Problem 67 - Chemistry: An Atoms-Focused Approach

Question

A lab technician cleaning glassware that contains residues of AgCl washes the glassware with an aqueous solution of ammonia. The \(\mathrm{AgCl}\), which is insoluble in water, rapidly dissolves in the ammonia solution. Why?

Step-by-Step Solution

Answer

Answer: AgCl dissolves rapidly in an aqueous solution of ammonia due to the formation of soluble complex ions. When AgCl comes in contact with ammonia (NH3), it reacts to form a soluble complex ion [Ag(NH3)_2]^+ and a chloride ion (Cl^-). The formation of these soluble complex ions stabilizes the silver ions and facilitates their dissolution in the ammonia solution, causing AgCl to rapidly dissolve.

Step 1: Write the chemical formulas for AgCl and ammonia solution

First, let's identify the chemical formulas for the given compounds. AgCl refers to silver chloride, and ammonia solution is an aqueous solution of NH3.

Step 2: Identify the reaction taking place

When AgCl comes in contact with ammonia solution (NH3), a complex ion formation reaction takes place, which solubilizes the AgCl.

Step 3: Write the complex formation reaction

Now let's write the complex formation reaction between AgCl and NH3: AgCl (s) + 2NH3 (aq) -> [Ag(NH3)_2]^+ (aq) + Cl^- (aq)

Step 4: Explain the solubility of AgCl in ammonia solution

In this reaction, AgCl, which is insoluble in water, reacts with ammonia (NH3) to form a soluble complex ion [Ag(NH3)_2]^+ and a chloride ion (Cl^-). The formation of soluble complex ions successfully causes the dissolution of AgCl in the ammonia solution. This is the reason why AgCl rapidly dissolves in the ammonia solution, even though it is insoluble in water. The solubility of AgCl in ammonia is due to the formation of soluble complex ions that stabilize the silver ions and facilitate their dissolution in the solution.

Key Concepts

Silver Chloride Solubility

Silver chloride (\( \text{AgCl} \)) is well-known for being one of those compounds that are poorly soluble in water. This means that when you try to dissolve silver chloride in water, it generally doesn't work well, and the compound will remain as a solid. Silver chloride forms a white precipitate, which indicates its lack of solubility. The chemistry behind this behavior lies in the ionic lattice structure of \( \text{AgCl} \).

Although \( \text{AgCl} \)is not soluble in water, under certain conditions it can become soluble. One such condition is when an ammonia solution is introduced. The addition of ammonia causes the formation of a complex ion with the silver ion, which essentially changes the chemistry of the solution and makes the \( \text{AgCl} \) dissolve.

• This is different from simply dissolving by breaking down the compound; rather, it's forming a new species that is soluble.
• Such transformations of solubility properties are often discussed in the context of complex ion formation.

Ammonia Solution Chemistry

Ammonia (\( \text{NH}_3 \)) in solution has distinct properties due to its ability to act as a ligand. This means that ammonia can form complexes with certain metal ions.

When ammonia is dissolved in water, it typically forms ammonium and hydroxide ions, but it also maintains some of its molecules as they are, which can interact with metal ions like silver. This interaction can alter the solubility of normally insoluble salts like silver chloride.

The mechanism of action rests on the coordination chemistry of ammonia.

• Ammonia acts as a ligand bond donor to the silver ions.
• This results in what's known as a chelating effect, where multiple ammonia molecules coordinate with a single silver ion.

These ammonia molecules surround the silver ion, forming a complex ion, namely \([\text{Ag(NH}_3)_2]^+ \). This reaction stabilizes the silver ions in solution, thereby increasing the overall solubility.

Chemical Reactions in Solutions

Chemical reactions in solutions often involve the interplay between reagents, solvents, and sometimes even unintended components from the surrounding environment. Solubility and complex ion formation are key topics in solutions chemistry that illustrate why certain reactions occur.

When \( \text{AgCl} \) is treated with an ammonia solution, a new soluble complex is formed, transforming the initially insoluble salt into dissolvable ions. The actual chemical reaction that takes place here can be expressed as:\[ \text{AgCl (s) + 2NH}_3 \text{ (aq) } \rightarrow [\text{Ag(NH}_3)_2]^+ \text{ (aq) } + \text{Cl}^- \text{ (aq) } \]

• Silver ions (\( \text{Ag}^+ \)) are coordinated by ammonia molecules, forming a diamine silver ion complex.
• The \( \text{Cl}^- \) ions are left free in the solution.

This process highlights how chemical reactions can modify the solubility of compounds, a key reason why certain seemingly insoluble compounds can actually dissolve when specific chemical conditions are met. This understanding is crucial for chemists, particularly those involved in solution chemistry, because it underpins many reactions used in laboratory and industrial processes.