Unit 7.5 Comparing
Intermolecular Forces
Teacher: Dr. Van Der Sluys
Objectives
• To determine
– Relative melting and boiling points
– Relative solubility of various combinations
of compounds
Vocabulary
• Solution - a homogeneous mixture of
two or more compounds.
• Solvent - The compound that comprises
the majority of a solution.
• Solute - A minor component of a
solution.
• Aqueous solution - A mixture of liquid
water and one or more solutes.
Types of Bonding
Intramolecular
Intermolecular
Nonpolar Covalent
London Dispersion Forces
Polar Covalent
London Dispersion when symmetric
Dipole-Dipole when asymmetric
Extremely Polar Covalent: H-N,
H-O or H-F bonds
Hydrogen Bonding
Weakly Ionic:
NaCl
Soluble in water due to
ion-dipole interactions
Extremely Ionic:
Fe2O3
Insoluble in water due to high
crystal lattice energies
Like Dissolves Like
• In order for a mixture to combine and become
homogeneous on the molecular level, the
compounds must have similar intermolecular
bonding properties, i.e. ethanol and water.
• Two compounds that have very different
intermolecular forces will not mix and will
produce a heterogeneous mixture, i.e. Italian
salad dressing.
Comparing Intermolecular
Forces
Compound
London
Dispersion
DipoleDipole
(AXE?)
Hydrogen
Bonding
(H-F, H-N,
H-O)
Weakly
Ionic
(Ions with
low
charges)
Strongly
ionic
(Both ions
have
charges >2
Usually
Insoluble in
H2O)
Comparing Intermolecular
Forces
Compound
CH4
methane
H 2O
water
London
Dispersion
DipoleDipole
(AXE?)
Hydrogen
Bonding
(H-F, H-N,
H-O)
Weakly
Ionic
(Ions with
low
charges)
Strongly
ionic
(Both ions
have
charges >2
Usually
Insoluble in
H2O)
Comparing Intermolecular
Forces
Compound
NH3
ammonia
H 2O
water
London
Dispersion
DipoleDipole
(AXE?)
Hydrogen
Bonding
(H-F, H-N,
H-O)
Weakly
Ionic
(Ions with
low
charges)
Strongly
ionic
(Both ions
have
charges >2
Usually
Insoluble in
H2O)
Comparing Intermolecular
Forces
Compound
Fe2O3
Iron(III)
oxide
C8H18
octane
London
Dispersion
DipoleDipole
(AXE?)
Hydrogen
Bonding
(H-F, H-N,
H-O)
Weakly
Ionic
(Ions with
low
charges)
Strongly
ionic
(Both ions
have
charges >2
Usually
Insoluble in
H2O)
Comparing Intermolecular
Forces
Compound
CO2
Carbon
dioxide
H2O
water
London
Dispersion
DipoleDipole
(AXE?)
Hydrogen
Bonding
(H-F, H-N,
H-O)
Weakly
Ionic
(Ions with
low
charges)
Strongly
ionic
(Both ions
have
charges >2
Usually
Insoluble in
H2O)
Melting and Boiling Points
• The relative melting and boiling of various
compounds are due to the strength of intermolecular
forces.
• If two compounds have the same types of
intermolecular forces, the total number of electrons
can usually be used to predict the degree of London
dispersion forces. The compound with more
electrons usually has the higher melting and boiling
points.
• For compounds with hydrogen bonding, the ratio of
hydrogen atoms to lone pairs can be important.
Comparing Intermolecular
Forces
Compound
CH4
methane
C 2H 6
ethane
London
Dispersion
(Total
number of
electrons?)
DipoleDipole
(AXE?)
Hydrogen
Bonding
(H-F, H-N,
H-O)
Weakly
Ionic
(Ions with
low
charges)
Strongly
ionic
(Both ions
have
charges >2
Usually
Insoluble in
H2O)
Comparing Intermolecular
Forces
Compound
CO2
Carbon
dioxide
H2O
water
London
Dispersion
(Total
number of
electrons?)
DipoleDipole
(AXE?)
Hydrogen
Bonding
(H-F, H-N,
H-O)
Weakly
Ionic
(Ions with
low
charges)
Strongly
ionic
(Both ions
have
charges >2
Usually
Insoluble in
H2O)
Comparing Intermolecular
Forces
Compound
NaCl
Sodium
chloride
C6H14
Hexane
London
Dispersion
(Total
number of
electrons?)
DipoleDipole
(AXE?)
Hydrogen
Bonding
(H-F, H-N,
H-O)
Weakly
Ionic
(Ions with
low
charges)
Strongly
ionic
(Both ions
have
charges >2
Usually
Insoluble in
H2O)
Comparing Intermolecular
Forces
Compound
CO2
Carbon
dioxide
H2O
water
London
Dispersion
(Total
number of
electrons?)
DipoleDipole
(AXE?)
Hydrogen
Bonding
(H-F, H-N,
H-O)
Weakly
Ionic
(Ions with
low
charges)
Strongly
ionic
(Both ions
have
charges >2
Usually
Insoluble in
H2O)
Summary
• If compounds have similar
intermolecular forces they tend to form
homogeneous solutions, “like dissolves
like.”
• The strength of intermolecular forces
can be used to predict relative melting
and boiling points