Physical Chemistry , 1st ed.

of ionic strength, activity, and activity coefficients help us correlate the amount

of charge with the behavior of the system.

It is also important to understand why ionic solutions behave the way they

do. A few simple assumptions lead us to the Debye-Hückel theory for the de-

scription of ionic solutions. Even brief descriptions of these ideas will help us

recognize why we devote an entire chapter to the interaction and chemistry of

charged solutes.

8.2 Charges

Perhaps one of the earliest understandings of the scientific world is the con-

cept ofcharge.In about the seventh century B.C., the Greek philosopher Thales

found that a resinous substance called elektron—which we call amber—attracted

light objects like feathers and thread after it had been rubbed. Through the

centuries, people learned that amber rods or glass rods repel each other after

being rubbed, but an amber rod and a glass rod attract each other. However,

after touching, they immediately lose their attraction. In or around 1752,

multitalented American Benjamin Franklin performed his (perhaps apoc-

ryphal) key and kite experiment with lightning, showing that it could induce

the same properties in amber as rubbing it. It was Franklin who suggested that

this phenomenon called electricity had opposing properties, which he labeled

positiveand negative.Franklin suggested that when one rubs a glass rod, elec-

tricity flows into it to make it positive. When one rubs an amber rod, electric-

ity flows out of it, making it negative. When two oppositely charged rods

touch, there is an exchange between the two until the amount of electricity is

equalized. Two rods of the same charge, positive or negative, would avoid, or

repel,each other. (Though amazingly prescient, Franklin was wrong about the

charge that actually moved. However, vestiges of Franklin’s definitions—espe-

cially with respect to the direction of current flow in an electrical circuit—are

still common today.)

In the century that followed Franklin, other researchers like Coulomb,

Galvani, Davy, Volta, Tesla, and Maxwell placed an understanding of electrical

phenomena on solid experimental and theoretical grounds. This section re-

views some of those grounds.

In 1785, the French scientist Charles de Coulomb (Figure 8.1) made very ac-

curate measurements of the force of attraction or repulsion between small

charged spheres. He found that the direction of the interaction—that is, attrac-

tion or repulsion—is dictated by the types of the charges on the spheres. If two

spheres have the same charge, either positive or negative, they repel each other.

If, however, the two spheres have different charges, they attract each other.

Coulomb also found that the magnitude of the interaction between any two

spheres is dependent on the distance between the two small spheres. The force

of attraction or repulsion,F, between two charged spheres varies inversely with

the square of the distance,r, between the spheres:

F

r

1

 2 (8.1)

It was found that the force between charged objects is also proportional to the

magnitude of the charges, represented by q 1 and q 2 , on the objects. Equation

8.1 becomes

F

q 1

r



2

q 2

 (8.2)

8.2 Charges 207

207

Figure 8.1 Charles-Augustin de Coulomb

(1736–1806) was a French physicist who used

very delicate (for the time) instrumentation to

make measurements on the force of attraction

between charged bodies.

AIP Emilio Segre Visual Archives, E. Scott Barr Collection