Intermolecular Forces of Attraction
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Transcript of Intermolecular Forces of Attraction
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Chem 16
Intermolecular Forces
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Non-polar vs Polar
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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
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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
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Induced Dipole
Distortion of electron cloud Bigger molecules are more polarizable
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London Forces
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Dipole-Induced Dipole Interaction
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Test Yourself
Which one has the highest boiling point? Explain your choice. CH4, SiH4, GeH4
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Boiling Points: Hydrides
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Dipole-Dipole Interaction
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Test Yourself
Which one has the highest boiling point? Explain your choice. C3H8, CO2, CH4 CN
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Test Yourself Which one has the highest boiling point? CH4, SiH4, GeH4 Which one has the highest boiling point? OH2 , SH2 , SeH2 , TeH2
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Boiling Points: Hydrides
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Ionic Interaction
Between Cations and Anions Example: NaCl
http://www.cem.msu.edu/~reusch/VirtualText/intro2.htm
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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.