1. INTERMOLECULAR FORCES OF ATTRACTION
Attraction existing between molecules in a given substance
Responsible for the phase of the substance at a given temperature; for example, strong IMFA – solid at room temperature; weak IMFA – gas at room temperature

2. COMPARISON OF MOLECULAR MASS, BOILING POINT
SUBSTANCE
MOLECULAR WEIGHT (amu)
BOILING POINT (K)
PROPANE CH3CH2CH3 44
231
DIMETHYL ETHER CH3OCH3 46
248
METHYL CHLORIDE CH3Cl 50
249
ACETALDEHYDE CH3CHO
294
ACETONITRILE CH3CN 41
355

3. Relative Magnitudes of Forces
The types of bonding forces vary in their strength as measured by average bond energy.
Strongest
Weakest
Covalent bonds (400 kcal)
Hydrogen bonding (12-16 kcal )
Dipole-dipole interactions (2-0.5 kcal)
London forces (less than 1 kcal)

4. TYPES OF IMFA (weakest to strongest)
1. VAN DER WAAL’S FORCES
a. London dispersion force
– caused by the movement of the electrons around the nucleus
- effect is very temporary
- present in all substances and only force present in non-polar molecules

5. LONDON DISPERSION FORCE

6. Induced dipole – temporary charges due to presence of electrons from neighboring atoms

7. BOILING POINTS OF HALOGENS
substance
Boiling point ( oC) F2
-188
Cl2
-35
Br2 60 I2
185
The higher the number of electrons, the stronger is the London dispersion force.

8. SHAPE OF THE MOLECULE AFFECTS BOILING POINT

9. b. dipole-dipole interaction
– present only in polar covalent molecules
- caused by the dipole moment (differences in the electronegativy of the elements present)

10. Polarity
A molecule, such as HF, that has a center of positive charge and a center of negative charge is said to be polar, or to have a dipole moment. H F + -

12. Ethanol is Polar

13. c. Hydrogen bonding
- present in polar covalent compounds where hydrogen is directly attached to Fluorine, Oxygen and Nitrogen ( the three most electronegative elements)
F O N

14. Hydrogen bonding only occurs if H is attached to F, O and N.

15. Hydrogen Bonding
Bonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen
Base pairing in DNA by hydrogen bonding

16. Hydrogen bonding in water

17. H bonding in ice

18. 2. IONIC BONDING –
present only in ionic compounds.
Electrostatic force of attraction.

19. An ionic compound dissolved in water.

20. FORMULA TO DETERMINE THE NUMBER OF RINGS AND/OR DOUBLE AND TRIPLE BONDS
2 + 2C + N – H – X 2
ρ =
where
C = number of carbon atoms
N = number of nitrogen atoms
H = number of hydrogen atoms
X = number of halogen (Group 17) atoms