Chemistry - A Molecular Science

Figure 7.10 Structure of ice The six-sided channels are responsible for the solid being less dense than the liquid. Water molecules are shown with space filling models (red spheres = O and blue spheres = H).

Table 7.1

Relative Strengths of Interactions

Interaction

Energy of interaction (kJ/mol)

Covalent Bond

100-1000

H-bond 10-50 Dipolar 5-25 Dispersion 1-50

Hydrogen bonding strongly affects the properties of water. When water freezes, the
molecules orient themselves so as to maximi

ze the hydrogen-bonding interactions. The

result is that each oxygen atom is surrounded

by four hydrogen atoms: two covalently

bound H atoms that make up the water molecule and two atoms from two other water molecules, which are hydrogen bonded through

oxygen lone pairs. The resulting structure

contains six-sided channels running through

out (Figure 7.10). These channels play an

important role in determining the properties of ice.

• 
  

As water freezes, the hydrogen bonds are optimized, which establishes the channels. Introduction of the void space into the struct

ure results in an expansion of the water, which

is why cars and outdoor plumbing

must be protected during the winter.

• 
  

The liquid is the denser phase, so ice melts when a pressure is applied.

• 
  

Ice floats because it is less dense than the

liquid. Thus, aquatic life

can survive the winter

because ponds freeze on the surface, which insulates the water below.

• 
  

The channels are six-sided, so ice crystals grow in hexagonal patterns, which is why snowflakes are six-sided.

Hydrogen bonding interactions can also dictate the structure of large molecules. In Chapter 13, we discuss the double helix structure of DNA, which is the result of the molecule coiling so as to maximize the st

rength of hydrogen bonding interactions.

Covalent bonds are much stronger than any

of the intermolecular interactions, but, as

shown in Table 7.1, the strengths of the inte

rmolecular interactions overlap. All molecules

interact through dispersion forces, others through both dispersion and dipolar forces, and some through dispersion, dipolar, and hydrogen bonding. Thus, water molecules interact through all three types of forces, but the dispersion forces in water are quite small and the hydrogen bonding interactions dominate and make water a liquid at room conditions. I

(^2)
molecules, on the other hand, interact only through dispersion forces, but those forces are so strong that I
is a solid at room conditions. 2
7.4
SOLIDS*
Intermolecular forces in the solid state ar
e much greater than thermal energy, so
the
particles
† in a solid take fixed positions relative to one another that maximize their
interactions. Molecules in the gas phase
are characterized by random motion, while
particles in the solid state are characterized by fixed positions with molecular motion restricted to small oscillati
ons about these positions. Thus,
solids
have their own shape and
volume, which are independent of the container.
†
The term “particles” is used for atoms, molecules, or ions.

• Solids are introduced only briefly
  here, the structure and properties of
  solids are taken up in much more detail in Chapter 8.
  Chapter 7 States of Matter and Changes in State
  © by
  North
  Carolina
  State
  University