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 (d) is correct: \\(\mathrm{K}_{2}[\mathrm{Cu}(\mathrm{CN})_{4}]\\) more stable, \\(\mathrm{K}_{2}[\mathrm{Cd}(\mathrm{CN})_{4}]\\) less stable.

Step 1: Understanding Complex Formation

When \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cd}^{2+}\) react with \(\mathrm{KCN}\), they form stable cyano complexes: \(\mathrm{Cu(CN)}_4^{3-}\) and \(\mathrm{Cd(CN)}_4^{2-}\). In this analysis, the stability of these complexes is crucial for separation.

Step 2: Evaluate Stability of Complexes

The relative stability of cyano complexes often depends on the metal involved. \(\mathrm{Cu(CN)}_4^{3-}\) is known to be more stable compared to \(\mathrm{Cd(CN)}_4^{2-}\). This relative stability prevents the copper complex from decomposing easily, enabling separation based on the difference in stability.

Step 3: Match to Given Options

From the provided options, option (d) states that \(\mathrm{K}_{2}[\mathrm{Cu}(\mathrm{CN})_{4}]\) is more stable, and \(\mathrm{K}_{2}[\mathrm{Cd}(\mathrm{CN})_{4}]\) is less stable. This matches the observed stability and allows for the effective separation of \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cd}^{2+}\).

Key Concepts

Separation Techniques

Separation techniques in chemistry are methods used to separate components in a mixture. These are crucial when dealing with complex mixtures. In qualitative analysis, especially when analyzing cations like \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cd}^{2+}\), separation techniques allow chemists to identify, and further analyze the components present. A common technique involves the use of selective precipitation and dissolution based on solubility differences.
This is where understanding the relative solubilities and stabilities of different compounds can aid separation.
By creating environments that allow for selective complex formation or precipitation, we achieve desired separations. In our context, differences in cyano complex stability enable efficient separation between similar cations.
Thus ensuring accurate qualitative analysis outcomes and preventing complications during analysis.

Cyano Complexes

Cyano complexes are coordination compounds in which cyanide ions (\(\mathrm{CN}^-\)) act as ligands, bonding to a central metal atom or ion. The ligand is versatile and binds firmly, utilizing lone pair electrons present on the carbon of cyanide to form strong metal-ligand bonds.
In transition metals, due to their incomplete d-orbitals, cyano complexes adjust the electronic structure, impacting the compound's properties.
In the presence of \(\mathrm{KCN}\), both copper and cadmium form cyano complexes: \[\mathrm{Cu}(\mathrm{CN})_4^{3-}\] and \[\mathrm{Cd}(\mathrm{CN})_4^{2-}\].
The \(\mathrm{Cu}\) complex happens to be a bit more stable, which hints at the stronger bonding ability of cyanide ligands with copper, showcasing their pivotal role in separations.

Complex Stability

In the context of cyano complexes, stability is often determined by the nature of the metal ion and the binding strength of the cyanide ligands. Stability heavily influences separation processes and is dictated by several factors:

• Charge on the metal ion: Higher charges typically lead to stronger ligand-metal bonds.
• Electron configuration: Transition metals with partially filled d-orbitals can form particularly stable complexes.
• Nature of ligands: Ligands like cyanide, which can form multiple bonds, enhance stability through pi-backbonding.

Copper complexes, with a \(\mathrm{CN}^{2-}\) ligand, exhibit heightened stability due to \(\pi\)-backbonding. This peculiarity makes cyano complexes invaluable in separations, as their stability assists in differentiating compounds such as separating copper from cadmium by avoiding decomposition.

Transition Metal Chemistry

Transition metals are unique elements characterized by d-orbitals that afford them their distinctive chemical properties. Due to partially filled d subshells, these metals exhibit high versatility in forming complex ions with various ligands.
The electron configuration leads to significant chemical diversity, affecting properties like color, magnetic behavior, and reactivity.
In separation processes, the complex chemistry of transition metals comes into play.
They form stable complexes, such as those formed with cyanide in qualitative analyses, making o them central to separation techniques.
For compounds like \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cd}^{2+}\), their ability to form unique stable complexes aids in their identification and separation, proving transition metal chemistry to be vital within analytical processes.