Problem 134 - A Complete Resource Book in Chemistry for JEE Main

Question

Compound \(\mathrm{X}\) (1) on strongly heating it gives two oxides of sulphur. (2) on adding aqueous \(\mathrm{NaOH}\) solution to its aqueous solution, a dirty green precipitate is obtained which starts turning brown on exposure to air. Identify X here. (a) \(\mathrm{CuSO}_{4} .5 \mathrm{H}_{3} \mathrm{O}\) (b) \(\mathrm{FeSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{ZnSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{Ag}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\)

Step-by-Step Solution

Answer

The compound X is \( \mathrm{FeSO}_{4} \cdot 7\mathrm{H}_2\mathrm{O} \).

Step 1: Analyze the Reaction on Strong Heating

When heated strongly, compound X releases two oxides of sulphur: SO2 and SO3. Sox soft sulfate compounds release such gases, which upon heating, decompose to yield SO2 or SO3.

Step 2: Examine the Reaction with NaOH

Upon adding aqueous NaOH solution to the compound X, a dirty green precipitate forms, which is identified as Fe(OH)2. This compound further oxidizes to form Fe(OH)3, which is brown, when exposed to air.

Step 3: Match Compound X with Options

The dirty green precipitate turning brown is characteristic of iron(II) compounds, particularly \(\mathrm{FeSO}_{4} \cdot 7\mathrm{H}_2\mathrm{O} \), which forms iron hydroxides in reactions with NaOH. None of the other choices release two oxides of sulphur and dirty green to brown precipitate. Hence, option (b) matches both reactions.

Key Concepts

Understanding Sulfate Compounds

Sulfate compounds are a fascinating group of chemical substances containing the sulfate ion, denoted as \( \text{SO}_4^{2-} \). This ion is composed of one sulfur atom centrally situated with four oxygen atoms surrounding it. The bonds between sulfur and oxygen create a highly stable structure. However, despite their stability, sulfate compounds can undergo various chemical reactions.

For instance, when sulfate compounds are heated, they can decompose to release gases such as sulfur dioxide (\( \text{SO}_2 \)) or sulfur trioxide (\( \text{SO}_3 \)). The release of these gases is a notable characteristic during the heating of specific sulfate materials like iron(II) sulfate.

• Sulfate ion: \( \text{SO}_4^{2-} \)
• Common gases released on heating: \( \text{SO}_2 \), \( \text{SO}_3 \)

This ability to release oxides of sulfur upon heating is pivotal in predicting certain chemical reactions, making sulfate compounds an essential topic in the study of chemistry.

Iron(II) Compounds and Their Characteristics

Iron(II) compounds are distinguished by the presence of iron in a +2 oxidation state. A prevalent example is iron(II) sulfate, written chemically as \( \text{FeSO}_4 \cdot 7 \text{H}_2\text{O} \). This form is often hydrated, which means it contains water molecules. These compounds are excellent for demonstrating certain chemical reactions.

When iron(II) compounds come into contact with strong bases like sodium hydroxide (NaOH), a reaction occurs, forming a precipitate of iron(II) hydroxide, \( \text{Fe(OH)}_2 \). This precipitate is initially a dirty green color, becoming an excellent visual identifier of such compounds.

Upon further exposure to air, the iron(II) hydroxide oxidizes to iron(III) hydroxide \( \text{Fe(OH)}_3 \), which is brown. These visual color changes are characteristic of reactions involving iron(II) compounds that are commonly used to identify or analyze different samples less ambiguously.

Exploring Precipitation Reactions

Precipitation reactions are a type of double displacement reaction where two aqueous solutions combine to form an insoluble solid, or precipitate. These reactions are found extensively in chemical analysis and synthesis.

For example, when the aqueous solution of iron(II) sulfate (\( \text{FeSO}_4 \)) reacts with potassium hydroxide (NaOH), it produces a characteristic dirty green precipitate of iron(II) hydroxide, \( \text{Fe(OH)}_2 \).

• Reactants: \( \text{FeSO}_4 (aq) + 2 \text{NaOH} (aq) \to \text{Fe(OH)}_2 (s) + 2 \text{Na}_2\text{SO}_4 (aq) \)
• Initial precipitate color: Dirty green \( \text{Fe(OH)}_2 \)

Over time and exposure to air, the \( \text{Fe(OH)}_2 \) oxidizes, changing to \( \text{Fe(OH)}_3 \), turning the macroscopic color brown, enhancing the visibility of the reaction’s transformation. This unique color shift is a hallmark of iron(II) compound reactions in precipitation.