Physical Chemistry Third Edition

6.2 Henry’s Law and Dilute Nonelectrolyte Solutions 249

0

0

100

200

P

/torr

(1 2 x 2 )P 2 * 5 x 1 P 1 *

(Raoult s law)

x 2 P 2 *

300

400

P 1 *

P 1

k 2 x 2

P 2 P 2 *

x 2

k 2

P 5 P 11 P 2

0.2 0.4 0.6 0.8 1

(Henry s law)

(Raoult s law)

Figure 6.9 Partial Vapor Pressures of Ethanol (Component 2) and Diethyl Ether in a

Solution at 20 ̊C, Showing Positive Deviation from Raoult’s Law.Drawn from data of

J. Timmermans,Physicochemical Constants of Binary Systems, Vol. 2, Interscience Pub-

lishers, Inc., New York, 1959, p. 401.

curve representingP 2 is nearly linear. This behavior corresponds toHenry’s law, which

is written:

Pikixi (Henry’s law, valid forxi 1) (6.2-1)

wherekiis called theHenry’s law constantfor substancei. The Henry’s law constant is

not a true constant. It does not depend on the mole fraction but it depends on temperature

and on the identities of all substances present.

Henry’s law is named for William Henry,
1774–1836, an English chemist who
was a friend and colleague of John
Dalton and who influenced Dalton’s
formulation of his atomic theory.

There is another feature of Figure 6.9 that is typical of nonionic substances: The curve

representingP 1 nearly coincides with the line representing Raoult’s law for values of

x 1 near unity, and similarly for theP 2 curve nearx 2 1. If one component in a solution

is present in a larger amount than the others, it is called thesolvent. We will generally

assign the solvent to be substance number 1. The other substances are calledsolutes.

Adilute solutionis one in which all solutes have small mole fractions. We can generalize