Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

A certain pale-green substance, \(X\) becomes dark brown on adding \(\mathrm{NaNO}_{2}\) in presence of dil. \(\mathrm{H}_{2} \mathrm{SO}_{4}\). its aqueous solution gives precipitates with (i) \(\mathrm{BaCl}_{2}\) and (ii) \(\mathrm{NaOH}\) in separate tests. The latter \(\mathrm{NaOH}\) in separate tests. The latter precipitate, Y, gradually changes colour from green to brown, on exposure to air. Identify X here? (a) \(\mathrm{NiSO}_{4}\) (b) \(\mathrm{FeSO}_{4}\) (c) \(\mathrm{ZnSO}_{4}\) (d) \(\mathrm{Cr}_{2}\left(\mathrm{SO}_{4}\right)_{3}\)

Step-by-Step Solution

Answer

The substance X is \(\mathrm{FeSO}_4\).

Step 1: Understand the Reaction with NaNO2 and Dil. H2SO4

The substance X becomes dark brown on adding \(\mathrm{NaNO}_2\) in the presence of dilute \(\mathrm{H}_2\mathrm{SO}_4\). This color change suggests the formation of a brown compound. In the case of \(\mathrm{FeSO}_4\), it can be oxidized by \(\mathrm{NaNO}_2\) to form brown \(\mathrm{Fe}_2\left(\mathrm{SO}_4\right)_3\).

Step 2: Reaction with BaCl2

The solution of substance X gives a precipitate with \(\mathrm{BaCl}_2\). This indicates the presence of the sulfate ion \(\mathrm{SO}_4^{2-}\) which reacts with barium ions to form barium sulfate \(\mathrm{BaSO}_4\), a well-known insoluble white precipitate.

Step 3: Reaction with NaOH

The solution of substance X gives a precipitate with \(\mathrm{NaOH}\), which suggests the formation of a metal hydroxide. For \(\mathrm{FeSO}_4\), \(\mathrm{Fe(OH)}_2\) forms which is initially green and turns brown on exposure to air due to oxidation to \(\mathrm{Fe(OH)}_3\).

Step 4: Consider Color and Chemical Changes

The change from a pale-green solution to brown and the presence of hydroxides that change from green to brown suggest iron compounds which oxidize in air, consistent with the behavior of ferrous sulfate \(\mathrm{FeSO}_4\).

Step 5: Identify the Substance X

Based on the observations: color changes on reaction with \(\mathrm{NaNO}_2\), precipitation reactions with \(\mathrm{BaCl}_2\) and \(\mathrm{NaOH}\), and oxidation behavior to form brown compounds, \(\mathrm{X}\) is identified as \(\mathrm{FeSO}_4\).

Key Concepts

Chemical Reactions

Chemical reactions are processes where one or more substances are converted into new products. Substances undergoing a chemical change display different properties afterwards because they rearrange their atoms to form new compounds.
For example, consider sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)) reacting with a metal compound like iron sulfate (\(\mathrm{FeSO}_4\)). Initially, these reactants might have a green color, which transforms as the reaction progresses to produce new substances with distinct characteristics.
Key characteristics of chemical reactions can include:

• Color change, like the transformation from green to brown.
• Formation of solids or precipitates.
• Evolution of gases.
• Release or absorption of energy as heat or light.

This specific reaction with iron sulfate when mixed with sodium nitrite (\(\mathrm{NaNO}_2\)) and sulfuric acid is a prime example, as it results in noticeable color and state changes indicating formation of new compounds.

Oxidation-Reduction

Oxidation-reduction or redox reactions are a vital class of chemical reactions where the transfer of electrons between substances results in changes to their oxidation states. Oxidation refers to loss of electrons, while reduction means gaining electrons.
In the problem involving iron sulfate, the switch from ferrous (\(\mathrm{Fe}^{2+}\)) to ferric (\(\mathrm{Fe}^{3+}\)) ions through oxidation is key. Sodium nitrite (\(\mathrm{NaNO}_2\)) plays a crucial role here, acting as an oxidizing agent, facilitating the oxidation process.
Identify oxidation and reduction steps in any reaction using these markers:

• An increase in oxidation state indicates oxidation.
• A decrease in oxidation state suggests reduction.
• Oxidizing agents are substances that accept electrons.
• Reducing agents are substances that donate electrons.

Through such transformations, iron goes from a stable \(\mathrm{Fe}^{2+}\) state to an oxidized \(\mathrm{Fe}^{3+}\), resulting in the characteristic brown color of ferric sulfate.

Precipitation Reactions

Precipitation reactions occur when two soluble salts react in solution to produce an insoluble solid known as a precipitate. This can be observed as a solid forming out of a solution.
In the case of identifying iron sulfate, the presence of sulfate ions (\(\mathrm{SO}_4^{2-}\)) can be confirmed by adding barium chloride (\(\mathrm{BaCl}_2\)), which leads to the formation of barium sulfate (\(\mathrm{BaSO}_4\)), a white precipitate.
The characteristics of a precipitation reaction include:

• Two reactants combine in aqueous solution.
• An insoluble solid appears as a distinct precipitate.
• Ionic compounds often participate in these reactions.

This reaction’s predictability makes it a valuable tool for identifying ions in solutions. Detecting a precipitate such as barium sulfate indicates the presence of sulfate ions in the original compound.

Transition Metal Compounds

Transition metals like iron are known for their variable oxidation states and ability to form colorful compounds. This variability is due to the electron structure in their d orbitals, allowing transition metals to participate in complex ion interactions and form various compounds.
Iron, for instance, can be found in both ferrous (\(\mathrm{Fe}^{2+}\)) and ferric (\(\mathrm{Fe}^{3+}\)) states, each exhibiting different properties. The transition from a pale green ferrous state to a brown ferric state is a signature of these metals’ diverse chemistry.
Key properties of transition metal compounds include:

• Variable oxidation states.
• Color variability linked to their electronic structure.
• Ability to form complex ions and participate in redox reactions.

Understanding these properties aids in predicting and identifying chemical behaviors in reactions, especially when dealing with changes like those seen in the given problem where the transition metal’s behavior confirms the identification of \(\mathrm{FeSO}_4\).