The Foundations of Chemistry

REAL GASES: DEVIATIONS FROM IDEALITY

Until now our discussions have dealt with idealbehavior of gases. By this we mean that
the identity of a gas does not affect how it behaves, and the same equations should work
equally well for all gases. Under ordinary conditions most realgases do behave ideally;
their Pand Vare predicted by the ideal gas laws, so they do obey the postulates of the
kinetic–molecular theory. According to the kinetic–molecular model, (1) all but a negli-
gible volume of a gas sample is empty space, and (2) the molecules of idealgases do not
attract one another because they are so far apart relative to their own sizes.
Under some conditions, however, most gases can have pressures and/or volumes that
are notaccurately predicted by the ideal gas laws. This tells us that they are not behaving
entirely as postulated by the kinetic–molecular theory.

Nonideal gas behavior (deviation from the predictions of the ideal gas laws) is most

significant at high pressuresand/or low temperatures,that is, near the conditions under

which the gas liquefies.

Johannes van der Waals (1837–1923) studied deviations of real gases from ideal
behavior. In 1867, he empirically adjusted the ideal gas equation

PidealVidealnRT

to take into account two complicating factors.

12-15

NH 3 gas (left)and HCl gas (right)

escape from concentrated aqueous

solutions. The white smoke (solid

NH 4 Cl) shows where the gases mix

and react.

NH 3 (g)HCl(g)88nNH 4 Cl(s)

12-15 Real Gases: Deviations from Ideality 471

(a) (b) (c)

A

B

Bell jar

Porous cup

Hydrogen gas

Air

Figure 12-14 Effusion of gases. (a) A molecular interpretation of effusion. Molecules are
in constant motion; occasionally they strike the opening and escape. (b) Latex balloons were
filled with the same volume of He (yellow),N 2 (blue),and O 2 (red). Lighter molecules, such
as He, effuse through the tiny pores of the latex balloons more rapidly than does N 2 or O 2.
The silver party balloon is made of a metal-coated polymer with pores that are too small to
allow rapid He effusion. (c) If a bell jar full of hydrogen is brought down over a porous cup
full of air, rapidly moving hydrogen diffuses into the cup faster than the oxygen and
nitrogen in the air can effuse out of the cup. This causes an increase in pressure in the cup
sufficient to produce bubbles in the water in the beaker.