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) \(\mathrm{CuSO}_{4} .5 \mathrm{H}_{2} \mathrm{O}\) (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 FeSO₄·7H₂O (option b).

Step 1: Identifying the Reaction with Heating Sulphates

When strongly heated, sulphates like FeSO₄ can decompose to release sulphur oxides which are SO₂ and SO₃. Other options do not release these oxides under strong heating.

Step 2: Observing the Reactivity with NaOH

Adding aqueous NaOH to FeSO₄ solution results in the formation of a green precipitate. This green precipitate is ferrous hydroxide, Fe(OH)₂, and it slowly oxidizes to a brownish compound, ferric hydroxide Fe(OH)₃, when exposed to air.

Step 3: Analyzing Other Options

CuSO₄ and ZnSO₄ do not produce a green precipitate under these conditions. Ag₂S₂O₃ is thiosulfate and, when reacted with NaOH, does not fit the described behaviors.

Step 4: Conclusion

Based on the characteristics of decomposition and reactivity with NaOH forming a green precipitate, compound X is FeSO₄·7H₂O.

Key Concepts

Thermal Decomposition of Sulfates

Thermal decomposition is a chemical reaction where a single compound breaks down into two or more elements or simpler compounds when heated. Iron(II) sulfate, or \( \text{FeSO}_4 \cdot 7 \text{H}_2\text{O} \), undergoes thermal decomposition when strongly heated. This process leads to the breakdown of the compound into iron(III) oxide \( \text{Fe}_2\text{O}_3 \) and two oxides of sulfur, namely sulfur dioxide \( \text{SO}_2 \) and sulfur trioxide \( \text{SO}_3 \).

The reaction can be represented as:\[2 ext{FeSO}_4 \rightarrow ext{Fe}_2\text{O}_3 + ext{SO}_2 + ext{SO}_3\]This reaction highlights the conversion from a hydrated compound to an anhydrous form along with the liberation of gases. It is essential to understand how heat affects such compounds in chemistry, as this knowledge can be applied to predict the behavior of other substances under similar conditions. Thermal decomposition not only helps identify the presence of sulfates like \( \text{FeSO}_4 \) but also aids in deducing the conditions of a reaction based on gas evolution.

Precipitation Reactions

Precipitation reactions occur when two soluble salts react in solution to form one or more insoluble products, known as precipitates. In the exercise, when aqueous sodium hydroxide \( \text{NaOH} \) is added to a solution of iron(II) sulfate \( \text{FeSO}_4 \), a dirty green precipitate of ferrous hydroxide \( \text{Fe(OH)}_2 \) is formed.

This can be represented by the equation:\[\text{FeSO}_4 + 2\text{NaOH} \rightarrow \text{Fe(OH)}_2 \downarrow + \text{Na}_2\text{SO}_4\]

• The "\( \downarrow \)" symbol denotes the formation of a solid precipitate in the mixture.
• Notably, other compounds like copper sulfate \( \text{CuSO}_4 \) or zinc sulfate \( \text{ZnSO}_4 \) do not form the same green precipitate under these conditions.

A thorough understanding of precipitation reactions can predict which ions will combine to form a solid and how to separate constituents from a solution effectively. This knowledge is particularly useful in fields like analytical chemistry and environmental science, where precipitates are often used to remove contaminants from wastewater.

Oxidation of Iron

Oxidation is a process where an element’s oxidation state increases, often involving the loss of electrons. For iron, this can be typically observed when \( \text{Fe(OH)}_2 \), the green precipitate formed in a reaction, is exposed to air. It undergoes oxidation to become \( \text{Fe(OH)}_3 \), a brown substance.

The reaction can be summarized as:\[4 \text{Fe(OH)}_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{Fe(OH)}_3\]This transformation indicates that exposure to oxygen significantly alters the properties of a compound. The oxidation of iron is a common occurrence and is important in understanding processes like rusting, where iron reacts with oxygen and water to form iron oxides.

Here are some additional points to consider about oxidation:

• Oxidation reactions are typically part of larger redox processes, where one species is oxidized, and another is reduced.
• Understanding oxidation is crucial in diverse applications, such as metallurgy, corrosion prevention, and energy storage.

By comprehending oxidation, students can appreciate why certain chemical reactions occur in everyday materials and industrial applications.