2. Dipole-dipole: IMF’s that occur between a neutral polar molecules (same or not the same molecules).

1. Ion-dipole: IMF’s that occur between neighboring an ion solution and a polar molecule (dipole) also in solution.

Na+

Intermolecular forces are classified into four major types.

4. London Dispersion Forces are attractive IMF’s that occur when spontaneous dipoles are formed randomly or induced by other charged species in neutral polarizable molecules.

3. Induced-dipoles: IMF’s that occur when a ion or a dipole induces a spontaneous dipole in a neutral polarizable molecule. +- +-

Ion or Dipole Induced Dipole

Energy of Attraction is:

Coulomb’s Law: The electric force acting on a point charge q1 as a result of the presence of a second point charge q2 some r meters away is given by:

Intermolecular forces arise from electrostatic forces that depend on distance between molecules.

F =k q1 q2

r2

E =k q1 q2

r

Because Energy = Force x Distance

+ -

k depends on medium (kair = 9.0 x 109 N•m2/C2)

units of coulombs

units of metersr

+-

If we can predict charge separation or polarity in molecules we can identify IMF’s in play for that molecule.

Covalent Bonding Ionic BondingPolar Covalent Bond

δ+ δ-

Differences in electronegativity between bonding atoms blurs the distinction between covalent, polar covalent and ionic bonding types.

δ symbols indicates partial charge on atoms.

F2 HF LiF

Dipole Moment

χ = |ENB - ENA|

0.0 3.02.0

Electronegativity is an element’s inherent property to draw electrons to itself when chemically bonded to another atom in a molecule.

F is the most electronegative element

Cs the least

3.0

2.0

0.0

χElectronegativity values from a table can be used to judge the extent of ionic, polar or covalent bond character in a chemical bond.

Mostly Ionic

Polar Covalent

Mostly Covalent

absolute value of electronegativity difference between2-bond atoms

χ = |ENB - ENA|

Polar-covalent bonds combined with a molecule’s geometry produce net dipole moments (dipoles) in molecules.

Net Dipole MomentNo Net Dipole Moment

Polar BondPolar Bond

Polar Bond Polar Bond

Think of the dipole moment as a molecule with separated charges + and - that can influence other charged molecules. This is the origin of IMF’s!

+- +-DipoleDipole

Recall your molecular and electron geometry from Chem 7.

2 EG

3 EG 4 EG

5 EG 6 EG

We identify polar bonds and use the molecular geometry to gauge if there is a net dipole moment (dipole) in a molecule.

Polar with net dipolemoment

δ+δ-

δ- δ-

δ-

δ-

δ-

δ-

δ+δ+δ+

non-polar with no net dipole moment

non-polar with no net dipolemoment

Polar with net dipolemoment

Polar with net dipolemoment

We need to connect some learning dots: electronegativity differences => polar molecules => dipole moments => gives rise to IMF => altering vapor pressure => boiling points, freezing points, viscosity, surface tension and more.

Summary of Intermolecular Types of Forces and Energies

Ion-dipole

H-Bond

Dipole-Dipole

Ion-induceddipole

Dipole-induceddipole

Dispersion(London)

+-+

Summary of Intermolecular Forces (van der waals forces)

--an ion interacts with a polar molecule (dipole).

--two polar molecules (dipoles) interact electrostatically.

--ion or a dipole induces a dipole in a polarizable non-polar molecule.

--occurs between non-polar molecules via polarizability--also called London Dispersion forces

Highest

Lowest

Type of Interaction

+-+Ion Dipole

Strength Energy (kJ/mol)

50-600

5-25

2-15

0.05-40

Interaction

DipoleDipole

+-+-

Induced Dipole

+-+Ion

Induced Dipole

+-Induced Dipole

+-

Dipole

+-

Ion-dipole intermolecular forces are attractive forces between an ion in solution and a neighboring polar molecule.

•The greater the dipole moment or ion charge the stronger the IMF. •The closer the ion is to the dipole the stronger the interaction:•Coloumb’s Law F= q1 q2/r2

An anionin solution

Molecule with a dipole moment

A cationin solution

- end of dipole attracted to + ions

+ end of dipole attracted to -ions

Dipole-Dipole intermolecular forces are attractive forces that occur between polar molecules (same or not the same).

Polar molecules give rise to dipole-dipole IMF’s

Boiling point differences are explained by dipole-dipole interactions

• More polar molecule => stronger dipole-dipole IMF’s => higher boiling points => lower vapor pressure.

Dipole-induced dipole are attractive intermolecular forces that occur when a polar molecule (dipole) induces a dipole by distorting an electron cloud in a polarizable molecule.

polar molecule induced dipolenon-polar molecule

As dipole moments increase in polar substances of the same mass, IMF’s increase as do boiling points (lower vapor pressure) and melting points.

Non-polar

Very polar

Polar

Hydrogen bonding is a special case of dipole-dipole intermolecular force that occurs between a hydrogen atom and an unshared pair of electrons in a polar N-H, O-H, or F-H bond.

Double H-bonds in acetic acidCH3COOH

Boiling points of two isomers: EtOH and DME

78.3°C -24.8°C

Water is a highly polar molecule due to oxygen inherent ability to attract electrons more so than hydrogen.

There is more electron density near the oxygen atom and less around the hydrogen (arrows).

This polarity property gives water its dissolving properties of other polar substances.

