Problem 1
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?
Step-by-Step Solution
Verified Answer
a) [Cu(NH3)4]2+, b) Cu(OH)2, c) [Cu(NH3)4]2+ is more stable.
1Step 1: Initial Reaction Observation
The student adds 6 drops of \(1 \mathrm{M} \mathrm{NH}_{3}\) to 6 drops of \(0.1 \mathrm{M} \mathrm{Cu}(\mathrm{NO}_{3})_{2}\). A blue precipitate initially forms. The possible reaction is: \[\mathrm{Cu}^{2+} + 2\mathrm{OH}^{-} \rightarrow \mathrm{Cu}(\mathrm{OH})_{2(s)}\] indicating the formation of copper(II) hydroxide, \(\mathrm{Cu}(\mathrm{OH})_2\).
2Step 2: Formation of Dark-Blue Solution in Excess NH3
Upon adding excess \(\mathrm{NH}_{3}\), the blue precipitate dissolves to form a dark-blue solution. This solution is likely \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\), which indicates the formation of the tetraamminecopper(II) complex: \[\mathrm{Cu}(\mathrm{OH})_{2(s)} + 4\mathrm{NH}_{3} \rightarrow [\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+} + 2\mathrm{OH}^-\].
3Step 3: Reaction with NaOH
Adding \(\mathrm{NaOH}\) to the dark-blue solution results in precipitation, indicating reformation of \(\mathrm{Cu}(\mathrm{OH})_2\) as \(\mathrm{Cu}(\mathrm{OH})_{2(s)}\) precipitates from the dissociation of the ammine complex due to the competition of \(\mathrm{OH}^-\) ions: \[[\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+} + 2\mathrm{OH}^- \rightarrow \mathrm{Cu}(\mathrm{OH})_{2(s)} + 4\mathrm{NH}_3\].
4Step 4: Stability Analysis
\([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\) is stable in the presence of excess \(\mathrm{NH}_3\), while \(\mathrm{Cu}(\mathrm{OH})_2\) precipitates in the presence of excess \(\mathrm{OH}^-\). The stability depends on the ligand and its concentration. In the case where \(\mathrm{NH}_3\) and \(\mathrm{OH}^-\) are present in equal concentrations without excess, \(\mathrm{Cu}(\mathrm{OH})_2\) later precipitates again, implying it's less stable in such conditions compared to \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\).
Key Concepts
Complex IonsLigand SubstitutionEquilibrium Reactions
Complex Ions
In coordination chemistry, complex ions are an essential concept when understanding the interactions between metal ions and ligands. A complex ion consists of a metal ion connected to several molecules or ions, known as ligands.
For instance, in our exercise, we observe the formation of the tetraamminecopper(II) complex \[[\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\]. This complex ion is formed when copper(II) hydroxide, \(\mathrm{Cu}(\mathrm{OH})_2\), reacts with an excess amount of ammonia, \(\mathrm{NH}_3\). Here, ammonia acts as a ligand, replacing the hydroxide ions initially coordinated to the copper ion.
These ligands can significantly influence the properties, such as color, solubility, and stability, of the metal complexes. In the case of the tetraamminecopper(II), the complex's deep blue color is a distinctive feature indicating the presence of this particular complex ion.
For instance, in our exercise, we observe the formation of the tetraamminecopper(II) complex \[[\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\]. This complex ion is formed when copper(II) hydroxide, \(\mathrm{Cu}(\mathrm{OH})_2\), reacts with an excess amount of ammonia, \(\mathrm{NH}_3\). Here, ammonia acts as a ligand, replacing the hydroxide ions initially coordinated to the copper ion.
These ligands can significantly influence the properties, such as color, solubility, and stability, of the metal complexes. In the case of the tetraamminecopper(II), the complex's deep blue color is a distinctive feature indicating the presence of this particular complex ion.
Ligand Substitution
Ligand substitution refers to the process where one ligand in a complex ion is replaced by another. This process is vital in coordinating chemistry as it can alter the properties of the metal complex significantly.
In the given exercise, as ammonia \(\mathrm{NH}_3\) is added to the initially formed copper hydroxide precipitate, the ligand substitution occurs. The hydroxide ligands in copper(II) hydroxide \(\mathrm{Cu}(\mathrm{OH})_2\) are replaced by ammonia ligands, resulting in the formation of the \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\) complex ion.
This type of reaction highlights the dynamic interactions that occur in solution involving metal complexes. The ability of a complex to undergo ligand substitution can be affected by various factors, including the nature of the metal ion, the identity of the ligands, and the conditions of the reaction environment, such as pH.
In the given exercise, as ammonia \(\mathrm{NH}_3\) is added to the initially formed copper hydroxide precipitate, the ligand substitution occurs. The hydroxide ligands in copper(II) hydroxide \(\mathrm{Cu}(\mathrm{OH})_2\) are replaced by ammonia ligands, resulting in the formation of the \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\) complex ion.
This type of reaction highlights the dynamic interactions that occur in solution involving metal complexes. The ability of a complex to undergo ligand substitution can be affected by various factors, including the nature of the metal ion, the identity of the ligands, and the conditions of the reaction environment, such as pH.
Equilibrium Reactions
Equilibrium reactions in coordination chemistry refer to a state where the formation and dissociation of complexes occur at the same rate. This balance influences the stability and predominant species in a solution.
In our exercise, the equilibrium reaction comes into play when ammonia is present in excess, converting copper(II) hydroxide into \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\). The system achieves an equilibrium where the complex is stable and prevalent.
However, when \(1 \mathrm{M} \mathrm{NaOH}\) is added, the system's equilibrium shifts. The hydroxide ions compete with ammonia, leading to the precipitation of \(\mathrm{Cu}(\mathrm{OH})_2\) once again as the dominant species. This outcome underscores the reversible nature of coordination complexes, where changing the conditions, such as ligand concentration, can significantly impact equilibrium and stability.
In our exercise, the equilibrium reaction comes into play when ammonia is present in excess, converting copper(II) hydroxide into \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\). The system achieves an equilibrium where the complex is stable and prevalent.
However, when \(1 \mathrm{M} \mathrm{NaOH}\) is added, the system's equilibrium shifts. The hydroxide ions compete with ammonia, leading to the precipitation of \(\mathrm{Cu}(\mathrm{OH})_2\) once again as the dominant species. This outcome underscores the reversible nature of coordination complexes, where changing the conditions, such as ligand concentration, can significantly impact equilibrium and stability.