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Difficulty: HardSalt Hydrolysis and Solution Acidity/Alkalinity

An aqueous solution of aluminium chloride (AlCl3AlCl_3) is acidic because the chloride ion (Cl−Cl^-) undergoes anion hydrolysis in water to produce hydrogen ions (H+H^+).

Answer: Answer

Answer

The statement is False. Chloride ions do not undergo salt hydrolysis; the acidity of an aluminium chloride solution arises from cation hydrolysis of the hydrated aluminium ion.
The statement is false because chloride ions are spectator ions in aqueous solution and do not hydrolyze. The acidic nature of an aluminium chloride solution is instead driven by cation hydrolysis, in which the high charge density of the hydrated aluminium ion causes it to act as a Brønsted-Lowry acid by donating protons to water.

Step-by-Step Solution

1
Analyze the dissolution of AlCl3AlCl_3 in aqueous medium.
AlCl3(s)+6H2O(l)→[Al(H2O)6]3+(aq)+3Cl−(aq)AlCl_3(s) + 6H_2O(l) \rightarrow [Al(H_2O)_6]^{3+}(aq) + 3Cl^-(aq)
Aluminium chloride dissociates fully in water to yield hydrated aluminium cations and chloride anions.
2
Evaluate the hydrolysis potential of the chloride anion (Cl−Cl^-).
Cl−(aq)+H2O(l)→No ReactionCl^-(aq) + H_2O(l) \rightarrow \text{No Reaction}
Chloride is the conjugate base of the strong acid HClHCl; hence, it is a spectator ion with negligible basicity and cannot undergo hydrolysis.
3
Evaluate the hydrolysis reaction of the hexaaquaaluminium(III) cation ([Al(H2O)6]3+[Al(H_2O)_6]^{3+}).
[Al(H2O)6]3+(aq)+H2O(l)⇌[Al(H2O)5(OH)]2+(aq)+H3O+(aq)[Al(H_2O)_6]^{3+}(aq) + H_2O(l) \rightleftharpoons [Al(H_2O)_5(OH)]^{2+}(aq) + H_3O^+(aq)
The high charge density of Al3+Al^{3+} weakens O-H bonds in coordinated water molecules, enabling proton transfer to free water molecules and increasing [H3O+][H_3O^+].

Key Concept

Cation Hydrolysis of Polyvalent Metal Ions
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Topics

Salt Hydrolysis and Solution Acidity/Alkalinity

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