Physical Chemistry , 1st ed.

precipitating from solution and ions dissociating from the solid and going into

solution:

PbCl 2 (s)→Pb^2 (aq) 2Cl(aq)

Pb^2 (aq) 2Cl(aq)→PbCl 2 (s)

No net change: chemical equilibrium

The rock on the side of the hill that becomes the rock at the bottom of the

hill is an example of an equilibrium, but this is an equilibrium where nothing

is happening. This is an example of a static equilibrium.Chemical equilibria

are different. The chemical reactions are still occurring, but the forward and

reverse reactions are occurring at just the same rate so that there is no overall

change in the chemical identity of the system. This is called a dynamic equilib-

rium.All chemical equilibria are dynamic equilibria. That is, they are con-

stantly moving, but going nowhere.

Example 5.1

Describe the following situations as either static or dynamic equilibria.

a.The level of water in a fishtank, as the water is constantly passing through

a filter

b.A rocking chair that has stopped rocking

c.Acetic acid, a weak acid, that is ionized only to the extent of about 2% in

aqueous solution

d.A bank account that maintains an average monthly balance of $1000 de-

spite numerous withdrawals and deposits

Solution

a.Since there is constant motion of the material at equilibrium—the water—

this is an example of a dynamic equilibrium.

b.A stopped rocking chair isn’t moving at all at the macroscopic level, so this

situation is an example of a static equilibrium.

c.The ionization of acetic acid is a chemical reaction, and like all chemical

reactions at equilibrium, it is a dynamic one.

120 CHAPTER 5 Introduction to Chemical Equilibrium

O

DD

O

DD

O

DD

O

DD

O

DD

O

HH

O

HH

O

HH

O

HH

O

HH

O

HD

O

DH

O

HD

O

HD

O

DH

O

DH

O

DH

O

HD

O

DH

O

DH

Figure 5.2 Sometimes it is difficult to know whether a system is at chemical equilibrium. An

equimolar mixture of H 2 O and D 2 O—water and heavy water—might appear to be at equilib-

rium when mixed initially, since both substances are simply water. But in reality, hydrogen ex-

change occurs to mix the isotopes of hydrogen among the water molecules. At equilibrium, the

predominant molecule is HDO.