Problem 124 - A Complete Resource Book in Chemistry for JEE Main

Question

In the separation of \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cd}^{2+}\) in 2 nd group qualitative analysis of cations, tetrammine copper (II) sulphate and tetrammine cadmium(II) sulphate react with \(\mathrm{KCN}\) to form the corresponding cyano complexes. Which one of the following pairs of the complexes and their relative stability enables the separation of \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cd}^{2+} ?\) (a) \(\mathrm{K}_{2}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]\) less stable and \(\mathrm{K}_{2}\left[\mathrm{Cd}(\mathrm{CN})_{4}\right]\) more stable (b) \(\mathrm{K}_{3}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]\) more stable and \(\mathrm{K}_{2}\left[\mathrm{Cd}(\mathrm{CN})_{4}\right]\) less stable (c) \(\mathrm{K}_{3}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]\) less stable and \(\mathrm{K}_{2}\left[\mathrm{Cd}(\mathrm{CN})_{4}\right]\) more stable (d) \(\mathrm{K}_{2}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]\) more stable and \(\mathrm{K}_{2}\left[\mathrm{Cd}(\mathrm{CN})_{4}\right]\) less stable

Step-by-Step Solution

Answer

Option (b): \(\mathrm{K}_{3}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]\) more stable and \(\mathrm{K}_{2}\left[\mathrm{Cd}(\mathrm{CN})_{4}\right]\) less stable, allowing separation based on stability differences.

Step 1: Understanding the Problem

We need to separate \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cd}^{2+}\) based on the formation of cyano complexes. The key lies in the relative stability of the cyano complexes formed with \(\mathrm{KCN}\): tetrammine copper(II) complex and tetrammine cadmium(II) complex.

Step 2: Recognizing Stability Patterns

In general, copper forms very stable cyanide complexes, while cadmium's cyanide complexes are less stable in comparison. The stability of these complexes allows for the selective precipitation or solubility of one cation over the other.

Step 3: Analyzing Given Options

We have four options. We need the one where the copper cyanide complex is more stable than the cadmium cyanide complex. This would allow copper to remain in solution, facilitating separation.

Step 4: Checking Option (a)

Option (a) states \(\mathrm{K}_{2}[\mathrm{Cu}(\mathrm{CN})_{4}]\) is less stable and \(\mathrm{K}_{2}[\mathrm{Cd}(\mathrm{CN})_{4}]\) more stable. This is incorrect based on general knowledge of cyanide complex stability.

Step 5: Checking Option (b)

Option (b) states \(\mathrm{K}_{3}[\mathrm{Cu}(\mathrm{CN})_{4}]\) is more stable and \(\mathrm{K}_{2}[\mathrm{Cd}(\mathrm{CN})_{4}]\) less stable. This supports the required separation of \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cd}^{2+}\) since \(\mathrm{Cu}^{2+}\) will remain in solution.

Step 6: Conclusion

Based on the stability required for separation, option (b) is the correct answer. \(\mathrm{Cu}^{2+}\) forms a more stable complex allowing it to remain dissolved, while \(\mathrm{Cd}^{2+}\) can be precipitated or detected separately.

Key Concepts

Qualitative Analysis

Qualitative analysis in inorganic chemistry is a powerful tool used to identify and distinguish between different cations present in a mixture. Essentially, it involves a series of separation and detection reactions to spot different metal ions based on their unique chemical properties. This method relies heavily on observing reactions that form precipitates, evolve gases, or change colors.

• Identification of cations: By exploiting each cation's characteristic reactions, we can systematically separate and identify ions like \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cd}^{2+}\). For instance, \(\mathrm{Cu}^{2+}\) can form complex ions with ammonia, recognizable by a deep blue color indicative of a coordination complex.
• Role of reagents: Specific reagents are introduced to trigger reactions that differentiate one cation from another. For example, adding \(\mathrm{KCN}\) induces distinct reactions in copper and cadmium due to their complexing abilities, as seen in this exercise.

Qualitative analysis thus helps decipher the components of an unknown sample, paving the way for further chemical understanding. It's a crucial practice in laboratory settings, aiding in research and practical applications.

Cation Separation

Cation separation is a critical process within qualitative analysis. In mixtures containing multiple metal ions, distinguishing between them involves understanding differences in chemical reactivity and stability of complexes. Here’s how separation works:

• Precipitation Reactions: The principle behind cation separation often involves precipitating one cation while keeping others in solution. By altering variables such as pH or adding specific reagents, selective precipitation becomes possible.
• Complexation Reactions: In this exercise, the use of \(\mathrm{KCN}\) allows copper and cadmium to form complexes, which differ in their solubility and stability. This difference enables separation as copper's complex remains soluble while cadmium can be precipitated.

Separation allows chemists to identify cations individually by analyzing their behavior in isolation. In essence, it relies on exploiting distinct chemical properties to achieve selective isolation, a pivotal step in qualitative analysis.

Coordination Complex Stability

Coordination complex stability plays a significant role in the separation of cations through qualitative analysis. Stability here means how well a complex ion stays intact when it forms.

• Factors Influencing Stability: Various factors determine the stability of coordination complexes, such as the metal ion size, charge, and the ligands involved. A smaller and highly charged metal ion tends to form more stable complexes.
• Stability and Separation: In the exercise, \(\mathrm{Cu}^{2+}\) forms a more stable cyanide complex compared to \(\mathrm{Cd}^{2+}\). This difference in stability allows copper to stay in solution, making it easier to separate from cadmium which precipitates.
• Ligand Effects: Ligands that have a strong binding affinity contribute to greater stability in complexes. Cyanide ions, in this case, create highly stable complexes with \(\mathrm{Cu}^{2+}\), aiding in distinguishing and separating it from \(\mathrm{Cd}^{2+}\).

Understanding complex stability is fundamental in qualitative analysis, enabling chemists to manipulate conditions for effective separation and identification of metal ions.