9.9 Salt Hydrolysis

Introduction to Salt Hydrolysis

When a salt is dissolved in water, it dissociates into its constituent cations and anions. These ions can react with water in a process called hydrolysis. This interaction can lead to the formation of $H^{+}$ or $O{H}^{-}$ ions, causing the resulting solution to be acidic, basic, or neutral.

Consider the following observations:

1. An aqueous solution of ammonium chloride ($\mathrm{N}{\mathrm{H}}_4\mathrm{Cl}$) turns blue litmus paper red, indicating an acidic solution.
2. An aqueous solution of potassium carbonate (${\mathrm{K}}_2\mathrm{C}{\mathrm{O}}_3$) turns red litmus paper blue, indicating a basic solution.
3. An aqueous solution of sodium chloride ($\mathrm{NaCl}$) has no effect on litmus paper, indicating a neutral solution.

The general reactions of a salt's ions ($M^{+}$ and $X^{-}$) with water are:

$M_{(aq)}^{+}+H_2{O}_{(l)}\rightleftharpoons MO{H}_{(aq)}+H_{(aq)}^{+}$

$X_{(aq)}^{-}+H_2{O}_{(l)}\rightleftharpoons H{X}_{(aq)}+O{H}_{(aq)}^{-}$

Hydrolysis is defined as the chemical reaction of cations and anions of a salt with water, which can alter the pH of the solution.

Note: Hydrolysis is different from hydration. In hydrolysis, the H-OH bond in the water molecule is broken. In hydration, water molecules surround the ions without any bonds being broken.

Hydrolysis of Anions (from Weak Acids)

Anions derived from weak acids are strong conjugate bases. They react with water to produce hydroxide ions ($O{H}^{-}$), making the solution basic.

Examples:

• Acetate ion ($\mathrm{C}{\mathrm{H}}_3\mathrm{CO}{\mathrm{O}}^{-}$): $\mathrm{C}{\mathrm{H}}_3\mathrm{CO}{\mathrm{O}}_{(\mathrm{aq})}^{-}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}\rightleftharpoons \mathrm{C}{\mathrm{H}}_3\mathrm{COO}{\mathrm{H}}_{(\mathrm{aq})}+\mathrm{O}{\mathrm{H}}_{(\mathrm{aq})}^{-}$
• Cyanide ion ($\mathrm{C}{\mathrm{N}}^{-}$): $\mathrm{C}{\mathrm{N}}_{(\mathrm{aq})}^{-}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}\rightleftharpoons \mathrm{HC}{\mathrm{N}}_{(\mathrm{aq})}+\mathrm{O}{\mathrm{H}}_{(\mathrm{aq})}^{-}$
• Carbonate ion ($\mathrm{C}{\mathrm{O}}_3^{2-}$): \mathrm{CO_3^{2-}_{(aq)} + 2H_2O_{(l)} \rightleftharpoons H_2CO_3_{(aq)} + 2OH^-_{(aq)}}

Anions from strong acids (e.g., $\mathrm{C}{\mathrm{l}}^{-}$, $\mathrm{N}{\mathrm{O}}_3^{-}$, $\mathrm{S}{\mathrm{O}}_4^{2-}$) are very weak conjugate bases and do not react with water.

Hydrolysis of Cations (from Weak Bases)

Cations derived from weak bases are strong conjugate acids. They react with water to produce hydronium ions ($H^{+}$ or ${\mathrm{H}}_3{\mathrm{O}}^{+}$), making the solution acidic.

Examples:

• Copper(II) ion ($\mathrm{C}{\mathrm{u}}^{2+}$): $\mathrm{C}{\mathrm{u}}_{(\mathrm{aq})}^{2+}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}\rightleftharpoons \mathrm{Cu}(\mathrm{OH}{)}_{2(\mathrm{aq})}+2{\mathrm{H}}_{(\mathrm{aq})}^{+}$
• Ammonium ion ($\mathrm{N}{\mathrm{H}}_4^{+}$): \mathrm{NH_4^+_{(aq)} + H_2O_{(l)} \rightleftharpoons NH_4OH_{(aq)} + H^+_{(aq)}}

Cations from strong bases (e.g., $\mathrm{N}{\mathrm{a}}^{+}$, ${\mathrm{K}}^{+}$, $\mathrm{C}{\mathrm{a}}^{2+}$) are very weak conjugate acids and do not react with water.

Types of Salts and Their Hydrolysis

Salts can be categorized into four types based on the strength of their parent acid and base.

1. Salts of Strong Acids and Strong Bases

   • Examples: $\mathrm{NaCl}$, $\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4$, $\mathrm{KN}{\mathrm{O}}_3$
   • Neither the cation nor the anion hydrolyzes.
   • The solution remains neutral ($pH=7$).

2. Salts of Weak Acids and Strong Bases

   • Examples: $\mathrm{C}{\mathrm{H}}_3\mathrm{COONa}$, $\mathrm{NaCN}$, $\mathrm{N}{\mathrm{a}}_2\mathrm{S}$
   • The anion hydrolyzes to produce $O{H}^{-}$ ions.
   • The solution is basic ($pH>7$).

3. Salts of Strong Acids and Weak Bases

   • Examples: $\mathrm{CuS}{\mathrm{O}}_4$, $\mathrm{N}{\mathrm{H}}_4\mathrm{Cl}$, $\mathrm{N}{\mathrm{H}}_4\mathrm{N}{\mathrm{O}}_3$
   • The cation hydrolyzes to produce $H^{+}$ ions.
   • The solution is acidic ($pH<7$).

4. Salts of Weak Acids and Weak Bases

   • Examples: $\mathrm{N}{\mathrm{H}}_4\mathrm{CN}$, $\mathrm{N}{\mathrm{H}}_4\mathrm{N}{\mathrm{O}}_2$
   • Both the cation and anion hydrolyze.
   • The nature of the solution (acidic, basic, or neutral) depends on the relative dissociation constants ($K_a$ and $K_b$) of the parent acid and base.
     • If $K_a>K_b$, the solution is acidic.
     • If $K_b>K_a$, the solution is basic.
     • If $K_a\approx K_b$, the solution is nearly neutral.

Summary of Salt Hydrolysis

The table below summarizes the behavior of different types of salts in aqueous solutions.

Significance of Hydrolysis

Hydrolysis is a crucial process in both plants and animals.

• In living systems, many biological reactions are hydrolysis reactions, including the hydrolysis of ATP (adenosine triphosphate) to release energy.
• Enzymes act as catalysts to facilitate the hydrolysis of complex molecules like fats, proteins, and carbohydrates into smaller units that can be used by the body.

Summary

• Salt Hydrolysis is the reaction of the ions of a salt with water, which can result in an acidic, basic, or neutral solution.
• Anions from weak acids hydrolyze to produce $O{H}^{-}$ ions (basic solution).
• Cations from weak bases hydrolyze to produce $H^{+}$ ions (acidic solution).
• Ions from strong acids or strong bases do not hydrolyze.
• For salts of weak acids and weak bases, the pH depends on the relative magnitudes of $K_a$ and $K_b$.
• Hydrolysis differs from hydration: in hydrolysis the O–H bond of water is broken; in hydration, water molecules merely surround the ions.