Ionization Constant
Definition and meaning of Ionization Constant in chemistry.
Ionization constant is the equilibrium constant that measures the extent to which a weak acid or weak base dissociates into ions when dissolved in water, denoted Ka for acids and Kb for bases.
In more detail
For a weak acid HA that ionizes as HA + H2O in equilibrium with H3O+ + A-, the acid ionization constant is Ka = [H3O+][A-]/[HA], with concentrations taken at equilibrium. A larger Ka means the acid ionizes more extensively and is therefore stronger, while a small Ka indicates that most of the acid remains un-ionized. Ionization constants are experimentally determined and vary with temperature, since they describe a true chemical equilibrium. Chemists often report pKa = -log10(Ka) instead, since it converts the tiny values typical of weak acids into a more convenient scale, where a lower pKa corresponds to a stronger acid.
Key facts
| Field | General Chemistry |
|---|---|
| Symbol | Ka (acid), Kb (base) |
| Expression | Ka = [H3O+][A-]/[HA] |
| Related scale | pKa = -log10(Ka) |
Acetic acid (CH3COOH) has Ka = 1.8 x 10^-5 at 25 degrees C, corresponding to a pKa of about 4.74, which classifies it as a weak acid that ionizes only slightly in water.
Frequently asked questions
How is Ka related to pKa?
pKa is defined as -log10(Ka). Because Ka values for weak acids are small and awkward to compare, pKa converts them to a simpler scale where a lower pKa always means a stronger acid.
Does the ionization constant change with temperature?
Yes. Because Ka and Kb are true equilibrium constants, their values depend on temperature, and tabulated values are typically reported at 25 degrees C.