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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-ShareAlike 4.0 International License.
Types of Intermolecular Forces and Relative Strengths
Ion-Dipole
Hydrogen bonding
Dipole-dipole
Dipole-induced dipole
Dispersion forces (or London forces)weakest
strongest
Dipole-Dipole Interactions
Polar molecules are electrostatically attracted to each other.
Imagine 2 HCl molecules…
H Cl H Cl
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.
H Cl H Cl
Dipole-Dipole Interactions
The most favorable arrangement for dipole-dipole interactions is head-to-tail.
These “chains” exist mainly in liquids and solids.
H Cl H Cl H Cl H Cl
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.
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.
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.
C
CC
C
CC
H
H
H
H
H
H
Polarizability
Types of molecules or ions that exhibit high polarizability:Large molecules (have lots of electrons)
CCl4 is more polarizable than CH4
C
Cl
Cl
Cl
ClC
H
H
HH
(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)
(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
Dipole-Induced-Dipole Attractions
A polar molecule polarizes the electron cloud on a neighboring nonpolar molecule.
Dipole-induced-dipole interactions exist between unlike molecules.
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.
H Cl Xe
Types of Intermolecular Forces and Relative Strengths
Ion-Dipole
Hydrogen bonding
Dipole-dipole
Dipole-induced dipole
Dispersion forces (or London forces)weakest
strongest
Types of Intermolecular Forces and Relative Strengths
Ion-Dipole
Hydrogen bonding
Dipole-dipole
Dipole-induced dipole
Dispersion forces (or London forces)weakest
strongest
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 liquidExample: NaCl dissolved in H2O
H
O HH
O
HH
OHH
O
H
Na+
HO
HCl-
HO
H
H
OH
HO
H
H
OH
Ion-Dipole Attractions in Solution
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”
+
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)
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.
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.
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
H
OH
H
O
H
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.
Next Up…
How to compare strengths of IM forces between molecules and predicting properties (bp, mp, vapor pressure, etc.)