Intermolecular Forces
Irresistible attraction…
ATTRACTIVE FORCES
Always electrostatic in nature
Intramolecular forces
bonding forces
These forces exist within each molecule.
They influence the chemical properties of the substance.
Intermolecular forces
nonbonding forces
These forces exist between molecules.
They influence the physical properties of the substance.
Why Are Molecules Attracted to 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
• 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
Dipole–Dipole Attractions
• Polar molecules have a permanent dipole
– because of bond polarity and shape
– dipole moment
– as well as the always present induced dipole
• The permanent dipole adds to the attractive
forces between the molecules
– raising the boiling and melting points relative to
nonpolar molecules of similar size and shape
Polar molecules and dipole-dipole forces.
solid
liquid
Hydrogen Bonding
• When a very electronegative atom is bonded to
hydrogen, it strongly pulls the bonding electrons toward
it
– O─H, N─H, or F─H
• Because hydrogen has no other electrons, when its
electron is pulled away, the nucleus becomes deshielded
– exposing the H proton
• The exposed proton acts as a very strong center of
positive charge, attracting all the electron clouds from
neighboring molecules
H-Bonding
HF
Drawing Hydrogen Bonds Between Molecules of a Substance
PROBLEM:
Which of the following substances exhibits H bonding? For
those that do, draw two molecules of the substance with
the H bonds between them.
O
(a)
C2H6
(b) CH3OH
(c)
CH3C NH2
Find molecules in which H is bonded to N, O or F. Draw H
bonds in the format -B:
H-A-.
SOLUTION:
(a) C2H6 has no H bonding
sites.
H
H
O
(b)
(c)
O
H N CH3C CH C
H C O H
3
H
H
N H
O
CH3C N H
H O C H
H
H
H
H
H N
CH3C
O
Dispersion Force
Dispersion forces among nonpolar molecules.
separated
Cl2
molecules
instantaneous
dipoles
Predicting the Type and Relative Strength of Intermolecular Forces
PROBLEM:
For each pair of substances, identify the dominant
intermolecular forces in each substance, and select the
substance with the higher boiling point.
(a) MgCl2 or PCl3
(b) CH3NH2 or CH3F
(c) CH3OH or CH3CH2OH
CH3
(d) Hexane (CH3CH2CH2CH2CH2CH3)
CH3CCH2CH3
or 2,2-dimethylbutane
CH3
Use the formula, structure
•Bonding forces are stronger than nonbonding(intermolecular)
forces.
•Hydrogen bonding is a strong type of dipole-dipole force.
•Dispersion forces are decisive when the difference is molar mass
or molecular shape.
Predicting the Type and Relative Strength of Intermolecular Forces
SOLUTION
: Mg2+ and Cl- are held together by ionic bonds while PCl is covalently
(a)
3
bonded and the molecules are held together by dipole-dipole interactions.
Ionic bonds are stronger than dipole interactions and so MgCl2 has the
higher boiling point.
(b) CH3NH2 and CH3F are both covalent compounds and have bonds which
are polar. The dipole in CH3NH2 can H bond while that in CH3F cannot.
Therefore CH3NH2 has the stronger interactions and the higher boiling
point.
(c) Both CH3OH and CH3CH2OH can H bond but CH3CH2OH has more CH
for more dispersion force interaction. Therefore CH3CH2OH has the
higher boiling point.
(d) Hexane and 2,2-dimethylbutane are both nonpolar with only dispersion
forces to hold the molecules together. Hexane has the larger surface area,
thereby the greater dispersion forces and the higher boiling point.
Summary diagram for analyzing the intermolecular forces in a sample.
INTERACTING PARTICLES
(atoms, molecules, ions)
ions present
ions only
IONIC BONDING
(Section 9.2)
ions not present
polar molecules only
DIPOLE-DIPOLE
FORCES
ion + polar molecule
ION-DIPOLE FORCES
H bonded to
N, O, or F
HYDROGEN
BONDING
nonpolar
molecules only
DISPERSION
FORCES only
polar + nonpolar
molecules
DIPOLEINDUCED DIPOLE
FORCES
DISPERSION FORCES ALSO PRESENT