquefied
indicating that if the kinetic energy is
reduced, some type of attractive force can
predominate.
Fritz London (1930) suggested that the motion
of electrons within an atom or non-polar
molecule can result in a transient dipole
moment
Intermolecular Forces
London Dispersion Force
An instantaneous dipole on one helium atom
induces an instantaneous dipole on a
neighbouring helium atom.
© 2013 K. Brown
Intermolecular Forces
London Dispersion Force
the magnitude of the induced dipole depends on
several factors
●
polarizability of the electrons
● volume of the electron cloud
● larger molar mass = more electrons = larger
electron cloud = increased polarizability =
stronger attractions
●
shape of the molecule
● more surface-to-surface contact = larger
induced dipole = stronger attraction
●
Intermolecular Forces
London Dispersion Force
The ease with which an external electric field
can induce a dipole (alter the electron
distribution) with a molecule is referred to as
the "polarizability" of that molecule.
The greater the polarizability of a molecule
the easier it is to induce a momentary dipole
and the stronger the dispersion forces.
Intermolecular Forces
London Dispersion Force
Larger molecules tend to have greater
polarizability
o Their electrons are further away from the
nucleus (any asymmetric distribution produces a
larger dipole due to larger charge separation)
o The number of electrons is greater (higher
probability of asymmetric distribution)
Intermolecular Forces
London Dispersion Force
As the molar mass
increases, the
number of electrons
increases.
Therefore the
strength of the
dispersion forces
increases.
Noble gas
He
Ne
Ar
Kr
Xe
molar mass
4.00
20.18
39.95
83.80
131.30
Bp(K)
4.2
27
87
120
165
Intermolecular Forces
Hydrogen Bonding
The most powerful intermolecular force
influencing neutral (uncharged) molecules is
the hydrogen bond.
If we compare the boiling points of methane
(CH4) -161oC, ammonia (NH3) -33oC, water (H2O)
100oC and hydrogen fluoride (HF) 19oC, we see a
greater variation for these similar sized
molecules than expected.
Intermolecular Forces
Hydrogen Bonding
Relationship between H-bonding
and Intermolecular Attraction
150
Boilin Point, °C
BP, H3X
BP, XH4
50
HF
H2Te
0
1
NH32
-100
-150
-200
BP, H2X
H2O
100
-50
BP, HX
H32S
SiH4
CH4
Period
H2Se4
GeH4
5
SnH4
Intermolecular Forces
Hydrogen Bonding
Intermolecular Forces
Hydrogen Bonding
When water molecules are close together, their
positive and negative regions are attracted to
the oppositely-charged regions of nearby
molecules.
Intermolecular Forces
Hydrogen Bonding
Intermolecular Forces
Summary
Dispersion forces are the weakest of the
intermolecular attractions.
●
Dispersion forces are present in all molecules
and atoms.
●
The magnitude of the dispersion forces
increases with molar mass
●
Polar molecules also have dipole-dipole
attractive forces
●
Intermolecular Forces
Summary
Hydrogen bonds are the strongest of the
intermolecular attractive force between like
molecules
●
Hydrogen bonds will be present when a molecule
has H directly bonded to either O , N, or F
atoms
●
Ion-dipole attractions are present in mixtures
of ionic compounds with polar molecules.
●
Intermolecular Forces
Summary
Ion-dipole attractions are especially
important in aqueous solutions of ionic
compounds
●
Ion-dipole attractions are the strongest
intermolecular attraction
●
Intermolecular Forces
Surface Tension
Molecules in the liquid state experience strong
intermolecular attractive forces. When those
forces are between the same molecules, they are
referred to as cohesive forces.
When the attractive forces are between unlike
molecules, they are called adhesive forces.
Intermolecular Forces
Surface Tension
The molecules of a water droplet are held
together by cohesive forces, and the especially
strong cohesive forces at the surface
constitute surface tension.
The molecules at the surface do not have other
like molecules on all sides of them and
consequently they cohere more strongly to those
directly associated with them on the surface.
Intermolecular Forces
Surface Tension
Intermolecular Forces
Surface Tension
Molecules on the surface have neighboring
molecules only on one side (the side facing the
interior) and thus experience an attractive
force which tends to pull them into the
interior.
