Cation hydrolysis is the reaction where the cation of a weak base reacts with water, leading to the formation of a less ionizable weak base and hydronium ions, thereby affecting the solution's pH.
Understanding Cation Hydrolysis
When a salt derived from a weak base and a strong acid is dissolved in water, the cation from the weak base undergoes hydrolysis. This process involves the cation reacting with water molecules, resulting in the formation of the conjugate acid (a weak base) and hydronium ions (H3O+). The generation of hydronium ions increases the acidity of the solution.
The Chemical Process
Consider a cation B+ from a weak base BOH. The hydrolysis reaction can be represented as:
B+(aq) + 2H2O(l) ⇌ BOH(aq) + H3O+(aq)
Here, the cation B+ reacts with water to form the weak base BOH and hydronium ions (H3O+). This process shifts the equilibrium, increasing the concentration of H3O+ ions and thus lowering the pH of the solution, making it acidic.
Example: Ammonium Chloride (NH4Cl)
A common example of cation hydrolysis is ammonium chloride (NH4Cl). Ammonium chloride is a salt formed from the weak base ammonia (NH3) and the strong acid hydrochloric acid (HCl). When ammonium chloride dissolves in water, the ammonium ion (NH4+) undergoes hydrolysis:
NH4+(aq) + 2H2O(l) ⇌ NH3(aq) + H3O+(aq)
In this case, the ammonium ion (NH4+) reacts with water to form ammonia (NH3) and hydronium ions (H3O+), increasing the acidity of the solution.
Factors Affecting Cation Hydrolysis
- Charge and Size of the Cation: Highly charged and small-sized cations are more likely to undergo hydrolysis due to their strong attraction to water molecules.
- Strength of the Weak Base: The weaker the base from which the cation is derived, the greater the extent of hydrolysis.
- Concentration: Higher concentrations of the salt can lead to a more pronounced hydrolysis effect.
Impact on pH
Cation hydrolysis leads to a decrease in the pH of the solution, making it acidic. The extent of the pH change depends on the hydrolysis constant (Kh) of the cation.
