Page 138 - Bruno Linder Elementary Physical Chemistry

August 19, 2010 10:40 9in x 6in b985-ch12 Elementary Physical Chemistry

Elements of Intermolecular Forces 123

Summary for neutral (non-ionic) molecules:

(1) London forces are always present, even in nonpolar molecules. If mole-
cules are polar, orientation and induction forces are also preset in gases.
The van der Waals forces are the weakest of the intermolecular forces.
(2) Dipole–dipole forces are present between polar molecules in liquids and
in solids (where rotation is restricted or hindered). These forces are the
next strongest of the intermolecular forces. Hydrogen bonding exists
only between molecules containing F–H, O–H and N–H bonds. These
are the strongest of the intermolecular forces.

Example 12.1.
1. What kind of intermolecular forces are present in the following sub-
stances?
(a) CH 4 ,
(b) chloroform (CHCl 3 ),
(c) butanol (CH 3 CH 2 CH 2 CH 2 OH)

Solution
(a) CH 4 is nonpolar; hence, only London forces.
(b) CHCl 3 is unsymmetrical and has polar bonds. Therefore, in the liquid
there are dipole–dipole and London forces.
(c) Butanol has H attached to O; it has hydrogen bonding, dipole–dipole
and London forces.

12.3. Intermolecular Forces and Liquid Properties

Properties that are dependent on intermolecular forces are:
(a) Vapor Pressure. The ease or difficulty with which molecules leave
the liquid depends on the strength of attraction to other molecules,
thus depending on intermolecular forces. Weak intermolecular forces
give high vapor pressure. Strong intermolecular forces give low vapor
pressure.
(b) Boiling Points. This is the temperature at which the vapor pressure
equals the atmospheric pressure. Thus, the higher the vapor pressure,
the lower the boiling point is. In other words, the boiling point is highest
for liquids with strong intermolecular forces (IF), and lowest for liquids
with weak IF.