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) \(\mathrm{Mg}^{2+}, \mathrm{Zn}^{2+}\)

Short answer

Option (b) \(\mathrm{Al}^{3+}, \mathrm{Cr}^{3+}\) will precipitate.

Step-by-step solution

Understand the role of NH4Cl and aqueous NH3

The addition of \(\mathrm{NH}_4\mathrm{Cl}\) and aqueous \(\mathrm{NH}_3\) is typically used to adjust the pH of the solution to promote the selective precipitation of certain metal hydroxides. \(\mathrm{NH}_4\mathrm{Cl}\) provides \(\mathrm{NH}_4^+\) ions which prevent the formation of hydroxide ions by common-ion effect, and \(\mathrm{NH}_3\) is a weak base that leads to slight precipitation of hydroxides of certain cations.

Identify cations that precipitate in ammonia medium

In an ammonia medium, the hydroxides of certain cations such as \(\mathrm{Al}^{3+}\), and \(\mathrm{Cr}^{3+}\) are less soluble, leading to their precipitation. \(\mathrm{Ca}^{2+}\), \(\mathrm{Mg}^{2+}\), and \(\mathrm{Zn}^{2+}\) do not readily precipitate under these conditions, as they form more soluble complexes or do not react in the ammonia medium.

Evaluate the provided answer options

We need to find which pair contains cations that both precipitate with \(\mathrm{NH}_4\mathrm{Cl}\) and \(\mathrm{NH}_3\). From Step 2, \(\mathrm{Al}^{3+}\) and \(\mathrm{Cr}^{3+}\) both form insoluble hydroxides and will precipitate. The correct option is therefore (b) \(\mathrm{Al}^{3+}, \mathrm{Cr}^{3+}\).

Confirm and conclude

Reagents \(\mathrm{NH}_4\mathrm{Cl}\) and aqueous \(\mathrm{NH}_3\) lead to the precipitation of \(\mathrm{Al}^{3+}\) and \(\mathrm{Cr}^{3+}\) as their hydroxides in aqueous solutions. These hydroxides are relatively insoluble compared to the other cations in the options given.

Key concepts

Metal Hydroxides Precipitation

Metal hydroxides are compounds formed by the reaction between a metal ion and hydroxide ions \(\text{OH}^-\). Precipitation occurs when these metal hydroxides are not soluble in a particular solution. When the concentration of ions in a solution exceeds the solubility product (\(K_{sp}\)) of a specific compound, that compound will precipitate out of the solution. For example, in the context of \(\text{Al}^{3+}\) and \(\text{Cr}^{3+}\), both form metal hydroxides that have low solubility in aqueous solutions, making them candidates for precipitation. In a solution, small changes in conditions like pH or the presence of other ions can lead to the precipitation of metal hydroxides as solids.

• Precipitation is a process of forming a solid from a solution.
• For precipitation, the product of ion concentrations must exceed the compound's solubility product.
• Precipitation is often influenced by temperature, concentration of ions, and pH of the solution.

Ammonia Medium

An ammonia medium is a solution containing ammonia (\(\text{NH}_3\)) which acts as a weak base. It is frequently used to manipulate the solubility of certain metal hydroxides in solutions. Ammonia can alter the pH of a solution which is vital since many metals precipitate as hydroxides at different pH levels. In particular, ammonia can complex with certain ions to form soluble amine complexes, thus preventing their precipitation. However, not all metal ions react with ammonia in this manner. For instance, \(\text{Al}^{3+}\) and \(\text{Cr}^{3+}\) do not easily form these complexes and their hydroxides remain insoluble, thus precipitating out of the solution in an ammonia medium.

• Ammonia acts as a base by accepting protons.
• Changes in pH due to ammonia can lead to selective precipitation of metals.
• Metal ions may form additional complexes with ammonia, affecting solubility.

Common-Ion Effect

The common-ion effect refers to the phenomenon where the solubility of a salt is reduced by the presence of another salt that contains a common ion. This effect is important in controlling precipitation reactions. In this context, the addition of \(\text{NH}_4\text{Cl}\) introduces \(\text{NH}_4^+\) ions, which is a common ion when working with ammonium hydroxide solutions. The presence of \(\text{NH}_4^+\) in the solution suppresses the ionization of \(\text{NH}_3 + \text{H}_2\text{O} \rightarrow \text{NH}_4^+ + \text{OH}^-\), limiting the availability of \(\text{OH}^-\) ions and lowering the solution's pH.

• The common-ion effect leads to decreased solubility of certain compounds.
• Presence of a common ion reduces ionization of a weak electrolyte.
• It is a useful tool in selective precipitation processes.

Cation Solubility

Cation solubility refers to the ability of a cation to dissolve in a solution. This property is influenced by various factors including the nature of the cation, temperature of the solution, presence of other ions, and the pH level. In aqueous solutions, cation solubility determines whether a particular ion will remain dissolved or will precipitate as an insoluble compound, like a hydroxide. For example, the solubility of \(\text{Al}^{3+}\) and \(\text{Cr}^{3+}\) in the presence of ammonium hydroxide is extremely low, causing their precipitation as hydroxides. But, other cations like \(\text{Mg}^{2+}\) may remain soluble or form soluble complexes under the same conditions.

• Cation solubility is influenced by other ions present in the solution.
• The chemical nature and charge of the cation play crucial roles.
• pH levels can drastically alter metal cation solubility.

Selective Precipitation

Selective precipitation is a process used to separate ions from a solution, by precipitating them as insoluble compounds, based on their differences in solubility. This technique is often useful in analytical chemistry to separate metals in a mixture. Through careful adjustment of conditions such as pH or addition of specific reagents, only certain cations will precipitate while others remain in solution. For instance, in an ammonia medium, \(\text{Al}^{3+}\) and \(\text{Cr}^{3+}\) can be selectively precipitated as their hydroxides, leaving behind more soluble cations like \(\text{Mg}^{2+}\) and \(\text{Zn}^{2+}\).

• Conditions are adjusted to favor the precipitation of specific ions.
• Selective precipitation helps in the separation and analysis of ions.
• It is a critical technique for purification and recovery of specific metals.