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Intermolecular Forces

Molecular CompoundsThe simplest molecule is H2:

The pair of shared electrons constitutes acovalent bond.

Increased electron densitydraws nuclei together


12.2

Intermolecular forces are attractive forces between molecules.

Intramolecular forces hold atoms together in a molecule.

Intermolecular vs Intramolecular

• 41 kJ to vaporize 1 mole of water (inter)

• 930 kJ to break all O-H bonds in 1 mole of water (intra)

Generally,intermolecularforces are muchweaker thanintramolecularforces.

“Measure” of intermolecular force

boiling point

melting point

ΔHvap

ΔHfus

ΔHsub

A Molecular Comparison of Gases,Liquids and Solids

• Converting a gas into a liquid or solid requires themolecules to get closer to each other:– cool or compress.

• Converting a solid into a liquid or gas requires themolecules to move further apart:– heat or reduce pressure.

• The forces holding solids and liquids together arecalled intermolecular forces.


• The covalent bond holding a molecule together is anintramolecular force.

• The attraction between molecules is an intermolecularforce.

• Intermolecular forces are much weaker thanintramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/mol forHCl).

• When a substance melts or boils the intermolecularforces are broken (not the covalent bonds).

• When a substance condenses intermolecular forces areformed.

Intermolecular Forces inSolutions


Dipole-Dipole Forces• There is a mix of attractive and

repulsive dipole-dipole forces asthe molecules tumble.

• If two molecules have about thesame mass and size, then dipole-dipole forces increase withincreasing polarity.


London Dispersion Forces• Weakest of all intermolecular forces.

• It is possible for two adjacent neutral molecules toaffect each other.

• The nucleus of one molecule (or atom) attracts theelectrons of the adjacent molecule (or atom).

• For an instant, the electron clouds becomedistorted.

• In that instant a dipole is formed (called aninstantaneous dipole).


London Dispersion Forces


London Dispersion Forces• One instantaneous dipole can induce another

instantaneous dipole in an adjacent molecule (oratom).

• The forces between instantaneous dipoles arecalled London dispersion forces.

• Polarizability is the ease with which an electroncloud can be deformed.

• The larger the molecule (the greater the number ofelectrons) the more polarizable.


• Ion-Induced Dipole:– An ion induces a dipole moment in an

adjacent molecule or atom.

• Interaction between an ion (e.g. Na+) anda dipole (e.g. water).

Intermolecular ForcesPolarizability & Periodic Table

• Polarizability increases down a group of atoms orions because size increases & larger electronclouds are more easily distorted

• Polarizability decreases from left to right across aperiod because the effective nuclear charge holdsthe electrons more tightly

• Cations are less polarizable than parent atombecause they are smaller, whereas anions aremore polarizable because they are larger


London Dispersion Forces• London dispersion forces increase as molecular weight

increases.

• London dispersion forces exist between all molecules.

• London dispersion forces depend on the shape of themolecule.

• The greater the surface area available for contact, thegreater the dispersion forces.

• London dispersion forces between spherical moleculesare lower than between sausage-like molecules.

Polar

Forces

Electronegativity• Polarity refers to a separation of positive and

negative charge. In a nonpolar bond, thebonding electrons are shared equally:

HCl:

• In a polar bond, electrons are sharedunequally because of the difference in Zeff.

H2,Cl2:

Chlorine - Cl2

Adding Dipole Moments

We know aCOVALENT bondcomes from sharing thebonding pair of electrons

F FShared pair

(bonding pair)

F FThe nucleus of each atompulls on the bonding pair.

F FBoth atoms have equal pull,

so the bonding pair is shared equally.

H ClIf two different atoms share a bond,

one will pull more stronglyon the bonding electrons.

H ClThe bonding electrons carry negative charge.

H ClThe closer they get to the chlorine atom,

the more negative it gets.The farther they get from the hydrogen,

the more positive it gets.

H Cl+

_

But the charge is only partial.Hydrogen has not lost the electronsas in the formation of an ion.

H ClThere is an unequal sharing of electrons.

H ClThe partial charge is denoted by a + or – and the Greek letter delta, δ

δ –δ +

The partial charge is denoted by a + or – and the Greek letter delta, δ

δ –δ +

δ –δ +

A polar bond is a bond in which thebonding electron pair is shared unequally.

A polar molecule is a moleculewith regions of partial negative (δ –)

and partial positive (δ +) charge.

The degree of sharing (equal to unequal)is determined by the electronegativitydifference between the two atoms.

Two atoms of equal electronegativitywill share the bond equally

F F

H ClTwo atoms with a small difference inelectronegativity will share unequally,

resulting in partial charge.

3.02.1

H ClTwo atoms with a small difference inelectronegativity will share unequally,

resulting in partial charge.

δ –δ +

H ClThis is a polar bond:

The bonding pair is, on average,closer to one atom.

δ –δ +

H Clδ –δ +

Is a polar bond a covalent bond?

FKTwo atoms with a large difference inelectronegativity will result in a loss

of an electron,resulting in a full charge.

4.00.8

+ _

FKpositive ion negative ion

A polar bond in a moleculemay make the entire moleculepolar, with one end slightlypositive and the other endslightly negative.

Hydrogen chloride is an exampleof a polar molecule.

H Clδ –δ +

The SHAPE of the moleculedetermines whether its polar

bonds make the molecule polar


Dipole-Dipole Forces

Attractive forces between polar molecules

Orientation of Polar Molecules in a Solid

In CF4 the fluorines are symmetricallyarranged around the carbon.

The fluorines all pull on the valenceelectrons in opposite directions,

effectively cancelling out the polarityof the bonds

CF4 has four polar bondsbut it is a non-polar molecule:There is no partial charge

on the molecule.

