Intermolecular Forces

• why is N2 a gas at room temp, but H2O is a liquid?– intermolecular forces

• intramolecular vs intermolecular forces

Intermolecular Forces

• Ion-dipole

H F

HF

Dissolution of an Ionic Solid

ions become surrounded by water dipoles

gain ion-dipole interactions

but lose ion-ion interactions

Na1+

HO

H

HO

H

H

O

H

H

O

H

Intermolecular Forces

• dipole-dipole

H Cl

H Cl

H Cl

HCl

Dipole-dipole Forces

Intermolecular Forces• hydrogen bonds

•special dipole-dipole interaction

a H atom which is covalently bound to O, N, or F interacts with another O, N, or F atom

H

O H O

H

H

Intermolecular Forces• hydrogen bonds

•direction matters

causes some interesting properties

Hydrogen Bonding for Water

ice

Intermolecular Forces• dispersion or London forces

•instantaneous dipole – induced dipole interaction

size of molecule or atom plays important role in strength of London forces

The Predictive Power of Intermolecular Forces

• melting and boiling points

• H2O vs N2

•ethanol and dimethyl ether

A Closer Look at Boiling

Surface Tension