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

X is \(\mathrm{FeSO}_{4}\) (Option b).

Step 1: Analyzing the First Reaction

The substance X turns dark brown when \(\mathrm{NaNO}_{2}\) is added in the presence of dilute \(\mathrm{H}_{2}\mathrm{SO}_{4}\). This reaction is characteristic for the oxidation of ferrous sulfate \((\mathrm{FeSO}_{4})\) where the iron (Fe) is oxidized from +2 to +3 oxidation state. The dark brown color is due to the formation of ferric ion \(\mathrm{Fe}^{3+}\). This suggests that X is likely \(\mathrm{FeSO}_{4}\).

Step 2: Examining Precipitate with BaCl2

The aqueous solution of X produces a precipitate with \(\mathrm{BaCl}_{2}\). The precipitate is probably barium sulfate \((\mathrm{BaSO}_{4})\), which is a common reaction when a sulfate compound is present. This supports the initial guess that X is a sulfate, more specifically, \(\mathrm{FeSO}_{4}\).

Step 3: Observing Reaction with NaOH

The aqueous solution also gives a precipitate with \(\mathrm{NaOH}\). \(\mathrm{FeSO}_{4}\), when reacted with \(\mathrm{NaOH}\), forms a green precipitate of \(\mathrm{Fe(OH)}_{2}\), which upon exposure to air oxidizes to the brown \(\mathrm{Fe(OH)}_{3}\). This color change from green to brown confirms the presence of \(\mathrm{Fe}^{2+}\) ions being oxidized to \(\mathrm{Fe}^{3+}\), further supporting that X is \(\mathrm{FeSO}_{4}\).

Key Concepts

Oxidation of Ferrous Sulfate

Ferrous sulfate, or \( \text{FeSO}_4 \), undergoes oxidation when exposed to specific conditions. In the context of our problem, we see this reaction facilitated by the addition of \( \text{NaNO}_2 \) in the presence of dilute sulfuric acid \( (\text{H}_2\text{SO}_4) \). Here, ferrous sulfate converts iron from a +2 oxidation state (\( \text{Fe}^{2+} \)) to a +3 state (\( \text{Fe}^{3+} \)).
The change from \( \text{Fe}^{2+} \) to \( \text{Fe}^{3+} \) manifests visually as a color change from pale green to dark brown. This occurs because \( \text{Fe}^{3+} \) ions form complexes that often have different colors compared to \( \text{Fe}^{2+} \) ions. The appearance of a brown color specifically suggests the formation of ferric ions \( (\text{Fe}^{3+}) \), known for its characteristic dark hue.
This type of oxidation is not only applicable in academic exercises but also has real-world implications like in the treatment of iron-rich water. It's a clear example of how a seemingly simple compound like \( \text{FeSO}_4 \) can undergo noticeable changes.

Precipitation Reactions

Precipitation reactions are central to understanding the behavior of ferrous sulfate when mixed with certain chemicals. When \( \text{FeSO}_4 \) is dissolved in water and reacts with barium chloride \( (\text{BaCl}_2) \), a precipitate forms.
This precipitate is barium sulfate \( (\text{BaSO}_4) \) and it is formed because sulfates highly favor forming insoluble compounds with barium ions \( (\text{Ba}^{2+}) \). This precipitation confirms the presence of sulfate ions, which aligns with the identification of \( \text{FeSO}_4 \) as compound X in the exercise.
Furthermore, when \( \text{FeSO}_4 \) reacts with sodium hydroxide \( (\text{NaOH}) \), another precipitation occurs. Initially, we observe the formation of ferrous hydroxide \( (\text{Fe(OH)}_2) \), a green solid. This reaction affirms the presence of ferrous ions \( (\text{Fe}^{2+}) \) and is a key component in identifying compound X.

Chemical Changes Upon Exposure to Air

Exposure to air can drive additional chemical changes, often modifying the properties and appearance of compounds. In this context, when the green precipitate of ferrous hydroxide \( (\text{Fe(OH)}_2) \) is left open to air, it undergoes a chemical transformation.
In the presence of oxygen, \( \text{Fe(OH)}_2 \) is oxidized further to ferric hydroxide \( (\text{Fe(OH)}_3) \). This not only represents the conversion from iron's +2 to +3 oxidation state but also showcases a distinct color change from green to brown. This color transition is reliably used in laboratory settings to indicate oxidation.
Such transformations underscore how reactive some compounds can be, even under seemingly inert conditions like ambient air. This chemical sensitivity is essential in processes such as rust formation, proving how environmental exposure can lead to substantial material changes.