Question

In testing the relative stabilities of Cu(II) species using a well plate, a student adds 6 drops \(1 \mathrm{M} \mathrm{NH}_{3}\) to 6 drops 0.1 \(\mathrm{M} \mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\). He observes that a blue precipitate initially forms, but that in excess \(\mathrm{NH}_{3}\) the precipitate dissolves and the solution turns blue. Addition of 6 drops \(1 \mathrm{M} \mathrm{NaOH}\) to the dark-blue solution results in the formation of a blue precipitate. a. What is the formula of Cu(II) species in the dark-blue solution? b. What is the formula of the blue precipitate present after addition of \(1 \mathrm{M} \mathrm{NaOH} ?\) c. Which species is more stable in equal concentrations of \(\mathrm{NH}_{3}\) and \(\mathrm{OH}^{-}\), the one in Part a or the one in Part b? _______

Short answer

a. The Cu(II) species in the dark-blue solution is \(\mathrm{[Cu}(\mathrm{NH}_{3})_{4}]^{2+}\). b. The blue precipitate after addition of \(1 \mathrm{M} \mathrm{NaOH}\) is \(\mathrm{Cu}(\mathrm{OH})_{2}\). c. In equal concentrations of \(\mathrm{NH}_{3}\) and \(\mathrm{OH}^{-}\), the species \(\mathrm{Cu}(\mathrm{OH})_{2}\) is more stable than \(\mathrm{[Cu}(\mathrm{NH}_{3})_{4}]^{2+}\).

Step-by-step solution

Reaction of copper with ammonia

When a student adds ammonia to the copper(II) nitrate solution, a blue precipitate forms initially. This is due to the formation of copper(II) hydroxide: \[ \mathrm{Cu}^{2+} + 2\mathrm{NH}_{3} + 2\mathrm{H}_{2}\mathrm{O} \longrightarrow \mathrm{Cu}(\mathrm{OH})_{2} + 2\mathrm{NH}_{4}^{+} \] However, in excess ammonia, the precipitate dissolves, and the solution turns dark blue. This indicates the formation of a soluble Cu(II)-ammine complex. The balanced chemical equation for this reaction is: \[ \mathrm{Cu}(\mathrm{OH})_{2} + 4\mathrm{NH}_{3} \longrightarrow \mathrm{[Cu}(\mathrm{NH}_{3})_{4}]^{2+} + 2\mathrm{OH}^{-} \]

Reaction of copper with hydroxide

When sodium hydroxide is added to the dark-blue solution, a blue precipitate is formed. This is due to the reaction of copper(II) ammine complex with hydroxide ions: \[ \mathrm{[Cu}(\mathrm{NH}_{3})_{4}]^{2+} + 2\mathrm{OH}^{-} \longrightarrow \mathrm{Cu}(\mathrm{OH})_{2} + 4\mathrm{NH}_{3} \] This equation represents the formation of a blue precipitate of copper(II) hydroxide after the addition of sodium hydroxide.

Answering the questions

a. The formula of the Cu(II) species in the dark-blue solution is \(\mathrm{[Cu}(\mathrm{NH}_{3})_{4}]^{2+}\). b. The formula of the blue precipitate present after the addition of \(1 \mathrm{M} \mathrm{NaOH}\) is \(\mathrm{Cu}(\mathrm{OH})_{2}\). c. To determine which species is more stable in equal concentrations of \(\mathrm{NH}_{3}\) and \(\mathrm{OH}^{-}\), let's examine the two reactions: 1. Formation of copper(II) ammine complex: \[ \mathrm{Cu}(\mathrm{OH})_{2} + 4\mathrm{NH}_{3} \longrightarrow \mathrm{[Cu}(\mathrm{NH}_{3})_{4}]^{2+} + 2\mathrm{OH}^{-} \] 2. Formation of copper(II) hydroxide: \[ \mathrm{[Cu}(\mathrm{NH}_{3})_{4}]^{2+} + 2\mathrm{OH}^{-} \longrightarrow \mathrm{Cu}(\mathrm{OH})_{2} + 4\mathrm{NH}_{3} \] Comparing these two reactions, we notice that they are the reverse of each other, and the species in Part b (\(\mathrm{Cu}(\mathrm{OH})_{2}\)) is more stable than the species in Part a (\(\mathrm{[Cu}(\mathrm{NH}_{3})_{4}]^{2+}\)) when equal concentrations of \(\mathrm{NH}_{3}\) and \(\mathrm{OH}^{-}\) are present.

