Problem 97 - A Complete Resource Book in Chemistry for JEE Main

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 material 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 correct answer is (c) a yellow filtrate and a brown residue.

Step 1: Understand the Reaction

To solve this exercise, we need to consider what happens when the given compounds are heated with excess Na2O2. Sodium peroxide acts as an oxidizing agent. FeSO4 will oxidize from iron(II) to iron(III), and the Al2(SO4)3 will remain unchanged as it is already in the highest oxidation state for aluminum.

Step 2: Evaluate the Components

Fe2+ ions from FeSO4 oxidize to Fe3+, which are typically yellow in solution. Chromic alum contains Cr3+ ions, which in the presence of an oxidizing agent like Na2O2 can be converted to chromate ions, ext{CrO}_4^{2-}, which are yellow in solution. Al3+ remains unchanged.

Step 3: Identify Filtrate and Residue

The oxidation of Cr3+ to the chromate ion will result in a yellow solution due to the formation of CrO4^2-. Iron(III) oxide, which is brown, will precipitate out as a residue. Sodium ions and sulfate ions will remain in the solution.

Step 4: Choose the Correct Option

Based on our evaluation: the filtrate will be yellow due to chromate ions, and the residue will be brown due to iron(III) oxide. Therefore, the correct option is (c) a yellow filtrate and a brown residue.

Key Concepts

Sodium Peroxide as an Oxidizing Agent

Sodium peroxide \(\text{Na}_2\text{O}_2\) is a powerful oxidizing agent, which means it can easily donate oxygen to other substances, making it possible for those substances to lose electrons. This characteristic is why sodium peroxide plays a significant role in oxidation reactions. When it interacts with other chemical compounds, sodium peroxide tends to transform them by altering their oxidation states.

• In oxidation reactions, sodium peroxide enables the conversion of substances by supplying oxygen or removing electrons.
• This chemical property makes it particularly effective when a transformation is needed, as in the case of transitioning iron(II) to iron(III) ions.

Sodium peroxide forms an alkaline solution when it dissolves in water, releasing oxygen and generating sodium hydroxide, which can further influence its reactivity in oxidation processes.
In the exercise, sodium peroxide was essential for oxidizing the iron ions and chromium ions present in the solution, leading to noticeable changes in the appearance of both the filtrate and residues.

Iron(II) to Iron(III) Oxidation

Iron \(\text{Fe}\) can exist in multiple oxidation states, but the most common states are iron(II) \(\text{Fe}^{2+}\) and iron(III) \(\text{Fe}^{3+}\). In oxidation, the iron(II) ion loses electrons to become iron(III). This process changes the chemical properties and often the visible color of the solution.

• The oxidation of \(\text{FeSO}_4\) involves the transformation of \(\text{Fe}^{2+}\) ions into \(\text{Fe}^{3+}\) ions.
• Upon oxidation, \(\text{Fe}^{3+}\) tends to form insoluble iron(III) oxide when combined with oxygen, which precipitates as a brown residue.

This oxidation reaction is crucial in many chemical processes and can be observed as a color change in the solution from green to yellow or brown, indicating the formation of new chemical species.
In the exercise, this oxidation led to the creation of a brown residue, highlighting the iron's role in altering the solution's characteristics due to the addition of sodium peroxide.

Chromate Ion Formation

Chromium ions in solution have multiple oxidation states, similar to iron. When chromium in chrome alum, typically trivalent chromium \(\text{Cr}^{3+}\), reacts with an oxidizing agent like sodium peroxide, it can transform into a chromate ion \(\text{CrO}_4^{2-}\). This conversion is significant due to the colorful display it provides.

• Chromate ions are yellow in color, which is a visual indicator of their presence in a solution.
• The process involves an oxidation increase, as chromium from its \(\text{Cr}^{3+}\) state accepts oxygen or loses electrons to become the chromate ion.

The formulation of chromate ions demonstrates a classic redox reaction, where the oxidizing agent enables the transformation of a compound into a more oxidized state.
In our exercise scenario, the shift from chromic ions to chromate ions led to a yellow filtrate, indicating successful oxidation and reaffirming the properties of sodium peroxide as a powerful oxidizing agent.