Hydrolysis Constant
Definition and meaning of Hydrolysis Constant in chemistry.
Hydrolysis constant (Kh) is the equilibrium constant that measures the extent to which an ion from a dissolved salt reacts with water to form its conjugate weak acid or weak base, releasing hydroxide or hydronium ions into solution.
In more detail
For the anion of a weak acid, Kh equals Kw divided by the acid's Ka; for the cation of a weak base, Kh equals Kw divided by the base's Kb. A larger Kh means the ion reacts with water to a greater extent, pushing the solution further from neutral pH. Salts formed from a strong acid and a strong base do not hydrolyze appreciably, since their ions are conjugates of strong electrolytes and have essentially no tendency to react with water. Using Kh, chemists can calculate the pH of a salt solution with the same equilibrium methods used for weak acids and bases.
Key facts
| Formula | Kh = Kw / Ka (anion of weak acid) or Kh = Kw / Kb (cation of weak base) |
|---|---|
| Field | General Chemistry |
| Related relationship | Kw = Ka x Kb for a conjugate acid-base pair |
| Example value | Kh = 5.6x10^-10 for acetate ion at 25 °C |
For sodium acetate, CH3COONa, the acetate ion hydrolyzes as CH3COO- + H2O <-> CH3COOH + OH-, with Kh = Kw/Ka = (1.0x10^-14)/(1.8x10^-5) = 5.6x10^-10, giving the resulting solution a pH above 7 at 25 °C.
Frequently asked questions
Why does a salt like sodium acetate form a basic solution?
The acetate ion is the conjugate base of a weak acid (acetic acid), so it hydrolyzes by accepting a proton from water, producing acetic acid and hydroxide ion. Because Kh is small but nonzero, enough OH- accumulates to make the solution basic.
How is Kh related to Ka and Kb?
Kh and the dissociation constant of the conjugate species are linked through the ion product of water: Kw = Ka x Kb. So Kh for an anion equals Kw/Ka, and Kh for a cation equals Kw/Kb.