Hydrolysis of Ions in a Salt
- When a salt such as ammonium chloride (NH$_4$Cl) dissolves in water, it dissociates into ions: NH$_4^+$ and Cl$^−$.
- While it may seem that these ions simply exist in solution without further interaction, the ammonium ion (NH$_4^+$) actually reacts with water, releasing hydrogen ions (H$^+$) and slightly lowering the pH.
This process, called hydrolysis, explains why some salt solutions are acidic, basic, or neutral.
Understanding Hydrolysis: The Basics
Definition
Hydrolysis of a salt
When a salt dissolves in water, its ions may undergo hydrolysis, reacting with water to form either H$^+$ (acidic) or OH$^−$ (basic) ions.
The pH of the resulting solution depends on the nature of the ions involved:
- Cations (positive ions): Certain cations, like NH$_4^+$, act as weak acids by donating protons to water.
- Anions (negative ions): Certain anions, like CH$_3$COO$^−$, act as weak bases by accepting protons from water.
Key Rule:
- Only ions from weak acids or weak bases undergo hydrolysis.
- Ions derived from strong acids (e.g., Cl$^−$) or strong bases (e.g., Na$^+$) are too weak to react with water and do not hydrolyze.
Predicting the pH of Salt Solutions
- To determine the pH of a salt solution, consider the parent acid and base from which the salt is derived.
- These combinations dictate whether the solution is acidic, basic, or neutral:
Salts of Strong Acids and Strong Bases
- Example: NaCl (sodium chloride)
- Parent acid: HCl (strong acid)
- Parent base: NaOH (strong base)
- Result: Neither ion hydrolyzes, so the solution is neutral (pH = 7).
