Intermolecular Forces (IMFs) What Makes Molecules Stick To One Another?

Intermolecular Forces (IMFs)

What Makes Molecules Stick

To One Another?

Intermolecular vs. Intramolecular

Intermolecular forces are the attractive forces between molecules.

Intramolecular forces (bonds) hold atoms together within a molecule and are much stronger

41 kJ to vaporize 1 mole of water (inter)

930 kJ to break all O-H bonds in 1 mole of water (intra)

Dipole-Dipole Interactions

The IMFs that hold polar molecules (permanent dipoles) together

H-bonding is one example…

X - Y ……….…... X - Y ………..…... X - Y

Attractive force

Hydrogen Bonds in Water

How Do Nonpolar Molecules Stick Together?

charge symmetric(no net +, -)

Dispersion (London) Forces

IMF = interaction of induced (temporary) dipoles Individual interaction is weaker than ordinary

dipole-dipole force

+ - + -

nonpolar molecules

attractive force

induced dipoles

dEcoulomb q1q2/d

•q for permanent dipoles > q for induced dipoles

Why Weaker? What’s Different?

Dispersion (London) Forces

Magnitude increases with molecular size (weight)

more e- to polarize large molecules where e-’s are far from

nuclei are easier to polarize than small molecules

Dispersion Forces Are Important In ~All Molecules

Only IMFs for nonpolar molecules Account for up to 85% of total IMF in

polar cmpds Minor only when H-bonding or ion-ion

forces are present

boiling point (l g)

melting point (s l)

Evaporization

Emelting

Esublimation

IMFs Influence Physical Properties and Phase Changes

Phase Changes

sublimatio

n

deposition

GAScondensation

vaporization

freezing or fusion

melting

SOLID LIQUID

E Relationships In Phase Changes

Energy vaporization condensation

gas

liquid

Evaporization= -Econdensation

endothermicexothermic

Energy of Vaporization

Energy to vaporize one mole Increases as IMFs get stronger

Compound Evap, kJ/molHF 25.2HCl 17.5H2O 40.7CH4 8.9He 0.08

IMFs?Hbonding, LondonDipole-dipole, LondonHbonding, LondonLondonLondon