Intermolecular Forces
Bonding
• Ionic
• Covalent
• Polar covalent
Intermolecular Forces
Intermolecular Vs. Intramolecular Forces
intramolecular forces ( “bonds”)
O
H
H
inter molecular forces
Figure 10.1 The Schematic
Representations of the Three States of
Matter
Intermolecular Forces
Vaporize: 40.7 kJ
Break O-H Bonds: 467 kJ
Table 10.6 Comparison of Atomic Separations
Within Molecules (Covalent Bonds) and
Between Molecules (Intermolecular
Interactions)
Intermolecular Forces
• Covalent and Ionic bonds are intramolecular forces
• Attractions between molecules are intermolecular forces
• Intermolecular forces are weaker than intramolecular
forces (vaporizing H2O vs. breaking OH bonds)
• 3 Possible Different Types of IM forces:
- Dipole-Dipole
-Hydrogen bonding
- London Dispersion forces
* ALL molecular substances’ molecules
will be attracted by London Dispersion
forces, but not of equal strength.
* NOT all molecular substances’
molecules will be attracted by DipoleDipole or Hydrogen bonding “forces”
• Properties of pure substances (in liquid or solid form) that
depend on the strength of intermolecular forces: Melting
point, boiling point, ΔHvap, ΔHfus, Vapor pressure, surface
tension
Dipole-Dipole Forces
If a molecule is polar, it has a
positive and negative end
• If a molecule is polar, the positive end of
one molecule will be attracted to the
negative end of another
– That’s basically dipole-dipole forces!
• If a molecule is nonpolar then it will not
have “dipole-dipole” forces of attraction
between its molecules
Figure 10.2 a-b
(a) The
Electrostatic
Interaction of
Two Polar
Molecules (b)
The Interaction
of Many
Dipoles in a
Condensed
State
Okay, so how do you know if a
substance is polar?
• See separate handout….later.
– One example now: If substance has only one
bond, and that bond is polar, then the whole
molecule is polar
• HCl is polar; ClF is polar
• H2 (H-H) is nonpolar; Cl2 (Cl-Cl) is nonpolar;
• Just remember that you need to first
“figure out” if a substance is polar before
you conclude it “has” dipole-dipole forces
acting between its molecules!
Polarity of Substances
HCl, CO2, NaCl
**Polarity only applies to molecular
substances. Ionic compounds are at the
“extreme”—the bonding is not
considered “polar (covalent)”, but simply
“ionic”!
**Ion-ion forces are comparable to intramolecular forces. VERY STRONG
Figure 10.3 a-b (a) The Polar Water
Molecule (b) Hydrogen Bonding Among
Water Molecules
Hydrogen Bonding is a Special
Case Version of Dipole-Dipole
Hydrogen Bonding
- Strong dipole-dipole forces
- Found in substances in which H is bound
to highly electronegative atom (N, O, F)
Why?
- Very polar bond
- H atom is really small
Example
Which substance has the strongest dipoledipole forces (between its molecules)?
HF, HCl, HBr
Figure 10.4 The Boiling Points of the
Covalent Hydrides of the Elements in Groups
4A, 5A, 6A, and 7A
Hydrogen Bonding
London Dispersion Forces
Figure 10.5
(a) An
Instantaneous
Polarization
can Occur on
Atom A (b)
Nonpolar
Molecules
Such as H2
also can
Develop
Instantaneous
and Induced
Dipoles
Figure 10.4 The Boiling Points of the
Covalent Hydrides of the Elements in Groups
4A, 5A, 6A, and 7A
Table 10.2 The Freezing Points
of the Group 8A Elements
London Dispersion Forces
• Present between any two molecules
• The only forces present between Nonpolar
molecules (& between Noble Gas Atoms)
• London dispersion forces increase as size
of atom or molecule increases
London Dispersion Forces
 Which substance has the strongest
London dispersion forces?
CH4 , CH3CH3, CH3CH2CH3
Properties
•
•
•
•
•
Boiling Point
Melting Point (same as Freezing Point)
Δ Hvap
Δ Hmelting
Vapor Pressure
Boiling Points
Explain the difference in boiling points:
1) n-pentane, C5H12 MW: 72.15
Neopentane, C5H12 MW: 72.15
B.P.= 36.2 C Vs.
B.P.= 9.5 C
2) dimethyl ether, CH3OCH3 MW: 46.07 B.P.= -25 C Vs.
ethanol, CH3CH2OH
MW: 46.07 B.P. = 79 C
3) Napthalene, C10H8
MW: 128.16
Acetic Acid, CH3CO2H MW: 60.05
B.P.= 218 C Vs.
B.P.= 118  C
Examples
What type of interparticle forces are present in these substances?
Ar, HCl, CaCl2
What is the most important interparticle force in teflon,
CF3(CF2CF2)nCF3?
Which substance has stronger intermolecular forces?
SeO2 Vs. SO2
Which has the highest boiling point?
NaCl or HCl
Which substance has the highest freezing point?
H2O, NaCl, or HF
Summary of IM Forces
• All substances have London Dispersion Forces
- London Dispersion Forces Increase w/ MW
• Dipole-Dipole Interactions
- Stronger when molecules are more polar
- Special Case: Hydrogen Bonding (N, O, F)
Take into account all forces as well as the size and
shape of molecule when deciding which substance has
the strongest IM forces
Intermolecular Force Trends
As IM Forces Increase:
•
•
•
•
•
Boiling Point Increases
Melting/Freezing Point Increases
Δ Hvap Increases
Δ Hmelting Increases
Vapor Pressure Decreases
Next Lecture
• Vapor Pressure
• Phase Diagrams