1. 8-3 Intermolecular forces
Intermolecular forces: are the forces of
attractions that exist between molecules in
a compound. 10~100KJ/mol
These affect the melting and boiling points of
compounds as well as the solubilities of one substance
in another, and cause the compound to exist in a
certain state of matter: solid, liquid, or gas.
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2. 8-3.1 The Polarity of Molecules
covalent bond
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3. Polarity of molecule A non-polar molecule has no net dipole.
This can be achieved by the molecule having :
(1) only non-polar bonds O2, Cl2
(2) polar bonds arranged symmetrically
so that the dipoles cancel out CH4, O=C=O
A polar molecule has a net dipole (has a partial
positive charge at one site and a partial negative
charge at another site). NH3, H2O, HCl
This is achieved in the molecule by the molecule
being made up of polar bonds arranged
unsymmetrically so that the dipoles do not cancel out.
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4. For diatomic molecule: H-H, Cl-Cl;
Molecular polarity is dependent on bond
polarity and the molecular geometry
For diatomic molecule: H-H, Cl-Cl;
HCl, CO, HF
For multiatomic molecule:
symmetrically: O=C=O, CH4, BF3 nonpolar molecules
unsymmetrically: NH3 , H2O polar molecules
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5. Dipole moment (μ): which is the product of the charge (Q) and
the distance d between charges:
μ= Q × d
Dipole moments are usually expressed in debye units (D) (after Peter Debye, 1884-l966).
1 D = 3.33 × 10 –30 C·m
where C is coulomb and m is meter.
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7. possesses a dipole moment nonpolar molecule: a molecule that
does not possess a dipole moment
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8. Polarizability of molecule
The charge distribution in a molecule can
be distorted by an external electric field
is called its polarizability.
Figure:
Instantaneous and
induced dipoles
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9. 8-3.2 Van Der Waals Forces Induced dipole Forces Van Der Waals Forces
Dipole-Dipole Forces
Induced dipole Forces
Van Der Waals
Forces
ion- induced dipole interaction
dipole-induced dipole interaction
Dispersion Forces
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10. Dipole-dipole force Dipole-Dipole Forces orientation force
Dipole-dipole forces are forces that act between polar molecules, that is, between molecules that possess dipole moments.
Dipole-dipole force
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13. Induced dipole forces
Induced dipole: because the separation of positive and negative charges in the neutral atom (or a nonpolar molecule).
The attractive interaction between an ion and the induced dipole is called ion-induced dipole interaction.
The attractive interaction between a polar molecule and the induced dipole is called dipole-induced dipole interaction.
polar molecule-nonpolar molecule
polar molecule-polar molecule
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14. This forces exists between the ion and the
induced dipole, or between the permanent
dipole and the induced diople
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15. dipole-induced dipole interaction
ion-induced dipole
interaction
dipole-induced dipole
interaction
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16. Dispersion Forces A kind of attractive interaction caused
by instantaneous dipole exists among any
two molecules.
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19. Van der Waals Forces polar-polar √ √ √ molecule polar-nonpolar √ √
Orientation Dipole-induced Dispersion
polar-polar √ √ √
molecule
polar-nonpolar √ √
nonpolar-nonpolar √
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20. 8-3.3 The Hydrogen Bond Decreasing molar mass Decreasing boiling point
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21. The Hydrogen Bond H bond is formed when an H atom
bonded to one highly electronegative
atom (F, O, or N) is simultaneously
attracted to a highly electronegative
atom (F, O, or N) of a neighboring
molecule.
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22. This interaction is written A­H······B or A­H······A
The hydrogen bond is a special type of dipole-dipole interaction between the hydrogen atom in a polar bond such as OH or NH, and the electronegative atoms O, N, or F.
This interaction is written
A­H······B or A­H······A
A and B represent O, N, or F;
A­H is one molecule and B is a part of another molecule, and the dashed line represents the hydrogen bond.
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23. A­H······B or A­H······A
Large electronegativity
Small radius
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25. In solid HF, for example, the molecules
HF molecule
In solid HF, for example, the molecules
do not exist as individual units; instead,
they form long zig-zag chains:
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27. Intermolecular hydrogen bonds
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28. Intramolecular hydrogen bonds
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