The two hydrogens of water arenot symmetrically positioned

around the oxygen.

The O-H polarities do not cancel,and the molecule carries a partial charge.

δ –δ +

δ +

Water is a liquid instead of a gasbecause the partial positives and negatives

attract each other.

δ +

δ –

δ + δ –

This attraction holds the molecules together,forming a liquid rather than thespread out molecules of a gas.

δ +

δ –

δ + δ –


Ion-Dipole Forces

Attractive forces between an ion and a polar molecule

Ion-Dipole Interaction

Intermolecular ForcesDispersion Forces

Attractive forces that arise as a result of temporarydipoles induced in atoms or molecules

ion-induced dipole interaction

dipole-induced dipole interaction

Dispersionforces amongnonpolarmolecules

separated Cl2molecules

instantaneousdipoles

Intermolecular ForcesDispersion Forces Continued

Polarizability is the ease with which the electron distributionin the atom or molecule can be distorted.

Polarizability increases with:

• greater number of electrons

• more diffuse electron cloud

Dispersionforces usuallyincrease withmolar mass.

Intermolecular ForcesHydrogen Bond

The hydrogen bond is a special dipole-dipole interactionbetween they hydrogen atom in a polar N-H, O-H, or F-H bondand an electronegative O, N, or F atom.

A H…B A H…Aor

A & B are N, O, or F

Intermolecular ForcesHydrogen Bonding• Special case of dipole-dipole forces.

• By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high.

• Intermolecular forces are abnormally strong.

• H-bonding requires H bonded to an electronegativeelement (most important for compounds of F, O, andN).– Electrons in the H-X (X = electronegative element) lie much

closer to X than H.

– H has only one electron, so in the H-X bond, the δ+ Hpresents an almost bare proton to the δ- X.

– Therefore, H-bonds are strong.

Intermolecular ForcesIntermolecular ForcesHydrogen BondingHydrogen Bonding

• Special case of dipole-dipole forces.

• By experiments: boiling points of compounds with H-F, H-O,and H-N bonds are abnormally high.

• Intermolecular forces are abnormally strong.

• H-bonding requires H bonded to an electronegative element(most important for compounds of F, O, and N).

– Electrons in the H-X (X = electronegative element) liemuch closer to X than H.

– H has only one electron, so in the H-X bond, the δ+ Hpresents an almost bare proton to the δ- X.

– Therefore, H-bonds are strong.

H-Bonding

Occurs when Hydrogen is attached to ahighly electronegative atom.

N-H… N- O-H… N- F-H… N-

N-H… O- O-H… O- F-H… O-

N-H… F- O-H… F- F-H… F-

δ+ δ-

Requires Unshared Electron Pairs of HighlyElectronegative Elements

Structure of IceObserve the orientation of the

Hydrogen Bonds

HB-ice

Why is the hydrogen bond considered a“special” dipole-dipole interaction?

Decreasing molar massDecreasing boiling point

SO

O

What type(s) of intermolecular forces exist betweeneach of the following molecules?

HBrHBr is a polar molecule: dipole-dipole forces. There arealso dispersion forces between HBr molecules.

CH4

CH4 is nonpolar: dispersion forces.

SO2SO2 is a polar molecule: dipole-dipole forces. There arealso dispersion forces between SO2 molecules.

SAMPLE PROBLEM Drawing Hydrogen Bonds Between Moleculesof a Substance

SOLUTION:

PROBLEM: Which of the following substances exhibits H bonding? Forthose that do, draw two molecules of the substance with the Hbonds between them.

C2H6(a) CH3OH(b) CH3C NH2

O

(c)

PLAN: Find molecules in which H is bonded to N, O or F. Draw Hbonds in the format -B: H-A-.

(a) C2H6 has no H bonding sites.

(c)(b)C O H

H

H

H

COH

H

H

H

CH3C N

O

H

H

CH3CN

O

H

H

CH3C

N

O

H

H

CH3C

N

O

H

H

Hydrogen BondsA hydrogen bond is an intermolecular force inwhich a hydrogen atom covalently bonded to anonmetal atom in one molecule is simultaneouslyattracted to a nonmetal atom of a neighboringmolecule

The strongest hydrogen bondsare formed if the nonmetalatoms are small and highlyelectronegative – e.g., N, O, F

Hydrogen Bonding EffectsSolid water is less dense than liquid water due tohydrogen bonding

Hydrogen bonding is also thereason for the unusually highboiling point of water

The structures of proteins,substances essential to life, aredetermined partly by hydrogenbonding Proteins

Hydrogen Bonding EffectsMany organic acids can form dimers due tohydrogen bonding

Certain organic molecules canalso form an intramolecularhydrogen bond

Interaction of Water and Oil• What do you know about oil

and water?

– “They don’t mix”

• Why?

– Because water is polarand oil is nonpolar

• Water molecules exert theirattractive forces on otherwater molecules

• Oil remains insoluble andfloats on the surface of thewater as it is less dense

Boiling Points of Liquidsand Melting Points of Solids

• Energy is used to overcome theintermolecular attractive forces in asubstance, driving the molecules into aless associated phase

• The greater the intermolecular force,the more energy is required leading to

– Higher melting point of a solid

– Higher boiling point of a liquid

Factors Influencing Boilingand Melting Points

• Strength of the attractive force holding thesubstance in its current physical state

• Molecular mass

• Larger molecules have higher m.p. and b.p. thansmaller molecules as it is more difficult toconvert a larger mass to another phase

• Polarity

• Polar molecules have higher m.p. and b.p. thannonpolar molecules of similar molecular massdue to their stronger attractive force

Melting and Boiling Points –Selected Compounds by Bonding Type

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