Intermolecular
Forces
SCH3U1
Matter, Chemical Trends and
Chemical Bonding
The bonds in the polyatomic
ion CO32– are classified as
A)
B)
C)
D)
ionic
metallic
nonpolar covalent
polar covalent
The bonds in the polyatomic
ion CO32– are classified as
A)
B)
C)
D)
ionic
metallic
nonpolar covalent
polar covalent
Which of the following is the
least polar?
A)
B)
C)
D)
CH3F
HCl
N2
CO
Which of the following is the
least polar?
A)
B)
C)
D)
CH3F
HCl
N2
CO
Intermolecular Forces
Intermolecular Forces: Attractions between
“molecules” that hold them together. These forces
are electrical in origin and result from the mutual
attraction of unlike charges or the mutual
repulsion of like charges.
Types of Intermolecular Forces:
•
Ion-Dipole Forces
•
Van der Waals Forces
•  dipole-dipole forces
•  London dispersion forces
•  hydrogen bonds
Intermolecular Forces
Ion-Dipole Forces: The result of electrical interactions
between an ion and the partial charges on a polar
molecule.
Which of the following
exhibits ion-dipole forces?
A) NaCl(s)
B) NaCl(aq)
C) Na(s)
D) Cl2(g)
Which of the following
exhibits ion-dipole forces?
A) NaCl(s)
B) NaCl(aq)
C) Na(s)
D) Cl2(g)
Intermolecular Forces
Dipole-Dipole Forces: The result of electrical
interactions between dipoles on neighboring molecules.
Dipole–Dipole Forces
•  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.
•  The more polar a molecule, the more
pronounced is the effect of dipole–dipole
forces on physical properties.
Dipole–Dipole Interactions
Opposites
attract!
Intermolecular Forces
Dipole-Dipole Forces
As the dipole forces increase
the intermolecular forces increase.
As the intermolecular forces increase,
the boiling points increase.
Which has the smallest dipoledipole forces?
A)
B)
C)
D)
CH3F
HCl
I2
CO
Which has the smallest dipoledipole forces?
A)
B)
C)
D)
CH3F
HCl
I2
CO
Intermolecular Forces
London Dispersion Forces: The result of motion of
electrons which gives the molecule a short-lived dipole
moment which induces temporary dipoles in
neighboring molecules.
Dispersion Forces
… exist between any two particles.
Also called London forces (after Fritz London,
who offered a theoretical explanation of these
forces in 1928).
Dispersion forces arise because the electron
cloud is not perfectly uniform.
Tiny, momentary dipole moments can exist
even in nonpolar molecules.
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 ________.
Intermolecular Forces
London Dispersion Forces
As the dispersion forces increase
the intermolecular forces increase.
As the intermolecular forces increase,
the boiling points increase.
Intermolecular Forces
London Dispersion Forces
Which is expected to have
the largest dispersion
forces?
A)
B)
C)
D)
C 2H 6
C8H18
N2
CO2
Which is expected to have
the largest dispersion
forces?
A)
B)
C)
D)
C 2H 6
C8H18
N2
CO2
Intermolecular Forces
Hydrogen Bond: An attractive force between a
hydrogen atom bonded to a very electronegative atom
(O, N, or F) and an unshared electron pair on another
electronegative atom.
Hydrogen Bonds
A hydrogen bond is an intermolecular force in
which:
–  a hydrogen atom that is covalently bonded to a
(small, electronegative) nonmetal atom in one
molecule …
–  is simultaneously attracted to a (small,
electronegative) nonmetal atom of a neighboring
molecule. Y ––– H - - - Z ~~~~
When Y and Z are small and
highly electronegative (N,
O, F) …
… this force is called a
hydrogen bond; a
special, strong type of
dipole–dipole force.
Hydrogen Bonds in Water
Hydrogen Bonding in
Acetic Acid
Hydrogen bonding
occurs between
molecules.
Hydrogen Bonding in
Salicylic Acid
Hydrogen
bonding
occurs within
the molecule.
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.
Intermolecular Forces
Hydrogen Bond
Intermolecular Forces
Hydrogen Bond
Intermolecular Forces
Which of the following
compounds exhibits
hydrogen bonding?
A)
B)
C)
D)
CH3Cl
HI
H3C-O-CH3
NH3
Which of the following
compounds exhibits
hydrogen bonding?
A)
B)
C)
D)
CH3Cl
HI
H3C-O-CH3
NH3
Which of the following
compounds exhibits only
dispersion and dipole-dipole
intermolecular interactions?
A)
B)
C)
D)
N2
HBr
CO2
H 2O
Which of the following
compounds exhibits only
dispersion and dipole-dipole
intermolecular interactions?
A)
B)
C)
D)
N2
HBr
CO2
H 2O