Question

A certain metal \(\mathrm{A}\) is boiled in dilute nitric acid to give a salt 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 A and 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) \(\mathrm{Fe}, \mathrm{Fe}_{2}\left(\mathrm{SO}_{4}\right)_{3}\)

Short answer

A is Ag and E is Ag2S2O3.

Step-by-step solution

Analyze the Reaction with Nitric Acid

The metal \( A \) reacts with dilute nitric acid to form a salt \( B \) and a nitrogen oxide \( C \). Silver is known to react with nitric acid to form silver nitrate \( (B = \text{AgNO}_3) \) and nitrogen dioxide \( (C = \text{NO}_2) \), which matches the given conditions.

Examining Reaction with Brine

An aqueous solution of \( B \) mixed with brine precipitates \( D \). \( \text{AgNO}_3 \) reacts with sodium chloride (in brine) to form a white precipitate of silver chloride \( (\text{D} = \text{AgCl}) \), which is known to dissolve in ammonia like specified.

Identifying Precipitate with Hypo Solution

Adding the solution of \( B \) to hypo solution (\( \text{Na}_2\text{S}_2\text{O}_3 \)) yields a white precipitate \( E \). When silver ions interact with thiosulfate ions, silver thiosulfate \( (\text{E} = \text{Ag}_2\text{S}_2\text{O}_3) \) forms, which fits the description of turning black when left standing due to photoreduction to silver sulfide.

Identify the Metal A and Compound E

Conclude from the steps that A must be silver (\( \text{Ag} \)) and \( E \) must be silver thiosulfate \( (\text{Ag}_2\text{S}_2\text{O}_3) \), consistent with the given conditions and observed reactions.

Key concepts

Reactions with Nitric Acid

When metals react with acids like nitric acid (\text{HNO}_3), they often form salts and gases. In this particular type of reaction, the metal donates electrons to hydrogen ions in the acid, leading to the formation of a salt and the release of hydrogen gas. However, nitric acid is a bit different because it acts as both an acid and an oxidizing agent.

In the reaction given, silver (\text{Ag}) reacts with dilute nitric acid to produce silver nitrate (\text{AgNO}_3) and nitrogen dioxide (\text{NO}_2). Here's the chemical equation for clarity:
\[\text{3Ag} + \text{4HNO}_3 \rightarrow \text{3AgNO}_3 + \text{2NO}_2 + \text{2H}_2\text{O}\]

This reaction is significant in inorganic chemistry because:

• Nitric acid can generate different nitrogen oxides depending on its concentration and reaction conditions.
• In this example, nitrogen dioxide is produced, which is a brown gas with corrosive properties.
• Silver nitrate is a soluble salt, making it an important compound in various chemical applications.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions combine to form an insoluble solid known as a precipitate. This is one of the many types of double displacement reactions in chemistry.

When an aqueous solution of silver nitrate (\text{AgNO}_3) is mixed with brine (a solution of sodium chloride, \text{NaCl}), a white precipitate forms. The equation for this reaction is:
\[\text{AgNO}_3 (aq) + \text{NaCl} (aq) \rightarrow \text{AgCl} (s) + \text{NaNO}_3 (aq)\]

The white precipitate formed is silver chloride (\text{AgCl}), which is known for its characteristic white color. Some useful information about this reaction:

• Silver chloride is insoluble in water, which is why it precipitates out of the solution.
• It can dissolve in ammonia (\text{NH}_4\text{OH}) solution, which provides a useful test to confirm its presence.
• Such tests are extremely helpful in qualitative analyses for identifying the presence of specific ions.

Silver Nitrate and Thiosulfate Reactions

Silver nitrate (\text{AgNO}_3) is a versatile reagent in inorganic chemistry. Another interesting reaction occurs when it's mixed with a hypo solution, which contains sodium thiosulfate (\text{Na}_2\text{S}_2\text{O}_3).

The interaction between silver ions and thiosulfate ions results in the formation of silver thiosulfate (\text{Ag}_2\text{S}_2\text{O}_3), a white precipitate as shown in this equation:
\[2\text{Ag}^+ (aq) + \text{S}_2\text{O}_3^{2-} (aq) \rightarrow \text{Ag}_2\text{S}_2\text{O}_3 (s)\]

This white precipitate undergoes a transformation when exposed to light, turning black, due to a process known as photoreduction. This causes the conversion of silver thiosulfate into silver sulfide (\text{Ag}_2\text{S}), giving it a dark appearance. This behavior is crucial in photographic processes and:

• Demonstrates the sensitivity of certain silver compounds to light.
• Highlights the versatility of silver as a reagent with numerous uses, including in photographic film development.
• Shows the complex reactions silver can undergo making it valuable in various applications of inorganic chemistry.