These have all been instances of homogeneous equilibria, where only one phase is involved.
Many equilibria however, involve more than one phase. Here's an example of a heterogeneous equilibrium...
ex: CaCO3(s) ⇌ CaO(s) + CO2(g)
The correct equilibrium expression is K = [CO2]
Because the concentrations of pure substances such as liquids and solids cannot change!
The take-home message here, is the following:
"Equilibrium expressions only include aqueous (aq) and gaseous (g) reactants and products."
Heterogeneous Equilibrium Examples
ex: Write equilibrium expressions for Kc and Kp
( K is the same thing as Kc )
a. P4(s) + 5O2(g) ⇌ P4O10(s)
b. NH4NO3(s) ⇌ N2O(g) + 2H2O(g)
c. NH4NO3(aq) ⇌ N2O(g) + 2H2O(l)
Coming up next as we continue with SECTION 13 - Chemical Equilibrium,
We'll talk about the Extent of a Reaction, and the Reaction Quotient, Q