Chem 2 - Intermolecular Forces & Phases of Matter II

1. Types of Intermolecular
Forces (Lecture Pt. 2)
By Dr. Shawn P. Shields
This work is licensed by Shawn P. Shields-Maxwell, Ph. D., under a Creative Commons Attribution-NonCommercial-
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2. Types of Intermolecular Forces and Relative Strengths
Ion-Dipole
Hydrogen bonding
Dipole-dipole
Dipole-induced dipole
Dispersion forces (or London
forces)
weakest
strongest

3. Dipole-Dipole Interactions
Polar molecules are electrostatically
attracted to each other.
Imagine 2 HCl molecules…
  

4. Dipole-Dipole Interactions
Polar molecules have partially-positive and
partially-negative ends.
These opposite charges attract each other.
Electrostatic attractions operate only at
short distances.
  

5. Dipole-Dipole Interactions
The most favorable arrangement for
dipole-dipole interactions is head-to-tail.
These “chains” exist mainly in liquids and solids.

6. Polarizability
In order to understand dispersion forces,
we need to discuss the concept of
polarizability.
The polarizability of a molecule (or ion)
depends on the ease with which the
electron cloud is distorted by a nearby
charge.

7. Polarizability
Types of molecules or ions that exhibit
high polarizability:
Heavy atoms (or ions) have lots of loosely-
held electrons, making the electron cloud
easy to distort.
Recall atomic radius… valence e for large
atoms are far from the nucleus and are not
tightly held.

8. Polarizability
Types of molecules or ions that exhibit
high polarizability:
Molecules that
have  bonds
The electrons in  bonds lie above and below
the bond axis making it easier to distort the
electron cloud with a nearby charge.

9. Polarizability
Types of molecules or ions that exhibit
high polarizability:
Large molecules (have lots of electrons)
CCl4 is more polarizable than CH4

10. (London) Dispersion Forces
Dispersion forces involve a momentary
random fluctuation in the electron density
of a molecule.
There are the weakest of all intermolecular
forces.
Also called van der Waals forces and
Instantaneous-Dipole-Induced-Dipole (idid)

11. (London) Dispersion Forces
A momentary random fluctuation in the
electron density of a molecule induces a
dipole in a neighbor.
Instantaneous
dipole
Induced
dipole
Xe Xe

12. Dipole-Induced-Dipole Attractions
A polar molecule polarizes the electron
cloud on a neighboring nonpolar molecule.
Dipole-induced-dipole interactions exist
between unlike molecules.

13. Dipole-Induced-Dipole Attractions
Can only be (potentially) present in a mixture.
A molecule with a permanent dipole induces a
dipole in a neighbor by distorting its electron
cloud.
 
Xe

14. Types of Intermolecular Forces and Relative Strengths
Ion-Dipole
Hydrogen bonding
Dipole-dipole
Dipole-induced dipole
Dispersion forces (or London
forces)
weakest
strongest

15. Types of Intermolecular Forces and Relative Strengths
Ion-Dipole
Hydrogen bonding
Dipole-dipole
Dipole-induced dipole
Dispersion forces (or London
forces)
weakest
strongest

16. Ion-Dipole Attractions
Ion-dipole attractions are electrostatic
interactions between an ion and some
other uncharged polar molecule.
The most common example of ion-dipole
attraction is the dissolution of a salt in a
polar liquid
Example: NaCl dissolved in H2O

17. Ion-Dipole Attractions in Solution
NaCl s → Na+ aq + Cl−(aq)
Na+(aq) cations are attracted
to the partial negative charges
on O atoms in water.
Cl (aq) anions are attracted to
the partial positive charges on
H atoms in water.
“partial
charges”


+




18. Ion-Dipole Attractions
Ion-dipole attractions are stronger
When the charge on the ion increases, or
With increased magnitude of the dipole
of the polar molecule.
Ion-dipole interactions are very important
in solution chemistry. (coming soon)

19. Hydrogen Bonding
Hydrogen Bonding is a special class of the
strongest “dipole-dipole” attractions
A very strong intermolecular attraction
Hydrogen bonds are strong because H is
small, allowing close approach of the
dipoles.

20. Hydrogen Bonding
The interaction is between the H-bond
donor dipole and the acceptor lone pair.
The lone pair must be on a highly
electronegative atom, such as N, O, or F,
which interacts with the bond dipole
associated with a H—N, H—O, or H—F
bond.

21. Picture of Hydrogen Bonding
Hydrogen bonds tend to be linear, with the
“shared” H closer to the donor.
H-bond acceptor
“accepts” an H atom
from the other H2O
molecule
H-bond donor
“donates” an H atom
to the other H2O
molecule

22. Hydrogen Bonding
Remember:
Hydrogen bonds are NOT chemical
(covalent) bonds…they are held together
by electrostatic attractions.
Hydrogen bonds are much weaker than
covalent bonds, but they are a very strong
intermolecular force.

23. Next Up…
How to compare strengths
of IM forces between
molecules and predicting
properties (bp, mp, vapor
pressure, etc.) 