Hydrolysis Constant
Definition and meaning of Hydrolysis Constant in chemistry.
The hydrolysis constant is an equilibrium value that measures how a dissolved salt reacts with water. It describes the extent to which a salt's ions form a conjugate weak acid or weak base. This chemical reaction releases extra hydroxide or hydronium ions, which changes the water's pH.
In more detail
When you dissolve a salt in water, the salt splits into positive and negative ions. Some of these specific ions will react directly with the surrounding water molecules. We use the hydrolysis constant, written as Kh, to measure this exact reaction.
For an anion from a weak acid, Kh equals the water constant (Kw) divided by the acid's Ka. For a cation from a weak base, Kh equals the water constant (Kw) divided by the base's Kb. A larger Kh value means the ion reacts with water to a much greater extent.
This stronger reaction pushes the final solution further away from a neutral pH of seven. Salts formed from a strong acid and a strong base do not react this way. Their ions have practically no tendency to react with water, so the pH stays neutral.
Chemists use the Kh value to calculate the exact pH of any salt solution. They apply the exact same math used for standard weak acids and bases. This helps them predict how a specific salt will behave in a laboratory experiment.
Key facts
| Field | General Chemistry |
|---|---|
| Symbol | Kh |
| Formula (weak acid anion) | Kh = Kw / Ka |
| Formula (weak base cation) | Kh = Kw / Kb |
| Related relationship | Kw = Ka x Kb for a conjugate acid-base pair |
| Example value | Kh = 5.6x10^-10 for acetate ion at 25 °C |
Sodium acetate is a common salt that creates a basic solution when dissolved in water. The acetate ion reacts with water to form acetic acid and a hydroxide ion. The exact reaction is CH3COO- + H2O <-> CH3COOH + OH-. To find the Kh, you divide Kw by the Ka value of acetic acid. This calculation gives a Kh value of 5.6x10^-10 at standard room temperature. This small but significant value proves the final solution will have a pH above seven.
Frequently asked questions
Why does a salt like sodium acetate form a basic solution?
The acetate ion steals a proton from water to form acetic acid. This leaves behind a hydroxide ion, which makes the entire solution basic.
How is Kh related to Ka and Kb?
Kh and the dissociation constant of the conjugate species are linked through water. You find Kh by dividing the water constant (Kw) by either Ka or Kb.
Do all dissolved salts undergo hydrolysis?
No. Salts made from a strong acid and a strong base do not hydrolyze. Their ions are too stable to react with water, so the solution remains neutral.