The Foundations of Chemistry

HETEROGENEOUS EQUILIBRIA

Thus far, we have considered only equilibria involving species in a single phase, that is,
homogeneous equilibria. Heterogeneous equilibriainvolve species in more than one
phase. Consider the following reversible reaction at 25°C.

2HgO(s) 34 2Hg(
)O 2 (g)

When equilibrium is established for this system, a solid, a liquid, and a gas are present.
Neither solids nor liquids are significantly affected by changes in pressure. The funda-
mental definition of the equilibrium constant in thermodynamics is in terms of the
activities of the substances involved.

For any pure solid or pure liquid, the activity is taken as 1 (Section 17-2), so terms

for pure liquids and pure solids do notappear in the Kexpressions for heterogeneous

equilibria.

Thus, for the reaction

2HgO(s) 34 2Hg(
)O 2 (g) Kc[O 2 ] and KPPO 2

These equilibrium constant expressions indicate that equilibrium exists at a given temper-
ature for one and only oneconcentration and one partial pressure of oxygen in contact with
liquid mercury and solid mercury(II) oxide.

17-11

17-11 Heterogeneous Equilibria 737

A photograph of the reaction

2HgO(s) 34 2Hg(
)O 2 (g)

The reaction is not at equilibrium here, because O 2 gas has been allowed to escape.