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 \(\left.\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}{ }^{2+} .\right]\)

Short answer

In summary, two balanced equations explaining the observations are: 1. Formation of the white precipitate (copper(II) hydroxide): \[Cu^{2+} + 2NH_3 + 2H_2O \rightarrow Cu(OH)_2 + 2NH_4^+\] 2. Dissolution of the precipitate (forming the tetraamminecopper(II) complex ion): \[Cu(OH)_2 + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+} + 2OH^-\]

Step-by-step solution

Formation of white precipitate

: Initially, when ammonia is added to copper(II) nitrate, a white precipitate forms. This is most likely due to the reaction between the copper(II) ions and hydroxide ions (formed from the reaction between ammonia and water) to form copper(II) hydroxide, which is a white precipitate. The reactions involved are: \[NH_3 + H_2O \rightarrow NH_4^+ + OH^-\] \[Cu^{2+} + 2OH^- \rightarrow Cu(OH)_2\] Combined, we can write the balanced equation for the formation of copper(II) hydroxide precipitate as: \[Cu^{2+} + 2NH_3 + 2H_2O \rightarrow Cu(OH)_2 + 2NH_4^+\]

Dissolution of the precipitate

: As more ammonia is added, the precipitate dissolves. This can be explained by the complexation of copper(II) ions with excess ammonia to form the tetraamminecopper(II) complex ion as hinted in the problem statement. The balanced equation for this reaction is: \[Cu(OH)_2 + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+} + 2OH^-\] So, when more ammonia is added to the solution, copper(II) hydroxide dissolves to form the tetraamminecopper(II) complex ion which remains in solution. In conclusion, the exercise requires us to write balanced equations for the formation and dissolution of a white precipitate upon the gradual addition of ammonia to aqueous copper(II) nitrate solution. Initially, copper(II) hydroxide precipitates are formed due to the reaction between copper(II) ions and hydroxide ions. As more ammonia is added, copper(II) hydroxide dissolves to form the soluble tetraamminecopper(II) complex ion.

Key concepts

Copper(II) Nitrate

Copper(II) Nitrate is a blue crystalline solid that dissolves in water to give copper ions (\((Cu^{2+}\)) and nitrate ions. In solution, copper(II) ions can interact with various substances, leading to different reactions.
When copper(II) nitrate is in an aqueous form, the copper ions are free to react with other ions or molecules added to the solution. This is an essential feature to understand when predicting or explaining chemical reactions involving copper solutions.
For instance, when adding ammonia (\(NH_3\)) to a copper(II) nitrate solution, you can witness some interesting transformations due to the copper ions and the properties of ammonia.

Ammonia

Ammonia is a small molecule with the formula \(NH_3\) and is known for its distinct, pungent smell. It is a weak base, meaning it accepts protons from water. This interaction produces ammonium ions (\(NH_4^+\)) and hydroxide ions (\(OH^-\)).

• The reaction can be represented by the equation: \[NH_3 + H_2O \rightarrow NH_4^+ + OH^-\]

These \(OH^-\) ions play a significant role in reactions with metal ions, precipitating as hydroxides initially.
In the case of copper(II) nitrate, the ammonia provides these hydroxide ions, leading initially to the formation of copper(II) hydroxide precipitate. As more ammonia is added, the nature of the reaction changes due to the ability of ammonia to form complex ions with metals.

Complex Ions

Complex ions are formed when a central metal ion binds with surrounding molecules or ions, known as ligands. These ligands donate electron pairs to the metal ion, enhancing its solubility and stability in solution.
In the exercise, with added ammonia, initially a white precipitate of copper(II) hydroxide forms. As more ammonia is added, these copper ions react further, forming the tetraamminecopper(II) complex ion: \([Cu(NH_3)_4]^{2+}\). This complex ion is soluble, explaining why the precipitate dissolves in the end.

• The reaction showing the formation of the complex ion is: \[Cu(OH)_2 + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+} + 2OH^-\]

This process highlights the significant role complex ion formation plays in solubility and helps in understanding the behavior of transition metal ions in solution.

Precipitation Reactions

Precipitation reactions occur when ions in a solution react to form a solid. The solid that forms is known as a precipitate, which is insoluble in the surrounding solution.
In the context of the exercise, a precipitation reaction happens when copper(II) ions (\(Cu^{2+}\)) react with hydroxide ions (\(OH^-\)) in the solution provided by ammonia:

• The balanced reaction equation is: \[Cu^{2+} + 2OH^- \rightarrow Cu(OH)_2\]

Copper(II) hydroxide is formed as a white precipitate when copper ions combine with the hydroxide ions.
However, as more ammonia is added, this solid dissolves because of the formation of a complex ion, a transition that reflects the balance between precipitation and complex formation reactions, showcasing dynamic chemical equilibria in solutions.