1. Intermolecular Forces

2. Intermolecular forces
12/10/99
QUESTIONS
Why do some solids dissolve in water but others do not?
Why are some substances gases at room temperature, but others are liquid or solid?
Why is water such a good solvent?
The answers have to do with …
Intermolecular forces

3. Intermolecular Forces
12/10/99
Intermolecular Forces
There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces
We have already looked at intramolecular bonds (ionic, polar, non-polar)
Intermolecular forces (IMF) have to do with the attraction between molecules (vs. the attraction between atoms in a molecule)

4. IONIC, DIPOLE - DIPOLE ATTRACTIONS
12/10/99 H Cl + –
Molecules are attracted to each other by the +ve and –ve forces created by the partial charges of a dipole.
This is called dipole-dipole attraction.
A dipole (and its partial charges) can also be attracted to the oppositely charged ion in an ionic compound.
This is called ion-dipole attraction. + –

5. H - BONDING
12/10/99
H-bonding is a special type of dipole - dipole attraction that is very strong
It occurs when F, O, or N are bonded to H
Q- Calculate the EN for HCl and H2O
A- HCl = = 0.8, H2O = = 1.4
The high EN of NH, OH, and HF bonds cause these to be strong forces (about 5x stronger than normal dipole-dipole forces)
They are given a special name (H-bonding) because compounds containing these bonds are important in biological systems

6. LONDON FORCES
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Non-polar molecules do not have dipoles like polar molecules. How, then, can non-polar compounds form solids or liquids?
London forces are named after Fritz London (also called van der Waal forces)
London forces are due to small dipoles that exist in non-polar molecules
Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical
The resulting tiny dipoles cause attractions between atoms/molecules

7. LONDON FORCES Induced dipole:
12/10/99
Instantaneous dipole:
Induced dipole:
Eventually electrons are situated so that tiny dipoles form
A dipole forms in one atom or molecule, inducing a dipole in the other

8. Do Intermolecular forces matter?
All molecules have intermolecular forces.
The strength of the attraction depends on the EN value of the dipoles for polar molecules.
For non-polar molecules the strength of the attraction depends on how many London forces there are and the size of the atom.
Intermolecular forces are responsible for the unique properties of some substances, boiling point, melting point, and are particularly important in biological function.