Homogeneous Equilibria
Definition and meaning of Homogeneous Equilibria in chemistry.
Homogeneous equilibria are chemical equilibria in which all reactants and products exist in the same physical phase, such as all gases or all species dissolved in one solution.
In more detail
Because every species shares one phase, all of them appear explicitly in the equilibrium constant expression (Kc or Kp), each raised to the power of its stoichiometric coefficient. This contrasts with heterogeneous equilibria, where pure solids and liquids are omitted from the expression since their concentrations, or activities, stay effectively constant. Homogeneous equilibria are common both in gas-phase reactions and in aqueous systems like acid-base ionization and complex-ion formation.
Key facts
| Field | Physical Chemistry |
|---|---|
| Classic example | N2(g) + 3H2(g) ⇌ 2NH3(g) |
| Contrast | Heterogeneous equilibria (multiple phases) |
| Constant used | Kc or Kp, all species included |
In the Haber process, N2(g) + 3H2(g) is in equilibrium with 2NH3(g), all species gaseous, giving Kc = [NH3]^2 / ([N2][H2]^3).
Frequently asked questions
How does homogeneous equilibrium differ from heterogeneous equilibrium?
In a homogeneous equilibrium every species is in the same phase and all appear in the equilibrium constant expression. In a heterogeneous equilibrium the species span multiple phases, and pure solids or liquids are left out of the expression because their concentration, or activity, does not change.
Do homogeneous equilibria only involve gases?
No. They also occur entirely in solution, for example the ionization of acetic acid, CH3COOH(aq) in equilibrium with CH3COO-(aq) and H+(aq), where every species is aqueous.