Problem 1 - Chemical Principles in the Laboratory

Question

In testing the relative stabilities of Cu(II) species using a well plate, a student adds 6 drops of \(1.0 \mathrm{MNH}_{3}\) to 6 drops of \(0.10 \mathrm{M} \mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2} .\) He observes that a blue precipitate initially forms (after the first few drops of \(1.0 \mathrm{M} \mathrm{NH}_{3}\), but that in excess \(\mathrm{NH}_{3}\) (five or six drops) the precipitate dissolves and the solution turns deep blue. Addition of 6 drops of \(1.0 \mathrm{M} \mathrm{NaOH}\) to the deep blue solution results in the formation of a blue precipitate. a What is the formula of the Cu(II) species in the deep blue solution obtained with excess \(\mathrm{NH}_{3} ?\) b. What is the formula of the blue precipitate present after addition of \(1.0 \mathrm{M} \mathrm{NaOH} ?\) c. Which species is more stable in equal concentrations of \(\mathrm{NH}_{3}\) and \(\mathrm{OH}^{-}\), the one in Part \(1(\mathrm{a})\) or the one in Part \(1(b) ?\)

Step-by-Step Solution

Answer

The formula of the deep blue species is [Cu(NH3)4]^{2+}. The blue precipitate is Cu(OH)2. The [Cu(NH3)4]^{2+} species is more stable with excess NH3.

Step 1: Identify species interaction with NH3

When a small amount of NH3 is added to the Cu(NO3)2 solution, a pale blue precipitate of Cu(OH)2 begins to form. However, as excess NH3 is added (5-6 drops), the precipitate dissolves, and the solution turns deep blue, indicating the formation of a complex ion.

Step 2: Determine the complex formed in excess NH3

In the presence of excess NH3, the deep blue solution is due to the formation of the [Cu(NH3)4]^{2+} complex. This occurs because NH3 acts as a ligand, coordinating with the Cu(II) ion to form the stable tetraamminecopper(II) complex.

Step 3: Identify the precipitate formed with NaOH

Adding NaOH to the deep blue solution containing [Cu(NH3)4]^{2+} causes the precipitation of Cu(OH)2. This occurs because OH^- ions present in NaOH have a stronger ability to form the insoluble copper(II) hydroxide precipitate than the NH3 ligands.

Step 4: Compare stability of species

The [Cu(NH3)4]^{2+} complex ion in part (a) is more stable than the Cu(OH)2 precipitate in part (b) because the NH3 ligands form a stronger coordination complex with Cu(II) than OH^- under the conditions where NH3 is in excess.

Key Concepts

Complex Ion Formation

The process of complex ion formation plays a vital role in the chemistry of copper(II) ions. When ammonia ( NH_3 ) is added to copper(II) nitrate ( Cu(NO_3)_2 ), a pale blue precipitate of copper(II) hydroxide ( Cu(OH)_2 ) initially forms. As more ammonia is added in excess, the scenario changes significantly.
Complex ion formation occurs when the ammonia molecules coordinate with the copper ions. Ammonia acts as a ligand, meaning it donates pairs of electrons to the copper ion. This results in the formation of a complex ion. In our case, the deep blue color observed indicates the formation of the tetraamminecopper(II) ion, [Cu(NH_3)_4]^{2+} .

• This complex ion is highly stable in solution.
• Ammonia molecules surround the copper ion, forming a unique structure.
• The coordination of ammonia makes the copper species more soluble.

Ligand Exchange

Ligand exchange is a common process in the formation of complex ions, such as the case with copper(II) chemistry. When ammonia is added to the copper solution, it replaces the weaker ligands or ions initially coordinated to copper. This exchange allows for the formation of a more stable complex.
In the reaction with Cu(NO_3)_2 and ammonia, the nitrate ions and hydroxide ions are partly replaced by ammonia, forming [Cu(NH_3)_4]^{2+} . This shows that ligand exchange can significantly affect the chemical and physical properties of a metal ion solution.

• Stronger ligands like NH_3 tend to replace weaker ones like OH^{-} .
• This exchange stabilizes the complex ion in the solution.

Precipitation Reactions

Precipitation reactions are essential in understanding compound formations and stability in solutions. Initially, when copper(II) nitrate reacts with a small amount of ammonia, Cu(OH)_2 precipitates out. This occurs because the reaction produces an insoluble compound in water.
When ammonia is present in excess, the Cu(OH)_2 precipitate dissolves back into the solution because of complex ion formation. However, adding sodium hydroxide ( NaOH ) to the complex solution re-initiates a precipitation reaction, leading to the reformation of Cu(OH)_2 .

• Precipitate formation is driven by low solubility in the solvent.
• Precipitation can be reversed in the presence of a strong ligand.

Chemical Stability

Chemical stability varies across different copper(II) complexes depending on the ligands involved. In our observations, the [Cu(NH_3)_4]^{2+} complex formed in excess ammonia is more stable than the Cu(OH)_2 precipitate. This is because the bonding between ammonia and the copper ion creates a more stable structure.
The stability of a complex can be understood by the ligands' strength in coordinating with a metal ion. NH_3 provides a stronger bonding environment compared to OH^{-} . Hence, the complex ion remains more stable in solution conditions.

• Stable complexes often have high solubility.
• Ammonia ligands potentiate stability via strong coordination bonds.

Coordinated Complexes

Coordinated complexes, such as [Cu(NH_3)_4]^{2+} , are formed through the interaction of metal ions with ligands. In the case of copper(II) ions with ammonia, multiple ammonia molecules coordinate around the copper ion.
Coordination creates a geometric structure, often leading to visible changes like color, due to different absorption and emission of light energy. For example, the deep blue color indicates a particular arrangement and number of ammonia molecules bonded to copper.

• Coordination involves donating electron pairs to a central metal ion.
• The stability and properties depend on the types of ligands and their arrangements.