1. Intermolecular Forces and
Liquids and Solids
Chapter 11
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2. Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
Enthalpy change = heat change at constant pressure
Stronger the intermolecular forces, larger the enthalpies of
vaporization, fusion, sublimation; generally, larger the
Boiling points & melting points.
“Measure” of intermolecular force
boiling point
melting point
Dhvap: enthalpy of vaporization
Dhfus: enthalpy of fusion
Dhsub: enthalpy of sublimation
11.2

3. Types of Intermolecular Forces
Ionic Bond: The electrostatic force of attraction between the cation and the anion of an ionic compound. This is the strongest type of force. Ionic compounds have generally very high melting points and boiling points.
Ion – dipole force (occurs between ions and dipoles=polar molecule). Polar molecules result when the electron distribution is not symmetrical. There is more electron density in one part of the molecule (negative pole) compared with another part of the molecule (positive pole).
H-bonding or hydrogen bonding: This is the force of attraction between the H atom of a O-H, or N-H or H-F with a lone pair of electrons on a O, or N or F of a neighboring molecule. This force is seen between HF, and polar molecules containing O-H or N-H bonds.
Dipole-Dipole Force: Attraction between two polar molecules.

4. 5. London Dispersion forces = van der Waals Forces = Induced Dipole Forces: Forces of attraction among nonpolar compounds, elements and mixtures of these.
The strength of the attraction increases with
The molecular weight (boiling points among the halogens: F2 < Cl2 < Br2 < I2; F2 & Cl2 are gases at room Temperature, Br2 is a liquid at room temperature, I2 is a solid at room temperature)
The surface area

5. Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2

6. Intermolecular Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
11.2

7. Intermolecular Forces
Dispersion Forces
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
ion-induced dipole interaction
dipole-induced dipole interaction
11.2

8. Induced Dipoles Interacting With Each Other
11.2

9. What type(s) of intermolecular forces exist between each of the following molecules?
HBr
HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces. S O
SO2
SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.
11.2

10. Intermolecular Forces
Hydrogen Bond
The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A H … B or
A & B are N, O, or F
11.2