Dissociation Constant
Definition and meaning of Dissociation Constant in chemistry.
Dissociation constant (K) is the equilibrium constant for a reaction in which a compound reversibly splits into two or more smaller species, most commonly a weak acid or base separating into ions in solution.
In more detail
For a generic dissociation HA ⇌ H+ + A-, the constant is K = [H+][A-]/[HA], with each concentration taken at equilibrium. A larger K means the equilibrium lies further toward the dissociated (ionized) products, indicating a stronger acid or base and greater ionization at a given concentration. Because K values span many orders of magnitude, chemists often report pK = -log10(K) instead, so a lower pKa signals a stronger acid. Dissociation constants are essential for predicting pH, buffer behavior, and the extent of ionization of drugs, enzymes, and other weak electrolytes.
Key facts
| General formula | K = [products]/[reactants] at equilibrium |
|---|---|
| Field | Physical Chemistry |
| Related quantity | pK = -log10(K) |
| Common types | Ka (acid), Kb (base) |
Acetic acid dissociates as CH3COOH ⇌ CH3COO- + H+ with Ka = 1.8 × 10⁻⁵ at 25°C, meaning only a small fraction of the acid ionizes in dilute aqueous solution, consistent with its classification as a weak acid.
Frequently asked questions
Is the dissociation constant the same as Ka?
Ka is a specific type of dissociation constant used for acid ionization; the general term 'dissociation constant' also covers base dissociation (Kb) and other reversible splitting equilibria.
What does a small dissociation constant mean?
A small K means the equilibrium favors the undissociated (associated) form, so only a small fraction of the compound separates into ions or fragments at equilibrium, characteristic of a weak acid, weak base, or weak complex.