INTERMOLECULINTERMOLECULAR FORCESAR 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
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
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
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
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
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
……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
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
ExampleExample
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.
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
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
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
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
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
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
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