Problem 64 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A hydrated metallic salt \(A\), light green in colour, on careful heating gives a white anhydrous residue \(B . B\) is soluble in water and its aqueous solution reacts with NO to give a dark brown compound \(C . B\) on strong heating gives a brown residue \(D\) and a mixture of two gases \(E\) and \(F\). The gaseous mixture when passed through acidified permanganate, discharges the pink colour and when passed through acidified \(\mathrm{BaCl}_{2}\) solution, gave a white precipitate. Identify \(A, B, C, D\), \(E\) and \(F\).

Step-by-Step Solution

Answer

A: FeSO₄ • 7H₂O, B: FeSO₄, C: [Fe(H₂O)₅(NO)]SO₄, D: Fe₂O₃, E: SO₂, F: SO₃.

Step 1: Identify the Hydrated Metallic Salt (A)

The initial clue is that the salt is light green in color. Common hydrated salts with this property include ferrous sulfate ( FeSO₄ • 7H₂O). Ferrous sulfate is light green when hydrated.

Step 2: Analyze Heating to Identify Anhydrous Salt (B)

On careful heating, hydrated ferrous sulfate ( A - FeSO₄ • 7H₂O) loses water molecules and becomes anhydrous, turning into white anhydrous ferrous sulfate ( B - FeSO₄).

Step 3: Identify Compound (C) Formed with NO Reaction

Ferrous sulfate ( B - FeSO₄) in aqueous solution reacts with nitric oxide ( NO) to form a dark brown complex known as [Fe(H₂O)₅(NO)]SO₄, which is compound C.

Step 4: Deduce Product from Strong Heating (D, E, F)

Upon strong heating, ferrous sulfate decomposes, giving iron(III) oxide ( D - Fe₂O₃), sulfur dioxide ( E - SO₂), and sulfur trioxide ( F - SO₃) as products.

Step 5: Confirm Gaseous Reaction with Acidified Solutions

The gases E (SO₂) and F (SO₃), when passed through acidified permanganate, discharge the pink color due to their reducing properties. When passed through acidified BaCl₂, they form a white precipitate of barium sulfate ( BaSO₄), confirming the presence of sulfur dioxide and sulfur trioxide.

Key Concepts

Hydrated Metallic Salts

Hydrated metallic salts are compounds in which a metallic salt is chemically bound to water molecules. These water molecules are not simply mixed with the salt but are an integral part of its crystalline structure. This hydration often affects the salt's color and properties.
- **Color and Appearance**: Hydrated salts can exhibit vibrant colors due to the presence of water. For instance, hydrated ferrous sulfate (\( ext{FeSO}_4 ullet 7 ext{H}_2 ext{O} \)) appears light green.
- **Composition**: The chemical formula of a hydrated salt shows the number of water molecules associated with each formula unit of the salt, e.g., the "7H₂O" in ferrous sulfate heptahydrate. Understanding this composition is crucial in predicting how the salt will behave upon heating or reacting with other substances.

Thermal Decomposition

Thermal decomposition is a chemical reaction where a compound breaks down into simpler substances when heated. This process is common with hydrated metallic salts, which lose their water content or even undergo further chemical changes upon heating.
- **Stages of Decomposition**: For ferrous sulfate (\( ext{FeSO}_4 ullet 7 ext{H}_2 ext{O} \)), initial heating causes the removal of water molecules, transforming it into anhydrous ferrous sulfate (\( ext{FeSO}_4 \)), a white powder.
- **Further Heating**: When heated strongly, anhydrous ferrous sulfate decomposes further into iron(III) oxide (\( ext{Fe}_2 ext{O}_3 \)) and gaseous products, such as sulfur dioxide (\( ext{SO}_2 \)) and sulfur trioxide (\( ext{SO}_3 \)). Each stage involves energy absorption and results in different chemical and physical changes.

Redox Reactions

Redox reactions involve the transfer of electrons between substances, characterized by the oxidation and reduction of species.
- **Oxidation and Reduction**: In the case of the reaction of \( ext{FeSO}_4 \) with \( ext{NO} \), it forms a dark brown complex. Iron typically acts as an oxidizing agent by accepting electrons.
- **Permanganate Reaction**: Gases like sulfur dioxide (\( ext{SO}_2 \)) and sulfur trioxide (\( ext{SO}_3 \)) can reduce permanganate ions, resulting in color changes (discharging the pink color), indicating their reducing property. These reactions are not just chemical transformations but also involve a change in the oxidation states of the elements involved.

Acid-Base Reactions

Acid-base reactions are interactions where an acid reacts with a base, often resulting in the formation of water and another product. These reactions can also involve salts forming precipitates.
- **Precipitate Formation**: For example, when gases such as \( ext{SO}_2 \) and \( ext{SO}_3 \) pass through barium chloride (\( ext{BaCl}_2 \)), a white precipitate of barium sulfate (\( ext{BaSO}_4 \)) forms. This process confirms the presence of sulfate ions.
- **Indicators and Color Changes**: Acid-base interactions can also involve indicators. For instance, the color change of acidified permanganate demonstrates a redox reaction often involving acids.