Page 140 - Bruno Linder Elementary Physical Chemistry

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

Elements of Intermolecular Forces 125

Variation of the boiling point of hydrogen-bonded molecules with
Fig. 12.3
molecular weight.

for H 2 Oand C 3 H 8 O 3 , the London force is dominant and increases
going down the list. Therefore the boiling point should increase,
and it does.
3) Surface tension should increase with increasing weight, and it does.
4) Viscosity should increase with increasing molecular weight and it
does. (It should be mentioned that there are other factors involved,
such as entanglements, but these are not discussed here.)

Figure 12.3 depicts the boiling point variation of several hydrogen-
containing compounds with molecular weight. Note that the boiling point
increases with increasing weight (larger polarizabilities, stronger London
forces) except for water. In water, there are in addition to the London forces
dipole–dipole forces and hydrogen bonding. Water has by far the strongest
intermolecular forces and should have a much higher boiling point. The
diagram clearly shows this.

12.5. Classification of Solids by Types
of Intermolecular Forces

All solids consist of structural units (atoms, molecules, ions) which are
strongly attracted to one another to give a rigid substance. In some
cases, the forces binding the structural units are chemical bonds (they
can be metallic, ionic, or covalent bonds). These bonds hold atoms
together. In other cases, the structural units are molecules held together
by intermolecular forces.