CHEM 1000 3.0Structure of liquids and solids3Van der Waal Forces•In the section on gases we said there wereforces between the molecules.Now weknow more about bonding and electronicstructure we can revisit the question.•What kinds of forces exist betweenmolecules?
CHEM 1000 3.0Structure of liquids and solids4Van der Waal Forces•Van der Waal forces are those attractiveforces between molecules that are responsiblefor:– Real gas behaviour (Van der Waal “a”)– Condensation•There are a number of such forces resulting indifferent strengths of interactions betweenmolecules.
CHEM 1000 3.0Structure of liquids and solids5Van der Waal ForcesLondon or Dispersion Force•The quantum concept of electronic structuretalks about the probability of an electronbeing in a certain region at a certain time.•Even for an atom or aperfectly symmetricalmolecule there is a finite chance that theelectronic charges are not uniformlydistributed.•This produces aninstantaneous dipole.
CHEM 1000 3.0Structure of liquids and solids6Van der Waal ForcesLondon or Dispersion Force•The instantaneous dipole can then influenceneighbouring molecules and induce a dipolein them. (aninduced dipole)•This gives two dipoles that can attract.Thisis usually called adispersion forceor aLondon force.Moleculeon averageInstantaneousdipole
CHEM 1000 3.0Structure of liquids and solids7Van der Waal ForcesLondon or Dispersion Forcea.Normalb.Instantaneous dipolec.Instantaneous dipole (left) induced dipole (right)
CHEM 1000 3.0Structure of liquids and solids8Van der Waal ForcesLondon or Dispersion Force•This force is going to be strongest in a moleculewith a large number of electrons or for anelongated molecule.•The ability or tendency of a molecule to havecharge separation occur is calledpolarizability.•The effect of London forces can often be seen inthe boiling points of similar compounds.
CHEM 1000 3.0Structure of liquids and solids9Effect of London Forces•Bpt(He) = 4K•Bpt(Rn) = 211KRn is larger____________________________________n-pentane is more elongatedCH3-CH2-CH2-CH2-CH3Bpt 36.1oCBpt 9.5oCCCH3CH3CH3CH3
CHEM 1000 3.0Structure of liquids and solids10Van der Waal ForcesDipole-Dipole interactions•Bonds where the elements have differentelectronegativity have a permanent chargeseparation.E.g. HCl•H→Cl•If this shows up in the molecule we say themolecule has a permanent dipole.
CHEM 1000 3.0Structure of liquids and solids11Van der Waal ForcesDipole-Dipole interactions•For molecules with permanent dipoles,Dipole-Dipole interactions can occur.
CHEM 1000 3.0Structure of liquids and solids12Van der Waal ForcesDipole-Dipole interactions
CHEM 1000 3.0Structure of liquids and solids13Van der Waal ForcesDipole-Dipole interactions•Dipole-dipole interactionsare in addition to theLondon forces.