The boiling points of covalent binary hydrides increase with increasing molecular mass down a Group but the hydrides of NH3, H2O and HF have abnormally high BP because of hydrogen bonding.

?Increased London Forces

London Dispersion Forces are attractive intermolecular forces that occur when temporary dipoles are formed due to random electron motions in all polarizable molecules.

2-neutral non-polar polarizable molecules

Induced-Dipole Momentand IMF between molecules

spontaneous movement of electrons forming an instantaneousdipole moment

ion-induced dipole interaction

dipole-induced dipole interaction

Polarizability is the ease with which an electron distribution (cloud) in the atom or molecule can be distorted by an outside ion or dipole.

non-polar molecule (electron cloud)

induced dipoledispersion

London Dispersion

Polarizability and London dispersion trends in the mirrors atomic size trends in the periodic table.

POLARIZABILITY

•Polarizability increases down a group

•Polarizability increases right to left across a period (bigger size more distortable.

•Cations are less polarizable than their parent atom because they are smaller and more compact.•Anions are more polarizable than their parent atom because they are larger.

•Polarizability increases with molar mass (# e-) of a molecule

Examples of increasing London dispersion forces with increase in size and polarizability.

Increasing Size/Polarizability

Increasing Size/Polarizability

Larger unbranched molecules are more polarizable than more compact branched molecules. More polarizable = higher London forces, and explains trends in boiling and melting points in non-polar molecules.

neopentane bp=10 oC tetrahedral

normal pentanebp=36 oC

extended structure

Alkanes: Molecular Weight vs Boiling Point

Alkane (CnH2n+2) Melting Point ˚C Boiling Point (˚C)

CH4 Methane -182 -164

CH3CH3 Ethane -183 -89

CH3CH2CH3 Propane -189 -42

C4H10 Butane -138 -0.5

C5H12 Pentane -130 36

C6H14 Hexane -95 69

C7H16 Heptane -91 98

C8H18 Octane -57 125

C9H20 Nonane -51 151

C10H22 Decane -30 174

1. Ion-Ion > Ion-Dipole > Dipole-Dipole > Dispersion

4. For non-polar molecules of the same molecular mass, longer less compact molecules are generally more polarizable and have greater dispersion forces and show higher boiling and melting points.

2. For polar molecules of approximately the same mass and shape and volume (i.e. polarizability the same), dipole-dipole forces dictate the difference in physical properties.

3. Hydrogen bonding occurs with polar bonds in particular H-F, H-O, H-N and an unshared pair of electrons on a nearby electrognegative atom usually F, O, or N.

5. For non-polar molecules of widely varying molecular mass, those with more mass are typically more polarizable and experience greater London dispersion forces and exhibit higher boiling points and melting points.

Some Generalizations About IMF’s

Comparing the strengths of Intermolecular Forces

Force Strength Energy-Distance Interactions

London Forces 1-10 kJ/mole 1/r6 All-molecules

Dipole-Dipole 3-40 kJ/mole 1/r3 Polar

molecules

Hydrogen Bonding 10-40 kJ/mole 1/r3 O-H

N-H H-F

Ion-Dipole 10-50 kJ/mole 1/r2 Ions-Polar molecules

Ion-Ion >>200 kJ/mole 1/r Cation-Anion

Flowchart for classifying intermolecular forces

In general, the bulk physical properties of liquids correlate the intermolecular forces between molecules. and their correlation to IMF’s

Three common physical properties of liquids that depend on the magnitude of IMF’s include surface tension, capillarity and viscosity.

surface tension

capillarity

The surface tension forms “a skin” like surface that can support mass.

Viscosity is a measure of resistance to flow of a fluid.

viscosity

We say that the leave has a “low surface energy” as water will not spread out (it takes energy to spread the water out)

molecules at the surface feel

a net force downward

molecules in the interior experience

equal force in 3-D

Surface tension is the amount of energy required to stretch or increase the surface area of a liquid. Units of energy per unit length (J/m).

Substance Formula Surface Tension (J/m2) at 200C

Major IMF’s

diethyl ether

ethanol

butanol

water

mercury

dipole-dipole; dispersion

H bonding

H bonding; dispersion

H bonding

metallic bonding

1.7x10-2

2.3x10-2

2.5x10-2

7.3x10-2

48x10-2

CH3CH2OCH2CH3

CH3CH2OH

CH3CH2CH2CH2OH

H2O

Hg

Surface Tension of Some Liquids

The surface tension of a liquid is a function of temperature.

Why might this be? (what forces compete here?)

Hot water works better than cold water in cleaning your clothes or hands as it can more effectively “wet” dirt (water gets into pores and not get “stuck”).

Capillary effect occurs when the adhesive IMF’s between a liquid and a substance are stronger than the cohesive IMF’s inside the liquid.

Cohesion is the intermolecular attraction between like molecules.

Adhesion is an attraction between unlike molecules

AdhesionCohesion

– Capillary rise implies that the:• Adhesive forces > cohesive forces

– Capillary fall implies that the:• Cohesive forces > adhesive forces

Water will rise to different heights in tubes depending on the diameter of the capillary. Can you surmise why and how?

Water will rise to different heights depending on the diameter of the capillary. Why?

In general, viscosity decreases as temperature increases and visa versa.

• Viscosity is the measure of a liquid’s resistance to flow relative to one another, and is thus related to the intermolecular forces.

Oil for your car is bought based on this property: 10W30 or 5W30 describes the viscosity of the oil at high and low temperatures (W means winter dudes higher the number the more viscous).