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

Question

Compound 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 \(\mathrm{X}\) here. (a) \(\mathrm{CuSO}_{4} .5 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{FeSO}_{4} .7 \mathrm{H}_{2} \mathrm{O}\) (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

Compound X is (b)  ext{FeSO}_4  ext{H}_2 ext{O}.

Step 1: Analyze Heating Reaction

When Compound X is strongly heated, it decomposes to produce two oxides of sulphur. These are typically sulphur dioxide (SO_2) and sulphur trioxide (SO_3). This behavior is characteristic of compounds containing hydrated sulfates, such as  ext{FeSO}_4 .

Step 2: Test Reaction with NaOH

When aqueous  ext{NaOH} is added to a solution of Compound X, it produces a dirty green precipitate. This precipitate is likely iron (II) hydroxide,  ext{Fe(OH)}_2, which is a common result when iron (II) sulfate reacts with  ext{NaOH}.

Step 3: Observing Precipitate Color Change

The green precipitate turns brown upon exposure to air, indicating the oxidation of iron (II) hydroxide to iron (III) hydroxide,  ext{Fe(OH)}_3. The color change and oxidation are indicative of the presence of iron compounds, specifically  ext{FeSO}_4 .

Step 4: Identify the Compound

Based on the formation of two oxides of sulfur and the behavior of the precipitate, Compound X is identified as hydrated iron (II) sulfate, or  ext{FeSO}_4  ext{H}_2 ext{O}. This matches with option (b).

Key Concepts

Hydrated Sulfates

Hydrated sulfates are fascinating compounds where molecules of water are associated with the sulfate. Imagine tiny water molecules nestled like friends around each sulfate ion. These water molecules are called "water of crystallization."

• They form a significant part of the structure.
• Removing this water can significantly change the properties of the compound.

In our exercise, we are looking at an example of a hydrated sulfate, specifically iron (II) sulfate, known as \( \text{FeSO}_4 \, \cdot \, 7 \text{H}_2\text{O} \). When you heat a hydrated sulfate, like in our exercise, the water evaporates first, and then you may get some interesting chemistry, like the decomposition of sulfate to produce oxides of sulfur.

Sulfate Compounds

Sulfate compounds are one of the more common inorganic structures around. These compounds have the sulfate ion, \( \text{SO}_4^{2-} \), as a key player. Let's explore this wondrous ion.

• The sulfate ion is made of one sulfur atom surrounded by four oxygen atoms.
• Overall, it carries a negative two charge.

This unique configuration gives sulfate compounds specific properties, like being water-soluble.
In the exercise, our compound, probably \( \text{FeSO}_4 \, \cdot \, 7 \text{H}_2\text{O} \), gives off oxides of sulphur upon heating. This sooty transformation results directly from its sulfate nature.

Oxidation Reactions

Oxidation reactions are chemical processes where a substance loses electrons. You might think of rust forming on metal or an apple turning brown. In our exercise, we see a similar process happening with iron.

• The dirty green \( \text{Fe(OH)}_2 \) turns brown as it oxidizes to \( \text{Fe(OH)}_3 \).
• This change tells us that the iron moves from a lower to a higher oxidation state.

Upon exposure to air, the iron (II) hydroxide converts. This is a classic scene in chemistry showcasing how air facilitates oxidation.

Precipitation Reactions

Precipitation reactions are always exciting because you see something new forming right before your eyes. These reactions occur when two solutions are mixed, resulting in an insoluble solid, called a precipitate, coming out of solution.
In the context of our exercise, when sodium hydroxide, \( \text{NaOH} \), is added to a solution of our compound, it results in the formation of a dirty green precipitate of iron (II) hydroxide, \( \text{Fe(OH)}_2 \).

• This type of reaction is essential to separate components in a mixture.
• It also helps identify ionic compounds in solution.

As expected, \( \text{Fe(OH)}_2 \) is not stable in air, showing its readiness to oxidize. It's truly a fascinating dance of chemistry!