Problem 65 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A mixture of two salts was treated as follows : (i) The mixture was heated with manganese dioxide and concentrated sulphuric acid when yellowish green gas was liberated. (ii) The mixture on heating with sodium hydroxide solution gave a gas which turned red litmus blue. (iii) Its solution in water gave blue precipitate with potassium ferricyanide and red colouration with ammonium thiocyanate. (iv) The mixture was boiled with potassium hydroxide and the liberated gas was bubbled through an alkaline solution of \(\mathrm{K}_{2} \mathrm{HgI}_{4}\) to give brown precipitate. Identify the two salts. Give ionic equations for reactions involved in the tests \((i),(i i)\) and (iii).

Step-by-Step Solution

Answer

The salts are ammonium chloride \(\text{NH}_4\text{Cl}\) and ferrous ammonium sulfate \(\text{FeSO}_4\cdot(NH_4)_2\text{SO}_4\cdot6\text{H}_2\text{O}\).

Step 1: Analyze Reaction (i)

When the mixture was treated with manganese dioxide (MnO₂) and concentrated sulfuric acid (H₂SO₄), a yellowish-green gas was liberated. This indicates the liberation of chlorine gas (Cl₂). This reaction is characteristic of chlorides, suggesting the presence of a chloride in the mixture. The ionic equation is: \[ 4 ext{Cl}^- + ext{MnO}_2 + 4 ext{H}^+ \rightarrow ext{Mn}^{2+} + 2 ext{H}_2 ext{O} + ext{Cl}_2 \]

Step 2: Analyze Reaction (ii)

Upon heating the mixture with sodium hydroxide (NaOH), a gas was produced that turned red litmus blue, indicating the liberation of ammonia gas (NH₃). This suggests the presence of an ammonium salt. The ionic equation for releasing ammonia from ammonium ions when heated with base is: \[ ext{NH}_4^+ + ext{OH}^- \rightarrow ext{NH}_3 + ext{H}_2 ext{O} \]

Step 3: Analyze Reaction (iii)

The water solution of the mixture gave a blue precipitate with potassium ferricyanide and a red color with ammonium thiocyanate. The blue precipitate indicates the formation of ferrous ferricyanide (Turnbull's blue), suggesting the presence of Fe²⁺ ions. The reaction with ammonium thiocyanate which gives a red color is due to the formation of iron(III) thiocyanate, confirming the presence of Fe³⁺ ions. This means both ferrous and ferric ions are present:- \[ ext{Fe}^{2+} + 3 ext{Fe(CN)}_6^{3-} \rightarrow ext{Fe}_3[ ext{Fe(CN)}_6]_2 \]- \[ ext{Fe}^{3+} + 3 ext{SCN}^- \rightarrow ext{Fe(SCN)}_3 \]

Step 4: Analyze Reaction (iv)

When boiled with potassium hydroxide (KOH), the gas liberated, bubbled through an alkaline solution of \( \text{K}_2\text{HgI}_4 \), produced a brown precipitate. This test is characteristic of nitrogen-based compounds like ammonia, suggesting the presence of ammonium salts which can liberate ammonium ions to react with Hg(II) ions forming a brown precipitate of mercuric amine iodide (\( \text{NH}_2\text{Hg}_2\text{I} \)): \[ 2 ext{NH}_3 + ext{K}_2 ext{HgI}_4 + 2 ext{KOH} \rightarrow ext{NH}_2 ext{Hg}_2 ext{I} + 4 ext{KI} + 2 ext{H}_2 ext{O} \]

Step 5: Identify the Salts

Based on all the reactions: - The release of chlorine gas and its characteristic reactions suggests one salt is a chloride, likely an ammonium chloride \(\text{NH}_4\text{Cl}\).- The reactions with potassium ferricyanide and ammonium thiocyanate, which show presence of ferrous and ferric ions, suggest the other salt is a mixed salt like ferrous ammonium sulfate \(\text{FeSO}_4\cdot(NH_4)_2\text{SO}_4\cdot6\text{H}_2\text{O}\). Thus, the two salts are likely \( \text{NH}_4\text{Cl} \) and \( \text{FeSO}_4\cdot(NH_4)_2\text{SO}_4\cdot6\text{H}_2\text{O} \).

Key Concepts

Ionic Equations

Understanding ionic equations is crucial for grasping many reactions in inorganic chemistry.
An ionic equation breaks down the reactants and products of a chemical reaction into ions, showing only those that participate in the reaction. The rest are called spectator ions and do not change during the reaction.
Ionic equations help simplify complex reactions by focusing on the main participants. For instance, when a chloride is heated with manganese dioxide and sulfuric acid, chlorine gas is released.
Here, the ionic equation helps us identify chloride ions ( ext{Cl}⁻) as part of the reaction, leading to the release of chlorine gas ( ext{Cl}_2).

• Writing ionic equations often involves identifying ions present before and after the reaction.
• They isolate the essence of the reaction, making it easier to predict similar outcomes in related reactions.

This approach offers a streamlined method to understand and predict chemical reactions.

Chlorides

Chlorides are compounds formed by the combination of chlorine gas with metals or other elements.
These compounds are typically salts and have diverse applications in chemical reactions and industries.
A common characteristic of chlorides is their solubility in water, making them widely used in various solutions and reactions.
For example, in the given exercise, the heating of a chloride compound in the presence of manganese dioxide and concentrated sulfuric acid resulted in chlorine gas being released.

• Chlorides are typically ionic in nature, meaning they are composed of ions joined by ionic bonds.
• Common chloride compounds include sodium chloride (table salt) and ammonium chloride, which are easy to find in everyday life.

Understanding the behavior of chlorides in reactions helps in predicting the release of gases like chlorine and completing reactions efficiently.

Ammonium Salts

Ammonium salts are formed when ammonia ( ext{NH}_3) reacts with acids.
They are notably soluble in water and essential in various chemical processes, both in laboratories and industrial applications.
A key feature of ammonium salts is their ability to release ammonia gas when heated with a base like sodium hydroxide.
This makes them useful for reactions where ammonia release is desired, such as fertilizers or condensation reactions.

• Examples of ammonium salts are ammonium chloride ( ext{NH}_4 ext{Cl}) and ammonium sulfate ( ext{(NH}_4 ext{)}_2 ext{SO}_4).
• They are recognized by their pungent smell when they decompose to release ammonia gas.

Their reactivity and functional versatility make ammonium salts invaluable in both synthetic and analytical chemistry.

Ferrous and Ferric Ions

Iron can form two important types of ions, ferrous ( ext{Fe}^{2+}) and ferric ( ext{Fe}^{3+}).
These ions play critical roles in chemistry due to their unique chemical characteristics and reactions.
Ferrous ions, being in a lower oxidation state, tend to form stable complexes that have significant industrial utility.
Meanwhile, ferric ions, which are higher in charge, will often participate in different kinds of complex reactions.
In the exercise, ferrous ions react with potassium ferricyanide to produce The famous Turnbull's blue, while ferric ions react with ammonium thiocyanate to give a characteristic deep red color.

• Ferrous ions tend to be more reactive and are often involved in redox reactions.
• Ferric ions, due to their higher oxidation state, engage in binding more strongly to ligands, enabling colorful and complex ions.

These properties of ferrous and ferric ions make them crucial for a wide range of chemical reactions and applications.