Question

When 6\(M\) ammonia is added gradually to aqueous copper(II) nitrate, a white precipitate forms. The precipitate dissolves as more 6\(M\) ammonia is added. Write balanced equations to explain these observations. [Hint: \(\mathrm{Cu}^{2+}\) reacts with \(\mathrm{NH}_{3}\) to form \(\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}^{2+} . ]\)

Short answer

The short answer for the formation of the white precipitate and its subsequent dissolving is as follows: 1. Formation of the white precipitate: \(\mathrm{Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)}\) 2. Dissolving of the white precipitate: \(\mathrm{Cu(OH)_2(s) + 4NH_3(aq) \rightarrow Cu(NH_3)_4^{2+}(aq) + 2OH^-(aq)}\)

Step-by-step solution

Formation of the white precipitate

When ammonia is added gradually to the aqueous copper(II) nitrate solution, it reacts with water to form ammonium ions (\(\mathrm{NH}_4^+\)) and hydroxide ions (\(\mathrm{OH}^-\)). The hydroxide ions then react with the copper(II) ions, forming the insoluble white precipitate copper(II) hydroxide. Balanced equation for the formation of the white precipitate: \(\mathrm{Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)}\)

Dissolving the white precipitate

When more ammonia is added, it reacts with copper(II) hydroxide to form the soluble complex ion, \(\mathrm{Cu}(\mathrm{NH}_3)_4^{2+}\), which dissolves in the solution. Balanced equation for the dissolving of the white precipitate: \(\mathrm{Cu(OH)_2(s) + 4NH_3(aq) \rightarrow Cu(NH_3)_4^{2+}(aq) + 2OH^-(aq)}\) So, the two balanced equations that explain the observations are: 1. \(\mathrm{Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)}\) 2. \(\mathrm{Cu(OH)_2(s) + 4NH_3(aq) \rightarrow Cu(NH_3)_4^{2+}(aq) + 2OH^-(aq)}\)

Key concepts

Complex Ion Formation

When substances dissolve in water, they sometimes engage in complex ion formation. A complex ion is a charged species consisting of a central metal ion bonded to molecules or anions, known as ligands. In this case, we have copper(II) ions (\(\mathrm{Cu}^{2+}\)), which can coordinate with ammonia (\(\mathrm{NH}_3\)). When excess ammonia is added to copper(II) hydroxide, the ammonia molecules act as ligands. They bond with copper through the nitrogen atoms in ammonia, replacing water or hydroxide ions.

• This process results in the formation of the complex ion \(\mathrm{Cu(NH}_3)_4^{2+}\).
• The tetraamminecopper(II) ion is soluble in water, explaining why the solution turns blue and why the precipitate dissolves.

Understanding how complex ions are formed is crucial in chemistry because it explains the solubility changes of certain compounds during reactions. It tells you about the specific conditions that influence these transformations, leading to various industrial and experimental applications.

Precipitation

Precipitation, a central concept in chemistry, involves the generation of a solid form from a solution during a chemical reaction. Initially, when ammonia is added to the copper(II) nitrate solution, a reaction occurs, leading to the formation of copper(II) hydroxide (\(\mathrm{Cu(OH)_2}\)), a white precipitate.

• This occurs when hydroxide ions (\(\mathrm{OH}^-\)) react with the dissolved \(\mathrm{Cu}^{2+}\) ions.
• The process can be understood by considering the solubility product, which is a constant that predicts when a solid will form in solution.

This is a reversible process. As more reagents are added, reactions can shift, creating or dissolving these solids. Recognizing the conditions that cause precipitation is vital for chemical experimentation and synthesis.

Copper(II) Ions

Copper(II) ions (\(\mathrm{Cu}^{2+}\)) play an essential role in the chemistry of transition metals. As part of the transition metal family, copper can form a variety of interesting coordination complexes and precipitates. In aqueous solutions, \(\mathrm{Cu}^{2+}\) ions usually present as their hydrated complex. They can undergo several reactions depending on the environmental conditions, such as pH or the presence of certain ligands.

• In the presence of \(\mathrm{OH}^-\) ions, copper(II) ions readily form insoluble \(\mathrm{Cu(OH)_2}\).
• When bound to ammonia, \(\mathrm{Cu}^{2+}\) ions form the soluble complex ion \(\mathrm{Cu(NH}_3)_4^{2+}\).

Understanding the behavior of copper(II) ions serves as a gateway to exploring the behaviors of other transition metals. It also illustrates how solutions' compositions can be manipulated for desired reactions, which is crucial in laboratory settings and practical applications.