Chapter 15 · Chemical Equilibrium
15.5Heterogeneous Equilibria: The Equilibrium Expression for Reactions Involving a Solid or a Liquid
9 min · two checks
Predict
Why doesn’t solid CaCO₃ appear in the K expression for CaCO₃(s) ⇌ CaO(s) + CO₂(g)?
The idea
- Omit pure solids and pure liquids from K.
- Write K for a heterogeneous reaction.
Heterogeneous means more than one phase. A pure solid or pure liquid does not change its concentration when you add more of it, as long as some is present. That constant gets absorbed into the value of K. You write only gases and solutes in the expression. For CaCO₃(s) ⇌ CaO(s) + CO₂(g), K = [CO₂]. Adding extra chalk does not shift this equilibrium the way adding extra CO₂ does. Removing all of the solid does stop the forward reaction, because the constant-concentration assumption needs some solid to be there.
Aqueous ions are not pure solids; they stay in the expression. Water, when it is the solvent in a dilute solution, is often left out for the same reason a pure liquid is left out. Do not leave water out of a gas-phase reaction where water is a product gas. The state symbol is doing real work.
Keep these
- Omit pure solids and pure liquids from the K expression.
- Include gases and dissolved species.
- Some solid must still be present for the equilibrium to exist.