R3.1.12 The pH of a salt solution (Higher Level only) Notes

Hydrolysis of Ions in a Salt

1. When a salt such as ammonium chloride (NH$_4$Cl) dissolves in water, it dissociates into ions: NH$_4^+$ and Cl$^−$.
2. 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

The pH of the resulting solution depends on the nature of the ions involved:

1. Cations (positive ions): Certain cations, like NH$_4^+$, act as weak acids by donating protons to water.
2. Anions (negative ions): Certain anions, like CH$_3$COO$^−$, act as weak bases by accepting protons from water.

Key Rule:

1. Only ions from weak acids or weak bases undergo hydrolysis.
2. 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

1. To determine the pH of a salt solution, consider the parent acid and base from which the salt is derived.
2. These combinations dictate whether the solution is acidic, basic, or neutral:

Salts of Strong Acids and Strong Bases

1. Example: NaCl (sodium chloride)
2. Parent acid: HCl (strong acid)
3. Parent base: NaOH (strong base)
4. Result: Neither ion hydrolyzes, so the solution is neutral (pH = 7).

Salts of Strong Acids and Weak Bases

1. Example: NH$_4$Cl (ammonium chloride)
2. Parent acid: HCl (strong acid)
3. Parent base: NH$_3$ (weak base)
4. Result: The NH$_4^+$ ion hydrolyzes:
   $$
   \text{NH}_4^+ + \text{H}_2\text{O} \leftrightharpoons \text{NH}_3 + \text{H}_3\text{O}^+
   $$
   The solution becomes acidic (pH < 7).

Salts of Weak Acids and Strong Bases

1. Example: CH$_3$COONa (sodium ethanoate)
2. Parent acid: CH$_3$COOH (weak acid)
3. Parent base: NaOH (strong base)
4. Result: The CH$_3$COO$^−$ ion hydrolyzes:
   $$
   \text{CH}_3\text{COO}^- + \text{H}_2\text{O} \leftrightharpoons \text{CH}_3\text{COOH} + \text{OH}^-
   $$
   The solution becomes basic (pH > 7).

Salts of Weak Acids and Weak Bases

1. Example: NH$_4$CH$_3$COO (ammonium ethanoate)
2. Parent acid: CH$_3$COOH (weak acid)
3. Parent base: NH$_3$ (weak base)
4. Result: Both ions hydrolyze:
   $$
   \text{CH}_3\text{COO}^- + \text{H}_2\text{O} \leftrightharpoons \text{CH}_3\text{COOH} + \text{OH}^-
   $$
   $$
   \text{NH}_4^+ + \text{H}_2\text{O} \leftrightharpoons \text{NH}_3 + \text{H}_3\text{O}^+
   $$
5. The pH depends on the relative strengths of the parent acid and base, determined by their dissociation constants ($K_a$ and $K_b$).