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INTERMOLECULAR INTERMOLECULAR FORCES FORCES

INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

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Did you know… Intermolecular forces are generally weaker than ionic or covalent bonds Intermolecular forces are generally weaker than ionic or covalent bonds Many properties such as boiling point reflect the strengths of the intermolecular forces Many properties such as boiling point reflect the strengths of the intermolecular forces The stronger the attractive force, higher the temp. when liquid boils The stronger the attractive force, higher the temp. when liquid boils Melting points of solids increase as strengths of IF increase Melting points of solids increase as strengths of IF increase

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Page 1: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

INTERMOLECULINTERMOLECULAR FORCESAR FORCES

Page 2: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

A Quick IntroductionA Quick Introduction Intermolecular forces exist Intermolecular forces exist

everywhereeverywhere Short-range attractive forces Short-range attractive forces

operating between the particles operating between the particles that make up the units of a liquid or that make up the units of a liquid or solid substancesolid substance

Intermolecular forces also cause Intermolecular forces also cause gases to liquefy or solidify at low gases to liquefy or solidify at low temperatures and high pressurestemperatures and high pressures

Page 3: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

Did you know…Did you know… Intermolecular forces are generally Intermolecular forces are generally

weaker than ionic or covalent bondsweaker than ionic or covalent bonds Many properties such as boiling Many properties such as boiling

point reflect the strengths of the point reflect the strengths of the intermolecular forcesintermolecular forces The stronger the attractive force, The stronger the attractive force,

higher the temp. when liquid boilshigher the temp. when liquid boils Melting points of solids increase as Melting points of solids increase as

strengths of IF increasestrengths of IF increase

Page 4: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

An Attraction Force: An Attraction Force: Ion-dipole ForcesIon-dipole Forces

Exist between an ion and the particle Exist between an ion and the particle charge on the end of a polar moleculecharge on the end of a polar molecule

Polar molecules = dipoles (has both Polar molecules = dipoles (has both poles + and -)poles + and -)

+ ion are attractive to – end of dipole…+ ion are attractive to – end of dipole…and vice versaand vice versa

Magnitude of attraction increases as the Magnitude of attraction increases as the magnitude or ion charge of the dipole magnitude or ion charge of the dipole moment increasesmoment increases

Page 5: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

Types of Types of Intermolecular ForcesIntermolecular Forces Dipole-dipole forcesDipole-dipole forces Ion – Dipole forcesIon – Dipole forces London Dispersion forcesLondon Dispersion forces Hydrogen-bonding forcesHydrogen-bonding forces

Page 6: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

Dipole-dipole forcesDipole-dipole forces Exist between neutral polar Exist between neutral polar

moleculesmolecules Weaker than ion-dipole forcesWeaker than ion-dipole forces Molecules with equal mass or size, Molecules with equal mass or size,

strengths of intermolecular strengths of intermolecular attraction increases with increasing attraction increases with increasing polaritypolarity

Page 7: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces
Page 8: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

A Few Queries…A Few Queries…

What about non-polar molecules?What about non-polar molecules? So how does a non-polar gas liquefy?So how does a non-polar gas liquefy? Because you need some sort of Because you need some sort of

attraction between particlesattraction between particles

Page 9: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

……well…well…

1930 – Fritz London proposed origin 1930 – Fritz London proposed origin of this type of attractionof this type of attraction

Recognized that motion of electrons Recognized that motion of electrons in atom/molecule can create an in atom/molecule can create an instantaneousinstantaneous dipole moment dipole moment

Page 10: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

ExampleExample Helium is spherically symmetricalHelium is spherically symmetrical Molecules -> non-polar; and possess no Molecules -> non-polar; and possess no

permanent dipole momentpermanent dipole moment However, instantaneous distribution of However, instantaneous distribution of

electrons is diff. from avg. distributionelectrons is diff. from avg. distribution If we freeze the electrons in helium If we freeze the electrons in helium

atom, both electrons atom, both electrons couldcould be on same be on same side of nucleus; giving it instantaneous side of nucleus; giving it instantaneous dipole momentdipole moment

Page 11: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

ExampleExample

Page 12: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

London Dispersion London Dispersion ForcesForces

Electrons repel one another, motion Electrons repel one another, motion of electrons on one atom influence of electrons on one atom influence the neighboring atoms.the neighboring atoms.

So temporary dipole moment on one So temporary dipole moment on one atom can induces a similar dipole on atom can induces a similar dipole on a adjacent atoma adjacent atom

This is significant only when This is significant only when molecules are molecules are closeclose together. together.

Page 13: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

Polarizability Polarizability Ease with which the charge Ease with which the charge

distribution can be distorted by an distribution can be distorted by an external electric fieldexternal electric field Think of it as “squashiness” of an Think of it as “squashiness” of an

electron cloudelectron cloud The greater the polarizability of The greater the polarizability of

molecule, the more easily its molecule, the more easily its electron cloud can be distorted to a electron cloud can be distorted to a momentary dipolemomentary dipole

Page 14: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

So…So… More polarizable molecules have More polarizable molecules have

stronger London dispersion forcesstronger London dispersion forces larger molecules have greater larger molecules have greater

polarizability- more electrons farther polarizability- more electrons farther from the nucleifrom the nuclei

So London dispersion forces So London dispersion forces increase with increasing size and increase with increasing size and molecular weightmolecular weight

Page 15: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

Other factors that Other factors that determine strength:determine strength:

ShapeShape Ex. N-pentane and neopentaneEx. N-pentane and neopentane

N-pentane attraction is higher because more N-pentane attraction is higher because more molecules come in contact- because it is more molecules come in contact- because it is more cylindrically shapedcylindrically shaped

Page 16: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

Summary of: Comparing Summary of: Comparing Strengths of Intermolecular Strengths of Intermolecular

ForcesForces Molecules that have comparable Molecules that have comparable

eights and shapes have dispersion eights and shapes have dispersion forces that are approx. equalforces that are approx. equal

When molecules differ greatly in When molecules differ greatly in weights, dispersion forces are weights, dispersion forces are accordingly differentaccordingly different

Page 17: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

Hydrogen BondingHydrogen Bonding Hydrogen bonding is a special Hydrogen bonding is a special

type of intermolecular type of intermolecular attraction that exists between attraction that exists between the hydrogen atom in a polar the hydrogen atom in a polar bond and an unshared electron bond and an unshared electron pair on a nearby small pair on a nearby small electronegative ion or atomelectronegative ion or atom

Tend to be the strongest type Tend to be the strongest type of intermolecular forcesof intermolecular forces

Page 18: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces
Page 19: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

More About Hydrogen More About Hydrogen BondingBonding

Can be considered unique dipole-Can be considered unique dipole-dipole attractionsdipole attractions

F, O, and N are so electronegative, F, O, and N are so electronegative, that a bond between them and that a bond between them and hydrogen is quite polar, with hydrogen is quite polar, with hydrogen on positive endhydrogen on positive end

Page 20: INTERMOLECULAR FORCES. A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces

The EndThe End http://chemed.chem.purdue.edu/genhttp://chemed.chem.purdue.edu/gen

chem/topicreview/bp/intermol/interchem/topicreview/bp/intermol/intermol.htmlmol.html

http://www.chem.unsw.edu.au/UGNohttp://www.chem.unsw.edu.au/UGNotes/hainesIMF/contents.htmltes/hainesIMF/contents.html

http://scidiv.bcc.ctc.edu/wv/08/0008-http://scidiv.bcc.ctc.edu/wv/08/0008-0012-interforce.html0012-interforce.html