5 Properties of liquids and solids.2015.pdf - Properties of...

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CHEM 1000 3.0Structure of liquids and solids1Properties of Liquids and SolidsPetrucci, Herring Madura and Bissonnette :Chapter 12
CHEM 1000 3.0Chemical Bonding2Problem setChapter 12questions 1, 22, 29, 36, 41, 51, 65, 87, 105,108, 125, 127, 139
CHEM 1000 3.0Structure of liquids and solids3Van der Waal ForcesIn the section on gases we said there wereforces between the molecules.Now weknow more about bonding and electronicstructure we can revisit the question.Whatkindsofforcesexistbetweenmolecules?
Van der Waal ForcesVan der Waal forces are the attractiveforces between molecules that areresponsible for:Real gas behaviour(Van der Waal “a”)Recall:𝑃 +𝑎𝑛2𝑉2𝑉 − 𝑛𝑏= 𝑛𝑅𝑇There are a number of such forces resultingin different strengths of interactionsbetween moleculesCHEM 1000 3.0Structure of liquids and solids4
CHEM 1000 3.0Structure of liquids and solids5Van der Waal ForcesLondon or Dispersion ForceThe 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 ForceThe 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 ForceNormalInstantaneousdipoleInstantaneous dipole(left) Induced dipole(right)
CHEM 1000 3.0Structure of liquids and solids8Van der Waal ForcesLondon or Dispersion ForceThis 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.
Effects of London ForcesCHEM 1000 3.0Structure of liquids and solids9
CHEM 1000 3.0Structure of liquids and solids10Effect of London ForcesBoiling Point (He) = 4KBoiling Point (Rn) = 211KRn is larger____________________________________n-pentane is more elongatedCH3-CH2-CH2-CH2-CH3Boiling Point 36.1oCBoiling Point 9.5oCCCH3CH3CH3CH3
Effects of London ForcesCompoundPolarizability, *10-25cm3Molar Mass, amuBoiling Point, KH27.902.015820.35O216.031.998890.19N217.628.013477.35CH426.016.04109.15C2H644.730.07184.55Cl246.170.906238.25C3H862.944.11231.05CCl4105153.81349.95CHEM 1000 3.0Structure of liquids and solids11Adapted from Petrucci et al., 2011
CHEM 1000 3.0Structure of liquids and solids12Van der Waal ForcesDipole-Dipole interactionsBonds where the elements have differentelectronegativity have a permanent chargeseparation.

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Term
Fall
Professor
Hempstead
Tags
Chem, Cubic crystal system, Crystal system, London Forces

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