L'addition d'acide nitrique dans une solution du sel...

L'addition d'acide nitrique dans une solution du sel complexe d'argent $\left[\mathrm{Ag}\left(\mathrm{NH}_3\right)_2\right]^{+} \mathrm{Cl}^{-}$provoque la formation d'un précipité ; pourtant le nitrate d'argent est soluble.
Quelles sont la nature et l'origine de ce précipité ?

Key Concepts

Coordination Complex

A coordination complex is a species in which a central metal ion is bonded to a set of molecules or ions called ligands. In this context, the silver ion (Ag+) forms a complex with ammonia (NH3) molecules, which stabilizes it in solution and alters its reactivity, particularly its tendency to form precipitates when interacting with other ions.

Acid-Base Equilibrium

Acid-base equilibrium refers to the balance between the deprotonated and protonated forms of substances in solution. When an acid is added, ammonia (a weak base) can become protonated to form ammonium ions (NH4+). This protonation reduces the concentration of free ammonia, thereby shifting equilibrium and destabilizing the silver-ammonia complex.

Precipitation Reaction

A precipitation reaction occurs when two soluble substances react to form an insoluble solid, known as a precipitate. In the scenario discussed, free silver ions, once released from the destabilized complex due to the protonation of ammonia, react with chloride ions (from the complex’s counterion) to form silver chloride (AgCl), an insoluble compound that precipitates out of solution.

Solubility Product (Ksp)

The solubility product is the equilibrium constant (Ksp) that governs the solubility of a sparingly soluble salt. For silver chloride, the low Ksp means that even a low concentration of silver and chloride ions will exceed the solubility product, resulting in precipitation. This concept explains why, despite the inherent solubility of silver nitrate, the formation of AgCl is favored in the presence of sufficient chloride ions.

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