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 d+ d- d+ d- d+ d- X - Y ……….…... X - Y ………..…... X - Y Attractive force The IMFs that hold polar molecules (permanent dipoles) together H-bonding is one example…

Hydrogen Bonds in Water

How Do Nonpolar Molecules Stick Together? charge symmetric (no net d+, d-)

Dispersion (London) Forces + - nonpolar molecules attractive force induced dipoles IMF = interaction of induced (temporary) dipoles Individual interaction is weaker than ordinary dipole-dipole force

Why Weaker? What’s Different? Ecoulomb a q1q2/d q for permanent dipoles > q for induced dipoles

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

IMFs Influence Physical Properties and Phase Changes boiling point (l ® g) melting point (s ® l) DEvaporization DEmelting DEsublimation

Phase Changes GAS LIQUID SOLID sublimation condensation deposition vaporization freezing or fusion LIQUID SOLID melting

E Relationships In Phase Changes gas Energy vaporization condensation endothermic exothermic liquid DEvaporization= -DEcondensation

Energy of Vaporization Energy to vaporize one mole Increases as IMFs get stronger Compound DEvap, kJ/mol HF 25.2 HCl 17.5 H2O 40.7 CH4 8.9 He 0.08 Hbonding, London Dipole-dipole, London London IMFs?