Chem 2 - Intermolecular Forces & Phases of Matter II

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.)