1. 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?
The answers have to do with …
Intermolecular forces

2. Intermolecular forces
12/10/99
Intermolecular forces
There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces
Intramolecular bonds are BONDS - ionic, polar covalent or non-polar covalent
Intermolecular forces (IMF) have to do with the attraction between molecules (vs. the attraction between atoms in a molecule)

3. Intermolecular vs Intramolecular
16 kJ to vaporize 1 mole of HCl (inter)
431 kJ to break H-Cl bonds in 1 mole of HCl (intra)

4. Electronegativity & IMFs
12/10/99
Electronegativity & IMFs
EN essentially defines the type of Bonding and IMF.
Ionic bonds form if the EN is 1.7 or greater and contains a metal or positive ion.
There are 3 Van Der Waals or Intermolecular Forces (IMF). They depend on the EN and symmetry of the molecule.

5. Ionic Bonding Ionic bonds form as previously described.
12/10/99
Ionic Bonding
Ionic bonds form as previously described.
The force is an intraparticle 3D force.
They have very high melting and boiling points and high solubility.
eg. mp(LiCl) is
801°C and
solubility is
84 g/100 g – + – +

6. lon – Dipole Attractions
Ionic compounds are generally water soluble due to the formation of attractions between the dipole of the water molecules and the charges of the ions.

7. London Dispersion Forces
exist in all molecules, (and noble gases), and in non-polar covalent molecules (where EN is 0) or they are symmetrical.
- weakest of the 3 IMFs.
strength depends on the # of electrons and how close the molecular packing is.
lowest melting, boiling points and water solubility. eg. mp (Cl2 is -161°C and solubility is 0.7g/100 g)

8. Dipole - Dipole attractions
12/10/99
Molecules are non polar if the EN of all bonds is 0 (compared between each set of bonded atoms in the molecule) or if it is symmetrical in shape
Much stronger than London Dispersion Forces.
Higher melting and boiling points and more soluble than non-polar molecules. H Cl + –
eg. mp(HCl) is -114°C and
solubility is
66 g/100 mL + – + – + – + –

9. Hydrogen Bonding Hydrogen bonding is a type of dipole-dipole.
- The H is covalently bonded to N, O or F in the compound.
- this forms a H Bonding IMF with ANOTHER molecule.
strongest IMF, about 5 times stronger than D-D
melting, boiling and water solubility is higher than to D-D.
eg. CH3OH mp is -97°C and solubility is complete.