A larger or smaller indicates a stronger acid, while a larger or smaller indicates a stronger base. These values measure the extent of ionization in water, not the concentration of the solution.
A larger or smaller indicates a stronger acid, while a larger or smaller indicates a stronger base. These values measure the extent of ionization in water, not the concentration of the solution.
For an acid,
The acid dissociation constant is
A larger means equilibrium lies further toward the ions, so the acid ionizes to a greater extent and is stronger. The base dissociation constant, , describes the equivalent equilibrium for a base accepting a proton from water.
The same reasoning applies to bases. A stronger base accepts protons more readily, so its equilibrium with water lies further toward its conjugate acid and hydroxide ions. Consequently, its base dissociation constant is larger. In both cases, the constant compares equilibrium concentrations and therefore describes equilibrium position rather than reaction rate.
The logarithmic quantities are defined as
Because of the negative logarithm, the direction is reversed:
| Quantity | Stronger species | Weaker species |
|---|---|---|
| Larger | Smaller | |
| Smaller | Larger | |
| Larger | Smaller | |
| Smaller | Larger |
For example, an acid with has . It is stronger than an acid with , whose .
Because p values are logarithmic, a difference of one pKa unit corresponds to a tenfold difference in Ka. A difference of two units corresponds to a hundredfold difference. Always compare acids using Ka with Ka or pKa with pKa; do not directly rank an acid’s Ka against a base’s Kb.
For a conjugate acid-base pair,
At , this gives . Therefore, a stronger acid has a weaker conjugate base, and vice versa.
A common misconception is that a strong acid must be concentrated. Strength is the extent of ionization, whereas concentration is the amount of solute per unit volume.
In an IB Chemistry HL response, state the correct numerical trend and explain it using equilibrium position or extent of ionization. When comparing conjugate pairs, explicitly link increasing acid strength to decreasing conjugate-base strength.