The surface of the liquid will rearrange until
the least number of molecules are present on
the surface
Intermolecular Forces
Surface Tension
In other words the surface area will be
minimized. A sphere has the smallest surface
area to volume ratio
The surface molecules will pack somewhat closer
together than the rest of the molecules in the
liquid
The surface molecules will be somewhat more
ordered and resistant to molecular disruptions
Intermolecular Forces
Surface Tension
The "inward" molecular attraction forces, which
must be overcome to increase the surface area,
are termed the "surface tension"
Intermolecular Forces
Surface Tension
the stronger the intermolecular attractive
forces, the higher the surface tension will be
●
raising the temperature of a liquid reduces
its surface tension
● raising the temperature of the liquid
increases the average kinetic energy of the
molecules
● the increased molecular motion makes it
easier to stretch the surface
●
Intermolecular Forces
Surface Tension
Intermolecular Forces
Viscosity
The resistance of a liquid to flow is called
its viscosity
The greater the viscosity, the more slowly it
flows
Viscosity is a measure of the ease with which
molecules move past one another and depends on
the attractive force between the molecules
Intermolecular Forces
Viscosity
It depends on whether there are structural
features which may cause neighbouring molecules
to become "entangled"
Viscosity decreases with increasing temperature
- the increasing kinetic energy overcomes the
attractive forces and molecules can more easily
move past each other
Intermolecular Forces
Viscosity
Intermolecular Forces
Viscosity
Viscosity of Water vs. Temperature
1.2
V iscosity, cP
1
0.8
0.6
0.4
0.2
0
0
20
40
60
Temperature, deg C
80
100
120
Intermolecular Forces
Solubility
Solubility depends on the attractive forces of
solute and solvent molecules
● Like dissolves Like
● miscible liquids will always dissolve in
each other
●
Intermolecular Forces
Solubility
polar substances dissolve in polar solvents
● hydrophilic groups = OH, CHO, C=O, COOH, NH ,
2
Cl
●
nonpolar molecules dissolve in nonpolar
solvents
● hydrophobic groups = C-H, C-C
●
Intermolecular Forces
Solubility
Intermolecular Forces
Polar Solvents
Water
Intermolecular Forces
Polar Solvents
Dichloromethane
(methylene chloride)
Intermolecular Forces
Polar Solvents
Ethanol
(ethyl alcohol)
Intermolecular Forces
Non-Polar Solvents
CH3
HC
n-hexane
HC
C
C
H
CH
CH
toluene
Cl
Cl
Cl
C
Cl
carbon tetrachloride
Intermolecular Forces
Capillary Action
capillary action is the ability of a liquid to
flow up a thin tube against the influence of
gravity
●
capillary action is the result of the two
forces working in conjunction, the cohesive and
adhesive forces
● cohesive forces attract the molecules
together
● adhesive forces attract the molecules on the
edge to the tube’s surface
●
Intermolecular Forces
Capillary Action
the adhesive forces pull the surface liquid up
the side of the tube, while the cohesive forces
pull the interior liquid with it
●
the liquid rises up the tube until the force
of gravity counteracts the capillary action
forces
●
Intermolecular Forces
Capillary Action
the curving of the liquid surface in a thin
tube is due to the competition between adhesive
and cohesive forces
●
the meniscus of water is concave in a glass
tube because its adhesion to the glass is
stronger than its cohesion for itself
●
Intermolecular Forces
Capillary Action
the meniscus of mercury is convex in a glass
tube because its cohesion for itself is
stronger than its adhesion for the glass
● metallic bonds stronger than intermolecular
attractions
●
Intermolecular Forces
Vapor Pressure
Ordinary evaporation is a surface phenomenon some molecules have enough kinetic energy
overcome intermolecular attractions and to
escape.
If the container is closed, an equilibrium is
reached where an equal number of molecules
return to the surface. The pressure of this
equilibrium is called the saturation vapor
pressure.
Intermolecular Forces
Vapor Pressure
Intermolecular Forces
Vapor Pressure
Since the molecular kinetic energy is greater
at higher temperature, more molecules can
escape the surface and the saturated vapor
pressure is correspondingly higher.
Intermolecular Forces
Vapor Pressure
Intermolecular Forces
Boiling
Ordinary evaporation is a surface phenomenon since the vapor pressure is low and since the
pressure inside the liquid is equal to
atmospheric pressure plus the liquid pressure,
bubbles of water vapor cannot form.
At the boiling point, the saturated vapor
pressure is equal to atmospheric pressure,
bubbles form, and the vaporization becomes a
volume phenomena.
Intermolecular Forces
Boiling
Intermolecular Forces
Boiling
The boiling point is defined as the temperature
at which the saturated vapor pressure of a
liquid is equal to the surrounding atmospheric
pressure.
For water, the vapor pressure reaches the
standard sea level atmospheric pressure of 760
mmHg at 100oC. This is the normal boiling point
of water.
Intermolecular Forces
Boiling
Intermolecular Forces
Boiling
What will happen to water at room temperature
if the atmospheric pressure is decreased to a
very low pressure?
Intermolecular Forces
The Critical Point
What happens if we try to boil a liquid in a
closed container?
As a liquid is heated in a closed container, it
does not boil but rather its density decreases
while the density of the vapor being formed
increases.
Intermolecular Forces
The Critical Temperature
The liquid and vapor densities become closer
and closer to each other until the critical
temperature is reached where the two densities
are equal and the liquid-gas line or phase
boundary disappears.
Intermolecular Forces
The Critical Point
Intermolecular Forces
Sublimation
Because there is some molecular vibration in
solids, surface molecules can occasionally
overcome the intermolecular forces from the
bulk of the solid and escape the surface into
the gas phase. This is sublimation.
The reverse process from gas to solid is
deposition.
Intermolecular Forces
Sublimation
Sublimation
Deposition
Intermolecular Forces
Deposition
Intermolecular Forces
Phase Diagram
Intermolecular Forces
Phase Diagram
CO2
Intermolecular Forces
Phase Diagram
H2O
Example Problems
1. Which of the following can form hydrogen
bonds with water: CH3OCH3, CH4, F-, Na+?
A substance can form hydrogen bonds with water
if it contains one of the three electronegative
elements, F, O or N or if it has an H bonded to
one of these three elements.
CH3OCH3 yes
CH4 no
F- yes
Na+ no
Example Problems
2. What types of intermolecular forces exist
between the following pairs: HBr and H2S, Cl2
and CBr4, H2O and NO3-, NH3 and C6H6?
There are four basic forces to choose from:
dispersion, dipole-dipole, hydrogen bonding and
ion-dipole.
HBr and H2S dispersion and dipole-dipole
Cl2 and CBr4, dispersion
H2O and NO3-, dispersion, hydrogen bonding and
ion-dipole
NH3 and C6H6 dispersion
Example Problems
3. Arrange in order of increasing boiling
point: NH3, Ne, C3H8, CH3OH.
Boiling points are intimately linked with the
type(s) of intermolecular forces involved. The
stronger the force the higher the boiling point
will be.
Ne < C3H8 < NH3 < CH3OH