Intermolecular Forces and Liquids Chap. 12

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© 2009 Brooks/Cole - Cengage

Intermolecular Forces and Intermolecular Forces and LiquidsLiquids

Chap. 12Chap. 12

Intermolecular Forces and Intermolecular Forces and LiquidsLiquids

Chap. 12Chap. 12

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Inter-Inter-moleculamolecular Forcesr Forces

Inter-Inter-moleculamolecular Forcesr Forces

Have studied Have studied INTRAINTRAmolecular molecular forces—the forces holding forces—the forces holding atoms together to form atoms together to form molecules. These are the forces molecules. These are the forces that hold COVALENT bonds that hold COVALENT bonds together.together.

Now turn to forces between Now turn to forces between molecules —molecules — INTERINTERmolecular molecular forces. forces.

Forces between molecules, Forces between molecules, between ions, or between between ions, or between molecules and ions.molecules and ions.

Electrons are NOT shared in these Electrons are NOT shared in these interactions!interactions!

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Ionic Bonds are an Ionic Bonds are an extreme case of extreme case of

intERmolecular forcesintERmolecular forces

Ionic Bonds are an Ionic Bonds are an extreme case of extreme case of

intERmolecular forcesintERmolecular forces

NaNa++—Cl—Cl-- in salt in salt

These are the strongest These are the strongest forces.forces.

Lead to solids with high Lead to solids with high melting temperatures.melting temperatures.

NaCl, mp = 800 NaCl, mp = 800 ooCC

Strength = 787kJ/molStrength = 787kJ/mol

MgO, mp = 2800 MgO, mp = 2800 ooCC

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Covalent Bonding Covalent Bonding intRAmolecular ForcesintRAmolecular Forces

for comparison of for comparison of magnitudemagnitude

Covalent Bonding Covalent Bonding intRAmolecular ForcesintRAmolecular Forces

for comparison of for comparison of magnitudemagnitude

C–H, 413 kJ/molC–H, 413 kJ/mol

C=C, 610 kJ/molC=C, 610 kJ/mol

C–C, 346 kJ/molC–C, 346 kJ/mol

CN, 887 kJ/molCN, 887 kJ/mol

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Dipole-Dipole Dipole-Dipole ForcesForces


Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule.

Strengths that range from 5 kJ to 20 kJ per mole

http://www.wisc-online.com/Objects/ViewObject.aspx?ID=GCH6804

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Influence of dipole-dipole forces is seen in Influence of dipole-dipole forces is seen in the boiling points of simple molecules.the boiling points of simple molecules.

CompdCompd Mol. Wt.Mol. Wt. Boil PointBoil Point

NN22 2828 -196 -196 ooCC

COCO 2828 -192 -192 ooCC

BrBr22 160160 59 59 ooCC

IClICl 162162 97 97 ooCC

IBrIBr 206206 116 116 ooCC

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London ForcesLondon ForcesLondon ForcesLondon Forces

London forces are exhibited by nonpolar molecules because of the random, correlated movements of the electrons in interacting molecules.

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Hydrogen BondingHydrogen BondingHydrogen BondingHydrogen Bonding

A special form of dipole-dipole attraction, A special form of dipole-dipole attraction, which enhances dipole-dipole attractions.which enhances dipole-dipole attractions.

H-bonding is strongest when X and Y are H-bonding is strongest when X and Y are N, O, N, O, or For F

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H-Bonding H-Bonding Between Between Methanol Methanol and Waterand Water

H-Bonding H-Bonding Between Between Methanol Methanol and Waterand Water

H-bondH-bondH-bondH-bond--

++

--

H-H-bondbondH-H-bondbond

--

++

--

H-Bonding H-Bonding Between Between Ammonia Ammonia and Waterand Water

H-Bonding H-Bonding Between Between Ammonia Ammonia and Waterand Water

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Hydrogen Bonding in Hydrogen Bonding in BiologyBiology

Hydrogen Bonding in Hydrogen Bonding in BiologyBiology

H-bonding is especially strong in biological H-bonding is especially strong in biological systems — such as DNA. systems — such as DNA.

DNA — helical chains of phosphate groups DNA — helical chains of phosphate groups and sugar molecules. Chains are helical and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, because of tetrahedral geometry of P, C, and O.and O.

Chains bind to one another by specific Chains bind to one another by specific hydrogen bonding between pairs of Lewis hydrogen bonding between pairs of Lewis bases.bases.

——adenine with thymineadenine with thymine

——guanine with cytosineguanine with cytosine

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Base-Pairing through H-Base-Pairing through H-BondsBonds

Base-Pairing through H-Base-Pairing through H-BondsBonds

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Liquids—Liquids—EvaporationEvaporation

To evaporate, molecules To evaporate, molecules must have sufficient must have sufficient energy to break IM forces.energy to break IM forces.

Breaking IM forces Breaking IM forces requires energy. The requires energy. The process of process of evaporation is evaporation is endothermicendothermic..

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Boiling Points of Boiling Points of Simple Hydrogen-Simple Hydrogen-

Containing Containing CompoundsCompounds

See Active Figure 12.8See Active Figure 12.8

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LiquidsLiquidsWhen molecules of liquid When molecules of liquid

are in the vapor state, are in the vapor state, they exert a they exert a VAPOR VAPOR PRESSUREPRESSURE

EQUILIBRIUM EQUILIBRIUM VAPOR VAPOR PRESSUREPRESSURE is the is the pressure exerted by a pressure exerted by a vapor over a liquid in a vapor over a liquid in a closed container when closed container when the the rate of evaporation rate of evaporation = the rate of = the rate of condensation.condensation. PLAY MOVIE

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Equilibrium Vapor PressureEquilibrium Vapor PressureSee Active Figure 12.17See Active Figure 12.17

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LiquidsLiquids

HEAT OF VAPORIZATIONHEAT OF VAPORIZATION is the heat is the heat reqreq’’d (at constant P) to vaporize the liquid.d (at constant P) to vaporize the liquid.

LIQ + heat LIQ + heat ff VAP VAP

Compd.Compd. ∆∆vapvapH (kJ/mol) H (kJ/mol) IM ForceIM Force

HH22OO 40.7 (100 40.7 (100 ooC)C) H-bondsH-bonds

SOSO22 26.8 (-47 26.8 (-47 ooC)C) dipoledipole

XeXe 12.6 (-107 12.6 (-107 ooC)C) London force London force induced induced

dipole dipole

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Intermolecular Forces and Liquids Chap. 12

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