Heterogeneous Equilibria

Reaction equilibria that involve different physical states of matter (different phases) are known as
heterogeneous equilibria. For example, for a reaction of the type

aA  (s) +   bB  (g) →   cC  (s) +   dD  (g),

one would expect to write the law of mass action as on the previous page:

But the molarity of a solid compound like A is just its density divided by its molar mass, which is a
constant that is characteristic of A. The numerical values of [A] and [C] (and consequently [A]a and
[C]c) can therefore be subsumed into the equilibrium constant:

Or, keeping in mind what we just discussed above, we would also write

The same holds for reactants and solids that appear as pure liquids. In short, concentrations of pure
solids and liquids do not appear in the equilibrium constant expression.

One final note on equilibrium constants: One might expect, from examining the law of mass action,
that the units for equilibrium constants would change depending on the reaction, since the mass