Problem 132 - A Complete Resource Book in Chemistry for JEE Main

Question

A certain metal \(\mathrm{A}\) is boiled in dilute nitric acid to give a salt \(\mathrm{B}\) and an oxide of nitrogen \(\mathrm{C}\). An aqueous solution of \(\mathrm{B}\) with brine gives a precipitate \(\mathrm{D}\) which is soluble in \(\mathrm{NH}_{4} \mathrm{OH}\). On adding aqueous solution of \(\mathrm{B}\) to hypo solution, a white precipitate \(\mathrm{E}\) is obtained. \(\mathrm{E}\) turns black on standing. Identify \(\mathrm{A}\) and \(\mathrm{E}\) here? (a) \(\mathrm{Zn}, \mathrm{Zn}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{Ag}, \mathrm{Ag}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\) (c) \(\mathrm{Ag}, \mathrm{Ag}_{2} \mathrm{SO}_{4}\) (d) \(\mathrm{Fe}, \mathrm{Fe}_{2}\left(\mathrm{SO}_{4}\right)_{3}^{3}\)

Step-by-Step Solution

Answer

Metal \( A \) is \( \mathrm{Ag} \); precipitate \( E \) is \( \mathrm{Ag}_2 \mathrm{S}_2 \mathrm{O}_3 \).

Step 1: Analyze the Metal Reaction with Nitric Acid

Metal \( A \) is boiled in dilute nitric acid, producing a salt \( B \) and an oxide of nitrogen \( C \). Typically, metals like silver \( \mathrm{Ag} \) react with nitric acid to form nitrous oxides and nitrates. Thus, suggesting either \( \mathrm{Ag} \) or iron \( \mathrm{Fe} \) could be \( A \).

Step 2: Examine Precipitate Formation in Brine

The aqueous solution of salt \( B \) with brine gives a precipitate \( D \). Brine is a salty solution rich in sodium chloride. Silver nitrate \( \mathrm{AgNO}_3 \) reacts with sodium chloride to form white silver chloride \( \mathrm{AgCl} \), which is soluble in ammonium hydroxide \( \mathrm{NH}_4\mathrm{OH} \). This suggests \( B \) could be \( \mathrm{AgNO}_3 \) and \( D \) could be \( \mathrm{AgCl} \).

Step 3: Identify White Precipitate in Hypo Solution

When \( B \) (likely \( \mathrm{AgNO}_3 \)) is added to hypo solution (sodium thiosulfate, \( \mathrm{Na}_2 \mathrm{S}_2 \mathrm{O}_3 \)), a white precipitate \( E \) forms, which is \( \mathrm{Ag}_2 \mathrm{S}_2 \mathrm{O}_3 \). \( E \) turns black upon standing due to decomposition to silver sulfide (\( \mathrm{Ag}_2 \mathrm{S} \)).

Step 4: Confirm Identification

The clues provided align with silver \( \mathrm{Ag} \) and silver thiosulfate \( \mathrm{Ag}_2 \mathrm{S}_2 \mathrm{O}_3 \). The nitrate from the reaction with nitric acid is \( \mathrm{AgNO}_3 \), which reacts with sodium thiosulfate to initially form \( \mathrm{Ag}_2 \mathrm{S}_2 \mathrm{O}_3 \), turning black over time. The behavior presented points to silver-related compounds.

Key Concepts

Metal and Nitric Acid Reaction

When certain metals are boiled in dilute nitric acid, they undergo fascinating chemical reactions that result in the formation of a salt and an oxide of nitrogen. In the case of silver (Ag) reacting with nitric acid, a typical reaction sequence can be observed:
- Silver reacts with nitric acid to form silver nitrate (AgNO₃), a type of salt.- The reaction also produces nitrogen oxides, usually in the form of nitrogen dioxide (NO₂).

This reaction broadly illustrates how metals can interact with acids to form different compounds. The extent of metal reaction may vary, depending on numerous parameters, including the metal itself, concentration of the acid, and temperature.
Here's the simplified chemical equation demonstrating this interaction:\[ \text{3Ag} + 4\text{HNO}_3 \rightarrow 3\text{AgNO}_3 + \text{NO} + 2\text{H}_2\text{O} \]

When teaching this topic, it's important to highlight that reactions with nitric acid can result in different nitrogen oxides, such as NO₂, N₂O₄, or NO, due to the acid's varying reduction potential. Understanding this concept lays the foundation for grasping more complex reactions.

Precipitation in Brine

Precipitation reactions are fascinating chemical processes where two soluble salts in solution mix and form an insoluble salt, known as a precipitate. When silver nitrate (AgNO₃), which is a salt formed from the reaction of silver with nitric acid, is added to brine—which is essentially concentrated sodium chloride (NaCl) solution—a white precipitate is formed.
- This white precipitate is silver chloride (AgCl).
- AgCl is known for its low solubility in water, making the precipitate readily visible.
- Interestingly, silver chloride is soluble in ammonium hydroxide (NH₄OH), a characteristic that can be used to further verify the compound.

The formation of \[ \text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} \downarrow + \text{NaNO}_3 \]
illustrates this precipitation process clearly.
When discussing precipitation, emphasize the role of solubility rules in determining whether or not a precipitate forms. Such knowledge is essential for predicting the outcomes of mixing various ions in solution.

Sodium Thiosulfate Interaction

The interaction between sodium thiosulfate and silver compounds unveils further interesting chemistry. When silver nitrate (AgNO₃) is mixed with sodium thiosulfate solution (also known as hypo, Na₂S₂O₃), a white precipitate of silver thiosulfate (Ag₂S₂O₃) forms. This reaction provides an insightful look into the interaction of silver compounds and thiosulfate ions.
- Initially, the precipitate appears white.
- Over time, it turns black due to the decomposition of silver thiosulfate into silver sulfide (Ag₂S).

The chemical reaction at play can be summarized as:\[ 2\text{AgNO}_3 + \text{Na}_2\text{S}_2\text{O}_3 \rightarrow \text{Ag}_2\text{S}_2\text{O}_3 \downarrow + 2\text{NaNO}_3 \]
And the subsequent decomposition is represented by:\[ \text{Ag}_2\text{S}_2\text{O}_3 \rightarrow \text{Ag}_2\text{S} (\text{black}) \]
This interaction not only highlights the behavior of these chemical species but also introduces key concepts such as solubility and stability of silver thiosulfate. Appreciating these nuanced chemical behaviors helps students make connections between observable reactions and the underlying chemical principles.