Problem 17 - Chemistry in 30 days

Question

Which of the following halides is most irsoluble in concentrated ammonia? (a) \(\mathrm{Ag} \mathrm{I}\) (b) \(\mathrm{AgF}\) (c) \(\mathrm{AgBr}\) (d) \(\mathrm{AgCl}\)

Step-by-Step Solution

Answer

AgI is the most insoluble in concentrated ammonia as it least forms complexes.

Step 1: Understand Solubility in Ammonia

To determine which silver halide is most insoluble in concentrated ammonia, we need to understand how these compounds interact with ammonia. Silver halides' solubility in ammonia generally relates to the formation of soluble complex ions.

Step 2: Relate Insolubility to Complex Formation

AgF is usually quite soluble in water and therefore also in ammonia, as it forms a complex with ammonia. AgCl, AgBr, and AgI have decreasing solubility in ammonia due to the lower tendency to form soluble complexes.

Step 3: Identify Least Complex Formation Tendency

The solubility of silver halides in concentrated ammonia can be ranked based on their ability to form the complex \[ ext{Ag(NH}_3)_2^+ \] ion. AgI is the least likely to form this complex, making it the most insoluble halide in concentrated ammonia.

Key Concepts

Ammonia Complex Formation

When silver halides are mixed with ammonia, they react by forming a complex ion. This process is known as ammonia complex formation. The important ion formed here is \( ext{Ag(NH}_3)_2^+\), a soluble complex ion.
A complex ion is essentially a compound consisting of a central metal atom, like silver, surrounded by other molecules or ions called ligands, such as ammonia. The ammonia molecules are attracted to the silver ion, which changes the solubility characteristics of the silver halide.
For some silver halides, the formation of this complex makes them more soluble leading to dissolution in ammonia solutions. However, not all silver halides equally engage in complex formation.

Silver Halides

Silver halides are a group of compounds composed of silver and halogen elements like chlorine, bromine, iodine, and fluorine.
These compounds include \( \mathrm{AgI},\ \mathrm{AgF},\ \mathrm{AgBr},\) and \(\mathrm{AgCl}\), characterized by their relatively low solubility in water.

• AgI (Silver iodide): Least soluble in ammonia.
• AgF (Silver fluoride): More soluble due to water solubility traits.
• AgBr (Silver bromide): Intermediate insolubility.
• AgCl (Silver chloride): Forms complex ion to some degree.

Each silver halide reacts differently with concentrated ammonia based on their individual ability to form soluble complexes.

Complex Ion Formation

Complex ion formation is central to understanding the behavior of silver halides in ammonia. Silver ions (Ag\(^+\)) have an affinity for ammonia molecules, forming the versatile complex icon \( ext{Ag(NH}_3)_2^+\).
The formation of these complex ions depends highly on the specific halide involved:

• Silver chloride (\( \mathrm{AgCl} \)): Moderately forms complex ions, increasing solubility.
• Silver bromide (\(\mathrm{AgBr}\)): Forms fewer complexes than AgCl, but still some.
• Silver iodide (\(\mathrm{AgI}\)): Least tendency to form complexes, making it more insoluble.
• Silver fluoride (\(\mathrm{AgF}\)): Readily forms complex, thus more soluble in water and ammonia.

Complex ion formation influences the ability of these compounds to dissolve in ammonia solutions.

Insolubility in Ammonia

The insolubility of silver halides in ammonia is crucial for identifying their capacity to refrain from forming complex ions. Silver iodide (\( \mathrm{AgI} \)) leads the series in terms of insolubility when introduced to concentrated ammonia.
For context, here's a succinct look into each halide's behavior in ammonia:

• \( \mathrm{AgF} \): Quite soluble due to complex formation.
• \( \mathrm{AgCl} \): Partially soluble with moderate complex ion creation.
• \(\mathrm{AgBr}\): Less soluble than \(\mathrm{AgCl}\), but more than \(\mathrm{AgI}\).
• \(\mathrm{AgI}\): Least reactive when it comes to forming complex ions, rendering it most insoluble.

Understanding these characteristics is important for predicting the solubility patterns of silver halides in various chemical environments.