Problem 50 - A Complete Resource Book in Chemistry for JEE Main

Question

Out of \(\mathrm{Ca}^{2+}, \mathrm{Al}^{3+}, \mathrm{Cr}^{3+}, \mathrm{Mg}^{2+}\), and \(\mathrm{Zn}^{2+}\), the reagents \(\mathrm{NH}_{4} \mathrm{Cl}\) and aqueous \(\mathrm{NH}_{3}\) will precipitate (a) \(\mathrm{Ca}^{2+}, \mathrm{Al}^{3+}\) (b) \(\mathrm{Al}^{3+}, \mathrm{Cr}^{3+}\) (c) \(\mathrm{Bi}^{3+}, \mathrm{Mg}^{2+}\) (d) \(\mathrm{Mg}^{2+}, \mathrm{Zn}^{2+}\)

Step-by-Step Solution

Answer

The correct choice is (b): Al^{3+}, Cr^{3+}.

Step 1: Understanding Reagent Interaction

Ammonium chloride (NH_4Cl) and aqueous ammonia (NH_3) together form a complex system where the ammonia can act as a complexing agent to precipitate certain metal ions as hydroxides. This combination decreases the concentration of hydroxide ions, affecting the precipitation of metal hydroxides selectively.

Step 2: Analyzing Metal Ion Precipitation with Hydroxides

Al^{3+} and Cr^{3+} ions can form precipitates with hydroxide ions, as they form insoluble hydroxides such as Al(OH)_3 and Cr(OH)_3 at specific pH levels. This behavior makes them capable of forming a precipitate when treated with excess NH_3, even in the presence of NH_4Cl which buffers the solution.

Step 3: Evaluating the Behavior of Each Ion

- Ca^{2+} and Mg^{2+} do not readily precipitate as hydroxides in presence of NH_4Cl and NH_3 solution. - Zn^{2+} forms soluble complexes with ammonia. - Meanwhile, Al^{3+} and Cr^{3+} will precipitate as insoluble hydroxides, Al(OH)_3 and Cr(OH)_3 respectively, making them the correct answer in a mixture.

Step 4: Choosing the Correct Answer

Given the analysis, the ions Al^{3+} and Cr^{3+} will form precipitates with the provided reagents. Thus, the correct choice is option (b): Al^{3+}, Cr^{3+}.

Key Concepts

Metal Hydroxides

Metal hydroxides are compounds formed when metal ions react with hydroxide ions ( OH^- ). These compounds are often considered insoluble in water, meaning they fall out of solution as a solid precipitate. This characteristic is crucial in precipitation reactions.

Upon adding a combination of NH_4Cl (ammonium chloride) and aqueous NH_3 (ammonia), we alter the pH of the solution because NH_4Cl acts as a buffering agent. This buffer prevents drastic changes in pH that would otherwise increase the concentration of hydroxide ions rapidly.

Several metal ions form metal hydroxides at specific pH levels, where their insolubility leads to precipitation.

• Aluminum ( Al^{3+} ): Forms Al(OH)_3 , a white gelatinous precipitate.
• Chromium ( Cr^{3+} ): Forms Cr(OH)_3 , another insoluble compound.

Different metal ions have various solubility products, guiding which metal hydroxides will form under given conditions. Correctly predicting these interactions is fundamental in carrying out successful precipitation reactions.

Complexation

Complexation is a chemical process where molecules or ions, typically in the form of ligands, bind to a central metal atom or ion. In these interactions, multiple bonds form between the ligands and the metal ion, resulting in a complex.

Ligands are molecules that can donate electron pairs, and they coordinate with the metal ion to create a stable structure. This complex formation can greatly affect the solubility and chemical behavior of the metal ions.

Some metal ions have a strong tendency to form these complexes, and the ability to do so depends on factors such as:

• The metal ion's electronic configuration.
• The nature of the ligands involved.
• The overall charge and polarity of the complex.

When metal ions like zinc ( Zn^{2+} ) encounter ligands like ammonia in solution, they can form soluble complexes. This behavior explains why zinc does not precipitate under these conditions but rather stays dissolved.

Ammonia as a Complexing Agent

Ammonia ( NH_3 ) serves as an effective complexing agent in precipitation reactions. It has a lone pair of electrons that can coordinate, or "attach," to a metal ion, thereby forming a complex. This interaction can prevent the precipitation of certain metal ions as hydroxides.

The presence of ammonia changes the behavior of the solution, making it possible to selectively precipitate certain metal ions while keeping others in solution.

Here's how ammonia exercises its role as a complexing agent:

• Ammonia binds to metal ions, such as Zn^{2+} , forming soluble Zn(NH_3)_2^{2+} complexes.
• These complexes remain dissolved in solution instead of forming metal hydroxides.
• This dual behavior increases the selectivity of the reaction, as only specific ions that do not form strong complexes with ammonia will precipitate.

Thus, understanding ammonia's role is crucial in controlling which ions are removed from a solution via precipitation.