Chapter 2

### LEARNING OBJECTIVES

After completing this chapter, the reader should be able to:

• ► Explain the potential energy diagram.

• ► Understand the types of intermolecular forces and their relative magnitudes.

• ► Identify the physical character-istics of each state of matter.

• ► Distinguish between the gaseous, liquid, and solid states of matter.

• ► Relate the heat of vaporization and the boiling point of liquids to the magnitude of intermolecular forces.

• ► Relate the heat of fusion and the melting point of solids to the magnitude of intermolecular forces.

• ► Demonstrate and understand the physical properties of gases, liquids, and solids.

• ► Describe the different types of solid materials used in pharmaceuticals.

• ► Differentiate chemical and physical stability.

### INTERMOLECULAR FORCES

#### Intermolecular Binding Forces

When molecules interact with each other, they do so by the forces of both attraction and repulsion. Forces of attraction are essential for molecules to come together. The two types of attractive forces are called cohesive forces and adhesive forces. For example, when like molecules are attracted to each other this represents cohesive forces of attraction. When different molecules are attracted to one another, these are adhesive forces of attraction. A good analogy would be the opposite poles of a magnet, which attract one another when in close proximity. In addition to attracting each other, molecules are acted on by repulsive forces, which act to separate molecules. Using the magnet analogy, bringing two positive ends of a magnet in close proximity leads to repulsion, as would two negative ends.

Consider two molecules coming together. When they are attracted to each other (i.e., when unlike charges are closer than like charges), the forces of attraction pull the molecules together. Attractive forces (FA) are inversely proportioned to the distance separating the molecules (r), as shown here:

$FA ∝ 1rn$

where n ∼ 6 or 7 for the interaction of two hydrogen atoms or nitrogen atoms and n ∼ 3 or 4 for chloroform molecules. The inverse relationship between the forces of attraction and the distance has been derived from the phenomena first described by Sir John Edward Lennard-Jones and is known as the Lennard-Jones potential.1 As described by the Lennard-Jones potential theory, attractive forces can be represented by means of a potential energy function. As the forces of attraction between the molecules increase, the potential energy becomes increasingly negative, as illustrated in Figure 2-1.

###### FIGURE 2-1

Potential energy diagram for attractive forces.

Attractive forces operate over a greater distance than do repulsive forces. The long-range attractive component of intermolecular force is significant when the overlap of electron clouds is small. Short-range repulsive forces operate when the molecules come close together ...

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