An important point about
forming the equilibrium constant expression is that:
Only
aqueous and gaseous substances are included in the
expression. Pure solids and
liquids are excluded.*
Intuitively, this makes sense because the
"concentration" of a pure solid or liquid
is defined by its density, and is therefore a constant at a
given temperature and pressure.
Equilibrium
Constant, Keq
Keq = [C]c [D]d [A]a [B]b*By excluded, we mean we take
their value to be 1 (not
0) and 1 to any power is still 1.
(not
0) and 1 to any power is still 1.