The reaction chemistry of aqueous copper(II) ions with ammonia is commonly used in both general chemistry and inorganic chemistry texts to illustrate the equilibria of complex ions in solution. Although the system initially seems simple, further analysis of the chemical species involved shows that it is in fact quite complicated. First of all, ammonia is a weak base and its basic equilibrium reaction must be taken into account. Second, although the aquated copper(II) ion is the most prevalent ion in solution before ammonia is added, this species is itself a weak acid. Third, a series of four coordination complex equilibria are established once ammonia is added. Finally, sparingly soluble copper(II) hydroxide precipitates as the hydroxide concentration rises due to the ammonia base equilibrium.
Typically, when the reaction chemistry of copper(II) with ammonia is discussed, the acid-base and precipitation equilibria are ignored. Although the mathematical description of the full system is unwieldy, it can be solved numerically using a software program such as MathCad, Mathematica or Maple. Such a treatment provides a thorough description of the real chemical system, complete with precipitation and redissolution of the copper hydroxide precipitate. This seemingly simple system encompasses three major classes of equilibrium chemistry commonly taught in inorganic chemistry: acid-base, complex ion formation, and solubility.
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