Chem 16

Intermolecular Forces

Non-polar vs Polar

Test yourself !

Arrange the following bonds in increasing order of polarity :

H-H, O-H, Cl-H, S-H, F-H Identify the type of compound : BaCl2, O2, NH3, CsF, CH4

Interparticle Forces of Attraction

London Dispersion or induced dipole Non-polar molecules

Dipole-dipole Polar molecules

H-bonding H attached to F, N, O,

Ionic bonding Cations and anions or electrostatic

Induced Dipole

Distortion of electron cloud Bigger molecules are more polarizable

London Forces

Dipole-Induced Dipole Interaction

Test Yourself

Which one has the highest boiling point? Explain your choice. CH4, SiH4, GeH4

Boiling Points: Hydrides

Dipole-Dipole Interaction

Test Yourself

Which one has the highest boiling point? Explain your choice. C3H8, CO2, CH4 CN

Test Yourself Which one has the highest boiling point? CH4, SiH4, GeH4 Which one has the highest boiling point? OH2 , SH2 , SeH2 , TeH2

Boiling Points: Hydrides

Ionic Interaction

Between Cations and Anions Example: NaCl

http://www.cem.msu.edu/~reusch/VirtualText/intro2.htm

Hydrogen Bonding

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Intermolecular Forces

Intermolecular forces are interactions that exist between molecules.

There are several types of intermolecular interactions.

Ionic compounds contain oppositely charged particles

held together by extremely

strong electrostatic inter-

actions. These ionic inter-

actions are much stronger

than the intermolecular forces

present between covalent

molecules.

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Covalent compounds are composed of discrete molecules.

The nature of the forces between molecules depends on the functional

group present. There are three

different types of interactions, shown

below in order of increasing strength:

van der Waals forces

dipole-dipole interactions

hydrogen bonding

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van der Waals Forces van der Waals forces are also known as London forces.

They are weak interactions caused by momentary changes in electron density in a molecule.

They are the only attractive forces present in nonpolar compounds.

Even though CH4 has no

net dipole, at any one

instant its electron density

may not be completely

symmetrical, resulting in a

temporary dipole. This can

induce a temporary dipole

in another molecule. The

weak interaction of these

temporary dipoles

constitutes van der Waals

forces.

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All compounds exhibit van der Waals forces.

The surface area of a molecule determines the strength of the van der Waals interactions between molecules. The larger the

surface area, the larger the attractive force between two

molecules, and the stronger the intermolecular forces.

Figure 3.1 Surface area and

van der Waals forces

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Dipole-Dipole Interactions

Dipoledipole interactions are the attractive forces between the permanent dipoles of two polar molecules.

Consider acetone (below). The dipoles in adjacent molecules align so that the partial positive and partial negative charges

are in close proximity. These attractive forces caused by

permanent dipoles are much stronger than weak van der Waals

forces.

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Hydrogen bonding typically occurs when a hydrogen atom bonded to O,

N, or F, is electrostatically attracted

to a lone pair of electrons on an O, N,

or F atom in another molecule.

Hydrogen Bonding

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Note: as the polarity of an organic

molecule increases, so does the

strength of its intermolecular forces.

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Physical PropertiesBoiling Point The boiling point of a compound is the temperature at which

liquid molecules are converted into gas.

In boiling, energy is needed to overcome the attractive forces in the more ordered liquid state.

The stronger the intermolecular forces, the higher the boiling point.

For compounds with approximately the same molecular weight:

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Consider the example below. Note that

the relative strength of the

intermolecular forces increases from

pentane to butanal to 1-butanol. The

boiling points of these compounds

increase in the same order.

For two compounds with similar

functional groups:

The larger the surface area, the higher the boiling point.

The more polarizable the atoms, the

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Consider the examples below which

illustrate the effect of size and

polarizability on boiling points. Figure 3.2

Effect of surface area and

polarizability on boiling point

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Which has the higher boiling point

and why?

O

A B

A has only VDW, while B

has both VDW and DD

interactions

O

B

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AB

O

OHH3CH2C

O

OCH3H3CH2C

A

O

OHH3CH2C

A had VDW, DDD and H-

bonding, while B lacks H-

bonding

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H3C

(CH2)5

H3C

H3C

(CH2)20

H3C

A B

H3C

(CH2)20

H3C

B

Both A and B only have VDW

interactions, but B has the

higher bp b/c of a larger

surface area.

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The melting point is the temperature at which a solid is converted to its liquid phase.

In melting, energy is needed to overcome the attractive forces in the more ordered crystalline

solid.

The stronger the intermolecular forces, the higher the melting point.

Given the same functional group, the more symmetrical the compound, the higher the

melting point.

Melting Point

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Because ionic compounds are held together by extremely strong interactions, they have very

high melting points.

With covalent molecules, the melting point depends upon the identity of the functional

group. For compounds of approximately the

same molecular weight:

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The trend in melting points of pentane, butanal, and 1-butanol

parallels the trend observed in their

boiling points.

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Symmetry also plays a role in determining the melting points of compounds having the same functional group and similar

molecular weights, but very different shapes.

A compact symmetrical molecule like neopentane packs well into a crystalline lattice whereas isopentane, which has a CH3 group

dangling from a four-carbon chain, does not. Thus, neopentane

has a much higher melting point.

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Which has the higher melting

point and why? NH2

BA

B has stronger intermolecular

forces (DD and HBZ).

NH2

B

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A B

A

Both only have VDW forces, so

A has the higher mp b/c it is

more symmetrical. Closer

packing means higher mp.