Question

When $6M$ ammonia is added gradually to aqueous copper(II) nitrate, a white precipitate forms. The precipitate dissolves as more $6M$ ammonia is added. Write balanced equations to explain these observations. [Hint: ${\mathrm{Cu}}^{2+}$ reacts with ${\mathrm{NH}}_3$ to form $\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4^{2+}\cdot ]$

Short answer

Initially, ammonia reacts with the copper ions to form a white precipitate, which is Cu(OH)_2: $${\mathrm{Cu}}^{2+}+2{\mathrm{NH}}_3+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{Cu}(\mathrm{OH}{)}_2+2{\mathrm{NH}}_4^{+}$$ Upon adding more ammonia, the precipitate dissolves due to the formation of the Cu(NH3)4^2+ complex: $$\mathrm{Cu}(\mathrm{OH}{)}_2+4{\mathrm{NH}}_3\to [\mathrm{Cu}({\mathrm{NH}}_3{)}_4{]}^{2+}+2{\mathrm{OH}}^{-}$$

Step-by-step solution

Write the equation for the initial reaction of Cu^2+ with NH3

Initially, ammonia reacts with the copper ions, Cu^2+, present in the copper(II) nitrate solution to form a white precipitate, which is likely to be Cu(OH)_2. The balanced equation for this reaction can be written as: $${\mathrm{Cu}}^{2+}+2{\mathrm{NH}}_3+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{Cu}(\mathrm{OH}{)}_2+2{\mathrm{NH}}_4^{+}$$

Write the equation for the dissolution of the precipitate in excess NH3

Upon adding more ammonia to the solution, the white precipitate dissolves. This is due to the formation of Cu(NH3)4^2+ complex. The balanced equation for this dissolution reaction can be written as: $$\mathrm{Cu}(\mathrm{OH}{)}_2+4{\mathrm{NH}}_3\to [\mathrm{Cu}({\mathrm{NH}}_3{)}_4{]}^{2+}+2{\mathrm{OH}}^{-}$$ Now, we have obtained both of the balanced equations explaining the observations in this exercise.

Key concepts

Ammonia and Copper Reactions

When you combine ammonia, which is a common household cleaner, with copper(II) ions, some interesting chemical reactions occur. Understanding these reactions lies at the heart of several industrial and chemical processes. Initially, when you add ammonia to a solution containing copper(II) nitrate, a reaction between the copper ions (${\mathrm{Cu}}^{2+}$) and ammonia (${\mathrm{NH}}_3$) takes place.

The fascinating part is that, when you begin adding ammonia, it first reacts with the water in the solution, ultimately forming a white precipitate of copper(II) hydroxide, $\mathrm{Cu}(\mathrm{OH}{)}_2$. This is a suspensive reaction where copper ions combine with hydroxide ions, creating this insoluble compound. The balanced chemical equation for this reaction is:

• ${\mathrm{Cu}}^{2+}+2{\mathrm{NH}}_3+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{Cu}(\mathrm{OH}{)}_2+2{\mathrm{NH}}_4^{+}$

This formation of copper(II) hydroxide is why you see a white solid being formed first.

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where compounds in solution form an insoluble solid. This formation is known as a 'precipitate.' In the reaction involving ammonia and aqueous copper(II) nitrate, the white cloudy appearance you initially see is a classic example.

Precipitation occurs here because copper ions (${\mathrm{Cu}}^{2+}$) react with hydroxide ions (${\mathrm{OH}}^{-}$), which have been produced from the reaction between ammonia and water. The new compound, copper(II) hydroxide ($\mathrm{Cu}(\mathrm{OH}{)}_2$), doesn't dissolve in water, causing it to become a visible solid in the solution.

The presence of hydroxide ions is crucial for this precipitation to occur. This is important for understanding how we can predict whether a certain compound will precipitate or remain in solution based on its solubility. Such reactions are useful in a variety of scientific fields, including water treatment and the isolation of certain compounds in chemistry.

Complex Ion Formation

As you add more ammonia to the copper-ammonia solution, an intriguing thing happens: the precipitated copper(II) hydroxide dissolves again. This is because of the formation of a complex ion between copper ions and ammonia molecules, namely the $[\mathrm{Cu}({\mathrm{NH}}_3{)}_4{]}^{2+}$.

In chemistry, complex ions are ions that combine a central metal with one or more molecules or ions. Here, copper serves as the central metal, and ammonia acts as the ligand—it's the molecule that coordinates to the metal.

This formation explains why the earlier white precipitate disappears. The additional ammonia provides more ligands that coordinate around the copper ion, forming a stable, soluble complex ion. The balanced chemical equation for this dissolution process is:

• $\mathrm{Cu}(\mathrm{OH}{)}_2+4{\mathrm{NH}}_3\to [\mathrm{Cu}({\mathrm{NH}}_3{)}_4{]}^{2+}+2{\mathrm{OH}}^{-}$

These kinds of complex ion formations are essential in areas such as metal refining and pharmaceutical manufacturing, as they offer pathways to dissolve metals and manipulate metal ions in solution for various applications.