Hydrolysis of Salts Summary

Hydrolysis of Salts

Introduction

• Ionic compounds (salts) dissociate into cations and anions in water.

• These ions can further react with water, behaving as acids or bases.

• The resulting solution can be acidic, basic, or neutral, depending on the relative strengths of the conjugate acids and bases.

Salt Solutions

• Ammonium chloride ($NH<em>4Cl$) dissociates into $NH</em>4^+$ and $Cl^-$.

• $NH<em>4^+$ is the conjugate acid of the weak base ammonia ($NH</em>3$), making it a weak acid.

• $Cl^-$ is the conjugate base of the strong acid HCl, so it does not undergo appreciable hydrolysis.

Conjugate Acid/Base Behavior

• Reaction occurs with the conjugate acid of a weak base.

• No reaction occurs with the conjugate acid/base of a strong acid/base.

• The stronger the acid or base, the less likely a reaction will occur.

Amphoteric Ions

• Amphoteric ions (e.g., bicarbonate, $HCO_3^-$) can act as both acids and bases.

• The relative Ka and Kb values determine whether the solution will be acidic or basic.

• If Kb > Ka, the solution is typically basic.

Metal Ion Hydrolysis

• Except for Group 1 and Group 2 metals, most metal ions undergo acid ionization when dissolved in water.

• Acid strength increases with increasing charge and decreasing size of the metal ions.

• Small, highly charged ions exhibit high acidity.

Metal Hydrates

• Metal ions can form covalent bonds with water molecules, forming complex ions.

• Example: Aluminum nitrate ($Al(NO<em>3)</em>3$) forms $[Al(H<em>2O)</em>6]^{3+}$ in water.

Polyprotic Behavior

• Metal hydrates can release protons in a stepwise fashion, acting as polyprotic acids.

• Example: $[Al(H<em>2O)</em>6]^{3+}$ can donate protons, forming various ions such as $[Al(H<em>2O)</em>5(OH)]^{2+}$.