Chem 2 - Analyzing Strengths of Intermolecular Forces III

1. Analyzing Strengths of
Intermolecular Forces
(Lecture Pt. 3)
By Dr. Shawn P. Shields
This work is licensed by Shawn P. Shields-Maxwell, Ph. D., under a Creative Commons Attribution-NonCommercial-
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2. Factors Affecting Strengths of IM Forces
Strengths of intermolecular forces
depend on many factors.
When comparing sets of molecules, one
needs to analyze the type and relative
strengths of IM forces present.
So how is this done?

3. Types of Intermolecular Forces and Relative Strengths
Ion-Dipole
Hydrogen bonding
Dipole-dipole
Dipole-induced dipole
Dispersion forces (or London
forces)
weakest
strongest

4. Factors Affecting Strengths of IM Forces
All matter has electrons, so (London)
dispersion forces are present in all
molecules (ions, etc.).
We also need to determine whether other
(stronger) forces are present, as well.

5. Determining Relative Strengths of
IM Forces
Is the molecule polar or nonpolar?
(Does the molecule have a permanent dipole?)

6. Determining Relative Strengths of
IM Forces
Is the molecule polar or nonpolar?
(Does the molecule have a permanent dipole?)

7. Identifying Hydrogen Bonding
Is H-bonding present? (Is there an H
directly bonded to F, N, or O present?)

8. Identifying Hydrogen Bonding
Is H-bonding present? (Is there an H
directly bonded to F, N, or O present?)
(Don’t be fooled by H bonded to C with a
highly electronegative atom “somewhere”.)

9. Relative Strengths of Dispersion Forces
Compare the presence and number of
heavy atoms in the molecules.

10. Relative Strengths of Dispersion Forces
Neither molecule has a permanent
molecular dipole.
Heavier atoms contribute to stronger
dispersion forces.

11. Effect of Molecular Shape
Extended versus compact structures

12. Effect of Molecular Shape
Extended versus compact structures
Extended molecules have more “opportunities”
for interaction.

13. Effect of Molecular Shape
Stronger cumulative interactions for extended
(spaghetti-like) structures

14. IM Forces and Physical Properties
Boiling point increases with increasing
strength of IM forces.
Vapor pressure decreases with increasing
strength of IM forces. (It is harder for
molecules to escape to the gas phase.)
Melting point increases with increasing
strength of IM forces.

15. Physical Properties and IM Forces
Which has the higher boiling point?
Is H-bonding present?

16. Physical Properties and IM Forces
Which is predicted to have the higher
melting temperature?

17. Physical Properties and IM Forces
Which is predicted to have the higher
melting temperature?

18. Physical Properties and IM Forces
Which is predicted to have the lower vapor
pressure?

19. Physical Properties and IM Forces
Which is predicted to have the lower vapor
pressure?

20. Example Problems
will be posted separately! 
(Remember, you must be able to determine if the
molecule has a permanent molecular dipole in
order to analyze IM forces. Please see my review
molecular dipole examples if you need help.)