Concise Physical Chemistry

c12 JWBS043-Rogers September 13, 2010 11:27 Printer Name: Yet to Come

186 SOLUTION CHEMISTRY

12.5 HENRY’S LAW

The shape of thepBcurve in Fig. 12.3 suggests another model similar to Raoult’s

law but with a different slope for the lower (solute) end of the curve. According to

Henry’s law, the partial pressure, is the linear function found at the limit of infinitely

dilute solutions (in practice, very dilute solutions). Like the ideal gas law, it is a

limiting law (Rosenberg and Peticolas, 2004).

At relatively high concentrations of solute B, Henry’s law is a poor approximation

but it is clear from Fig. 12.4 that in the very dilute region near the vertical marker,

it is a better descriptor of the partial vapor pressure of B than Raoult’s law. On the

contrary, atXB→1 (toward the right in Fig. 12.4), B follows Raoult’s law. Now we

have a combined model:Raoult’s lawfor B acting as a solvent in high concentrations

andHenry’s lawfor B acting as a solute in low concentrations.

12.5.1 Henry’s Law Activities

We have encounteredactivitiesandactivity coefficientsbefore. A Henry’s law activity

of solute B in solvent A is shown as a vertical line well to the left on theXBaxis of

Fig. 12.4, whereXBis small. The activity coefficientγis the ratio of realpBto the

ideal, as determined by Henry’s law. For the behavior of B shown in Fig. 12.4, the

activity coefficient will beγ<1 in all cases because real behavior is always less than

ideal. It will approachγ=1 in the limit of infinite dilution. If the partial pressure

curve in Fig. 12.4 were below the Raoult’s law dotted line, all Henry’s law activity

coefficients would beγ>1.

The model we have described so far is for binary solutions in which the components

are mutually miscible (soluble in each other in all proportions). Everything we have

Α Β

XB

pB

pBo

FIGURE 12.4 Henry’s law for the partial pressure of component B as the solute. The solid

line is a Henry’s law extrapolation to infinite dilution of B. The dotted line is Raoult’s law.