Key concepts

Copper(II) Complexes

Copper(II) complexes are interesting species and often play a vital role in transition metal chemistry. One common example is the copper(II)-ammine complex. When copper ions interact with ammonia, they can form a deep blue complex known as \([\text{Cu}(\text{NH}_3)_4]^{2+}\). This complex is significant because it showcases transition metals' ability to form coordination compounds with various ligands. The blue color of the complex is due to electronic transitions within the copper ions, which is a characteristic property of transition metals. Copper in this complex acts as a central metal ion surrounded by nitrogen atoms from ammonia, which act as ligands. This ability to form such colorful complexes makes copper unique and of great interest for various applications, including analytical chemistry and qualitative analysis.

Learning about copper(II) complexes is essential because it helps understand how copper interacts with different chemical species and forms stable compounds displayed in vivid colors during chemical reactions.

Precipitation Reactions

Precipitation reactions are an important part of chemistry and occur when two soluble substances react to form an insoluble product known as a precipitate. In the given experiment, copper(II) hydroxide \(\text{Cu}(\text{OH})_2\) precipitates when copper nitrate is treated with ammonia and later with sodium hydroxide. This blue precipitate formation is visible and helps in identifying the copper species in solution.

These reactions are essential for separating and identifying ions in a solution. When ammonia is in excess, copper(II) hydroxide dissolves to form a soluble complex, demonstrating how equilibrium can shift based on the conditions and concentrations of reactants. Repetition of this phenomenon with sodium hydroxide showcases the reversible nature of precipitation reactions and the importance of concentration and solubility in determining the final state of the chemical system.

Understanding precipitation reactions allows students to see practically how reactions proceed and how products are formed and altered by changing conditions.

Chemical Stability

Chemical stability refers to a compound's tendency to remain unchanged over time under specific conditions. In the experiment, two copper(II) species are compared: the copper(II) ammine complex \([\text{Cu}(\text{NH}_3)_4]^{2+}\) and copper(II) hydroxide \(\text{Cu}(\text{OH})_2\).

The stability of these species is context-dependent, emphasizing the formation of stable products under given conditions. Here, copper(II) hydroxide precipitates in the presence of sodium hydroxide, indicating a more stable form under these particular conditions compared to the copper(II) ammine complex. Stability can depend greatly on external factors like pH, temperature, and the presence of ligands.

The understanding of chemical stability is crucial in predicting reaction outcomes and is intimately linked with concepts like solubility, ligand concentration, and the general chemical environment.

Coordination Chemistry

Coordination chemistry studies the structure and properties of complexes formed through the bond between a central atom and surrounding molecules or ions, known as ligands. It is a central aspect of transition metal chemistry. In the experiment, copper acts as a central metal atom forming a coordination compound with ammonia, a ligand.

Coordination compounds like \([\text{Cu}(\text{NH}_3)_4]^{2+}\) arise when ammonia molecules use their lone electron pairs to bond with copper ions. The number and type of ligands attached to the central atom define the complex's geometry and properties.

Understanding coordination chemistry is vital for many fields, including catalysis, material science, and bioinorganic chemistry. It applies the principles of Lewis acid-base interactions, where the central metal ion accepts electrons from ligands, forming stable complexes with distinct physical and chemical properties.