Heterogeneous Equilibria
Definition and meaning of Heterogeneous Equilibria in chemistry.
Heterogeneous equilibria are chemical equilibria in which the reactants and products exist in two or more distinct physical phases, such as solid-gas, solid-liquid, or liquid-gas systems.
In more detail
Because the concentration of a pure solid or pure liquid stays constant as long as some of that phase remains present, these species have an activity of 1 and are omitted entirely from the equilibrium constant expression. Only gases (as partial pressures) and dissolved species (as molar concentrations) appear in K, since their amounts actually change during the reaction. A practical consequence is that adding or removing extra pure solid or liquid does not shift the equilibrium position, only the amounts of gas or solute present matter. Heterogeneous equilibria are common in thermal decompositions, dissolution equilibria, and reactions of gases with solid surfaces.
Key facts
| Field | Physical Chemistry |
|---|---|
| Classic example | CaCO3(s) ⇌ CaO(s) + CO2(g) |
| Excluded terms | Pure solids and pure liquids |
| Rule | Activity of a pure solid or liquid is defined as 1 |
For the decomposition CaCO3(s) ⇌ CaO(s) + CO2(g), the equilibrium constant is written as Kp = P(CO2), because the pure solids CaCO3 and CaO are excluded from the expression.
Frequently asked questions
Why are solids and liquids left out of the equilibrium constant expression?
Their concentration (moles per unit volume) does not change as long as some of that phase is present, so their activity is constant and equal to 1, contributing nothing variable to K.
Does the amount of solid present affect the equilibrium?
No. As long as some solid of each type remains, changing how much is present has no effect on the equilibrium position at all, because the amount of a pure solid does not appear in the equilibrium constant expression; the gas pressure or solute concentration stays fixed by K and temperature the entire time.