The aqueous solution of aluminium chloride is acidic due to the

Options

A. formation of `AI(OH)_(3)`
B. hydrolysis of cation and anion
C. hydrolysis of anion
D. hydrolysis of cation

Answer

D

Explanation

Salts that contain small, highly charged metal cations (for example, `AI^(3+), Cr^(3+), Fe^(3+), Bi^(3+)`, and `Be^(2+)`) and the conjugate bases of strong acids (e.g, `CI^(-))` produce an acidic solution because these cations hydrolyze to produce excess hydronium ions. Consider aluminium chloride `(AICI_(3))` as a typical example. When solid anhydrous `AICI_(3)` is added to water, the becomes very warm as the `AI^(3+)` ions become hydrated in solution to form the hydrated ion `[AI(H_(2)O)_(6)]^(3+)`. Each of these species is octahedral, meaning that the metal ion `(AI^(3+))` is located at the center of a regular octachedron, and the O atoms in six `H_(2)O` molecules are located at the corners. Consider one bond between the metal ion and the oxygen atom of one of the six water molecules in `[AI(H_(2)O)_(6)]^(3+)`:

The attraction of the small `AI^(3+)` ion for the lone pairs on the oxygen atoms is so greater that in the metal-oxygen bonds of the hydrated cation, electron density is decreased around the O end of each `H_(2)O` molecule (as `AI^(3+)` ion draws electron density towards itself). The O atom in turn draw electron density of the `O-H` bond toward itself. This weakens the O-H bonds in coordinated `H_(2)O` molecules relativer to the O-H bond in noncoordinated `H_(2)O` molecules. Consequently, the coordinated `H_(2)O` molecule `H_(2)O` molecule can donate `H^(+)` to the solvent `H_(2)O` molecules to form `H_(3)O^(+)` ions. This produces acidic solutions. The equation for the hydrolysis of hydrated `AI^(3+)` solutions. The equation for the hydrolysis of hydrated `AI^(3+)` is written as follows:
`[A(H_(2)O)_(6)]^(3_)(aq.)+H_(2)O(I)hArr[AI(OH)(H_(2)O)_(5)]^(2+)(aq.)H_(3)O^(+)(aq.)`
or simply `[AI(H_(2)O)_(6)]^(3+)(aq.)hArr[AI(OH)(H_(2)O)_(5)]^(2+)(aq.)+H^(+)(aq.)`
The hydrolysis of hydrated small, highly charged cations may occur beyound the first step, i.e., the `[AI(OH)(H_(2)O)_(5)]^(2+)` species can undergo further ionization as
`[AI(OH)(H_(2)O)_(5)^(2+)](aq.)hArr[AI(OH)_(2)O)_(4)]^(+)+H^(+)(aq.)` and so on. The extent of hydrlysis is the greatest for the smallest and most highly charges ions because a compact highly charged ion is more effective in polarizing the O-H bond and facilitating ionization. This is the reason that relatively large ions of low charge such as `Na^(+)` and `K^(+)` do not undergo hydrolysis.