Acid Ionization Constant
Definition and meaning of Acid Ionization Constant in chemistry.
The acid ionization constant (Ka) is the equilibrium constant for the ionization of an acid in water. It measures how strongly an acid donates protons: a larger Ka means a stronger acid.
In more detail
For a weak acid HA that ionizes to H⁺ and A⁻, the constant is written Ka = [H⁺][A⁻] / [HA], using the equilibrium concentrations. A small Ka means the equilibrium lies toward the un-ionized acid, so the acid is weak. Because Ka values span many orders of magnitude, acid strength is often reported as pKa = −log10(Ka), where a lower pKa indicates a stronger acid.
Key facts
| Symbol | Ka |
|---|---|
| Expression | Ka = [H⁺][A⁻] / [HA] |
| Related measure | pKa = −log10(Ka) |
| Field | Physical Chemistry |
Acetic acid has a Ka of about 1.8 × 10⁻⁵ (pKa ≈ 4.76), showing that it is a weak acid that only partially ionizes in water.
Frequently asked questions
What does a small Ka value mean?
A small Ka means the acid ionizes only slightly in water, so the equilibrium favors the un-ionized acid and the acid is weak.
What is the difference between Ka and pKa?
pKa is simply −log10(Ka). It compresses the wide range of Ka values into an easier scale, where a lower pKa means a stronger acid.