Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

Ammonia forms the complex ion \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\) with copper ions in the alkaline solutions, but not in acidic solutions. What is the reason for it? (a) in acidic solutions, hydration protects copper ions (b) the acidic solutions, protons coordinate with ammonia molecules forming \(\mathrm{NH}_{4}^{+}\)ions and \(\mathrm{NH}_{3}\) molecules are not available (c) in alkaline solutions, insoluble \(\mathrm{Cu}(\mathrm{OH})_{2}\) is precipitated which is soluble in excess of any alkali (d) copper hydroxide is an amphoteric substance

Step-by-Step Solution

Answer

In acidic solutions, ammonia forms \( \mathrm{NH}_4^+ \) ions, reducing availability for complex formation (Option b).

Step 1: Understanding the complex ion formation

The complex ion \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\) is formed when copper ions combine with four ammonia molecules. This reaction is favorable in alkaline solutions where ammonia molecules are readily available to form the complex.

Step 2: Analyzing conditions in acidic solutions

In acidic solutions, there are a high number of hydrogen ions (protons). These protons can react with ammonia molecules to form \(\mathrm{NH}_4^+\) ions, effectively reducing the concentration of free \(\mathrm{NH}_3\) available to form the complex with copper ions.

Step 3: Reviewing other options

Option (a) suggests hydration protects copper ions, but complex formation is about ligand availability rather than protection mechanisms. Option (c) talks about precipitation of \(\mathrm{Cu(\mathrm{OH})}_2\) in alkaline solutions, which doesn't prevent complex formation but instead is another separate reaction that can occur. Option (d) refers to amphoteric behavior, which is unrelated to ion complex formation in this context.

Step 4: Choosing the correct explanation

Since ammonia is protonated to form \(\mathrm{NH}_4^+\) in acidic solutions, reducing the availability of \(\mathrm{NH}_3\) to form the complex, the reason \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\) does not form in acidic solutions aligns with option (b).

Key Concepts

Ammonia Chemistry

Ammonia, represented chemically as \( \text{NH}_3 \), plays a crucial role in many chemical processes, particularly in forming complex ions. Ammonia molecules are known as ligands. A ligand is a molecule that can donate a pair of electrons to a metal ion, forming a coordination compound. Ammonia is particularly good at this due to its lone pair of electrons found on the nitrogen atom.
This makes ammonia highly effective in forming stable complexes. One typical example involves the copper ion \( \text{Cu}^{2+} \), which can combine with four ammonia molecules to create a complex ion \( [\text{Cu}(\text{NH}_3)_4]^{2+} \) in alkaline environments. In this process, ammonia "surrounds" the copper, effectively stabilizing it.

Ligand Availability

The formation of complex ions heavily depends on the availability of ligands like ammonia. In alkaline solutions, ammonia molecules are plentiful, and nothing impedes their ability to bind with metal ions like copper. This is because alkaline solutions generally have a low concentration of protons \( (\text{H}^+) \), allowing ammonia to remain in its molecular form \( (\text{NH}_3) \).
In contrast, in acidic solutions, the high concentration of protons reacts with ammonia molecules, forming ammonium ions \( (\text{NH}_4^+) \). This reaction reduces the number of free ammonia molecules available to act as ligands. Thus, in acidic environments, the formation of the complex ion \( [\text{Cu}(\text{NH}_3)_4]^{2+} \) is less favorable because the necessary ammonia ligands have been protonated.

Acidic and Alkaline Solutions

The nature of a solution—whether acidic or alkaline—can greatly influence chemical reactions, including complex ion formation. Acidic solutions are characterized by a high concentration of hydrogen ions \( (\text{H}^+) \), while alkaline solutions are dominated by hydroxide ions \( (\text{OH}^-) \).
In acidic solutions, the abundance of hydrogen ions competes with metal ions for interactions with ammonia. This leads to the protonation of ammonia, forming \( \text{NH}_4^+ \), and making ammonia unavailable for complex formation with metals like copper. Conversely, in alkaline solutions, these competing reactions do not occur, allowing ammonia to participate freely as a ligand.
Understanding these environmental factors is key to predicting when and where complex ion formation can occur effectively.

Protonation of Ammonia

Protonation refers to the addition of a proton \( (\text{H}^+) \) to a molecule. In the context of ammonia chemistry, protonation transforms ammonia \( (\text{NH}_3) \) into ammonium \( (\text{NH}_4^+) \). This process is significant as it changes the chemical reactivity and availability of ammonia molecules. When ammonia takes up a proton in acidic solutions, it loses its ability to function effectively as a ligand.
Once protonated, ammonia cannot donate its lone pair of electrons to metal ions, which prevents the formation of metal-ammonia complexes. Consequently, the reduction in free ammonia molecules impedes reactions that would lead to the creation of complex ions, illustrating the critical role of protonation in determining ammonia's behavior and effectiveness in different environments.