Problem 41 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An aqueous solution of \(\mathrm{FeSO}_{4}, \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}\) and chrome alum is heated with excess of \(\mathrm{Na}_{2} \mathrm{O}_{2}\) and filtered. The materials obtained are : (a) a colourless filtrate and a green residue (b) a yellow filtrate and a green residue (c) a yellow filtrate and a brown residue (d) a green filtrate and a brown residue

Step-by-Step Solution

Answer

The answer is (c): a yellow filtrate and a brown residue.

Step 1: Identify the components in the mixture

The mixture includes three salts: \( \mathrm{FeSO}_4 \), \( \mathrm{Al}_2(\mathrm{SO}_4)_3 \), and chrome alum (which is \( \mathrm{KCr(SO}_4)_2 \)). These are all sulfates of iron, aluminum, and chromium.

Step 2: Determine the effect of Na2O2 on the mixture

\( \mathrm{Na}_2 \mathrm{O}_2 \) is a strong oxidizing agent that can convert \( \mathrm{Fe}^{2+} \) to \( \mathrm{Fe}^{3+} \), \( \mathrm{Cr}^{3+} \) to \( \mathrm{CrO}_4^{2-} \), giving it a yellow color, and \( \mathrm{Al}^{3+} \) remains unaffected in terms of color. The reaction with chromium ions is crucial here as it produces the characteristic yellow chromate ions.

Step 3: Analyzing the results after oxidation

After the oxidation, we expect the iron remains as \( \mathrm{Fe}_2\mathrm{O}_3 \), which is brown. Chromium transforms to chromate \( \mathrm{CrO}_4^{2-} \), which results in a yellow filtrate. Aluminum forms \( \mathrm{Al(OH)}_3 \) as a part of the residue, but it's not colored.

Step 4: Conclude the final observations

Based on the analysis, a reaction with \( \mathrm{Na}_2 \mathrm{O}_2 \) results in oxidation forming a yellow chromate solution and a brown residue primarily due to the iron. Hence, the final result is: a yellow filtrate due to \( \mathrm{CrO}_4^{2-} \) and a brown residue due to \( \mathrm{Fe}_2\mathrm{O}_3 \).

Key Concepts

Oxidation

In chemistry, oxidation refers to a process where a molecule, atom, or ion loses electrons. In our exercise, this is evident when

• Iron ions (\( \mathrm{Fe}^{2+} \)) are oxidized to \( \mathrm{Fe}^{3+} \).
• Chromium ions (\( \mathrm{Cr}^{3+} \)) are converted to chromate ions (\( \mathrm{CrO}_4^{2-} \)).

The substance that helps in this electron loss is known as an oxidizing agent.

The chemical \( \mathrm{Na}_2 \mathrm{O}_2 \) acts as a strong oxidizing agent, facilitating these transformations.
This process is essential for forming the yellow chromate ions and the brown iron oxide residue.
By understanding oxidation, it's easier to predict the colors formed during chemical reactions because these changes often indicate the formation of new compounds.

In this case, the oxidation involves substantial electron transformation leading to these observable color changes in solutions and residues.

Chromate Ion

Chromate ions, \( \mathrm{CrO}_4^{2-} \), are bright yellow in color and are produced from chromium (\( \mathrm{Cr} \)) compounds during specific chemical reactions.
The oxidation of \( \mathrm{Cr}^{3+} \) ions to chromate ions is an important part of the reaction in our exercise.
These ions are responsible for the yellow color of the filtrate after the reaction.
Understanding the chemistry of chromate ions is crucial in many fields:

• They are used in pigments and dyes because of their vivid color.
• Chromates are also important in the tanning of leather and corrosion resistance.

The formation of chromate ions in this reaction showcases how changes at atomic levels translate to visual cues.

This transformation is yet another application of the fundamental oxidation process.

Iron Sulfate

Iron sulfate, \( \mathrm{FeSO}_4 \), starts as ferrous sulfate when iron is in its +2 oxidation state.
During the reaction with \( \mathrm{Na}_2 \mathrm{O}_2 \), it undergoes oxidation to form ferric ions (\( \mathrm{Fe}^{3+} \)).
The ferric ions then react further in the solution to produce iron oxide (\( \mathrm{Fe}_2 \mathrm{O}_3 \)), which appears as a brown residue.
This brown iron oxide has various implications:

• In nature, it relates to rust formation.
• In industrial processes, it's an important pigment and supplement input.

The change from \( \mathrm{Fe}^{2+} \) to \( \mathrm{Fe}^{3+} \) is one of the classic examples of oxidation in action.

Notably, the color shift from the initial green or blue tinge of ferrous sulfate solutions to the brown color indicates the progress of the oxidation reaction.

Aluminum Sulfate

Aluminum sulfate, \( \mathrm{Al}_2(\mathrm{SO}_4)_3 \), is involved in the reaction but doesn't undergo any notable color change.
Upon heating with \( \mathrm{Na}_2 \mathrm{O}_2 \), aluminum ions (\( \mathrm{Al}^{3+} \)) result in forming aluminum hydroxide (\( \mathrm{Al(OH)}_3 \)).
This substance is part of the residue, but it remains colorless.
The properties of aluminum sulfate are noteworthy:

• It is widely used in water purification.
• It is also a key ingredient in some antiperspirants and food processing.

In chemical reactions, aluminum sulfate contributes more in terms of forming precipitates rather than color changes.

Here, its conversion to a non-colored solid shows how not all reaction products result in vivid visual cues, but they still play an important role in reaction dynamics.