Problem 67 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is not correct regarding the titration of ferrous ammonium sulphate and \(\mathrm{KMnO}_{4}\) ? (a) Here \(\mathrm{KMnO}_{4}\) acts like an oxidant and a self indicator (b) Here high temperature is maintained during the titration (c) Here ferrous sulphate is oxidized into ferric sulphate. (d) Both (b) and (c)

Step-by-Step Solution

Answer

The incorrect statement is (b).

Step 1: Understand the Titration Process

In the titration of ferrous ammonium sulphate with potassium permanganate (\(\mathrm{KMnO}_{4}\)), \(\mathrm{KMnO}_{4}\) acts as an oxidizing agent. It oxidizes ferrous ion (\(\mathrm{Fe}^{2+}\)) to ferric ion (\(\mathrm{Fe}^{3+}\)). This reaction occurs under acidic conditions and \(\mathrm{KMnO}_{4}\) serves as a self-indicator due to its pink color in acidic medium.

Step 2: Evaluate Statement (a)

Statement (a) is correct because \(\mathrm{KMnO}_{4}\) is an oxidant; it can change its oxidation state while oxidizing other species. It is also a self-indicator since it imparts a pink color to the solution after the endpoint of the titration is reached.

Step 3: Evaluate Statement (b)

Statement (b) is incorrect. High temperature is not maintained during this titration, as the reaction proceeds adequately at room temperature.

Step 4: Evaluate Statement (c)

Statement (c) is correct. The ferrous ion (\(\mathrm{Fe}^{2+}\)) in ferrous ammonium sulphate is oxidized to ferric ion (\(\mathrm{Fe}^{3+}\)), forming ferric sulphate.

Step 5: Evaluate Statement (d)

Statement (d) suggests that both (b) and (c) are incorrect. Since statement (c) is correct, statement (d) as a whole must be incorrect.

Step 6: Identify the Incorrect Statement

Given the analysis above, the incorrect statement with respect to the titration process is statement (b).

Key Concepts

Oxidation-Reduction Reactions

In chemistry, oxidation-reduction reactions, also known as redox reactions, involve the transfer of electrons between two substances. In these reactions, one species undergoes oxidation (loses electrons) and the other undergoes reduction (gains electrons). Redox reactions are fundamental processes that drive many chemical reactions.
During the titration of ferrous ammonium sulphate with potassium permanganate (\(\mathrm{KMnO}_{4}\)), the ferrous ions (\(\mathrm{Fe}^{2+}\)) are oxidized to ferric ions (\(\mathrm{Fe}^{3+}\)). Meanwhile, the \(\mathrm{KMnO}_{4}\) is reduced, showcasing a classic redox process. The key in redox titrations is to identify the oxidizing and reducing agents and observe the electron shift.

• Oxidation: Loss of electrons
• Reduction: Gain of electrons
• Oxidizing agent: Substance that gains electrons
• Reducing agent: Substance that loses electrons

Acidic Medium

An acidic medium plays a crucial role in many redox reactions, especially in titrations involving potassium permanganate. The environment is adjusted to be acidic by adding acids like sulfuric acid.
The acidity enhances the ability of \(\mathrm{KMnO}_{4}\) to function effectively as an oxidizing agent. In the presence of sulfuric acid, \(\mathrm{KMnO}_{4}\) can oxidize ferrous sulphate to ferric sulphate smoothly and with a clear endpoint. The endpoint is the moment when all the reducing agent has reacted, and is often indicated by a persistent color change.
Utilizing an acidic medium ensures that the redox reaction proceeds to completion, without interferences from other possible reactions, such as the formation of manganese dioxide, which could occur in neutral or basic conditions. This specific setup helps maintain consistency and accuracy in the titration process.

Self-Indicator

In some titrations, a self-indicator can serve as an internal cue for the endpoint of the reaction. A self-indicator means that the titrant itself changes color when the endpoint is reached, eliminating the need for an external indicator.

Potassium permanganate (\(\mathrm{KMnO}_{4}\)) acts as a self-indicator due to its distinct color changes in different oxidation states. In an acidic medium, \(\mathrm{KMnO}_{4}\) is a deep purple color, but when it is reduced, the solution becomes pale or colorless. The appearance of a persistent faint pink color, after all the ferrous ions have reacted, signifies the endpoint of the titration.

• Distinct color for easy verification
• Eliminates need for additional chemical indicators

Oxidizing Agent

An oxidizing agent is a substance that gains electrons in a chemical reaction, causing another substance to lose electrons. In oxidation-reduction reactions, the oxidizing agent undergoes reduction itself.

In the titration of ferrous ammonium sulphate with \(\mathrm{KMnO}_{4}\), \(\mathrm{KMnO}_{4}\) is the oxidizing agent. It accepts electrons from the \(\mathrm{Fe}^{2+}\) ions, turning them into \(\mathrm{Fe}^{3+}\) ions.
This process is crucial, as it drives the overall redox reaction. Potassium permanganate is particularly effective as an oxidizing agent due to its unique ability to exhibit different oxidation states, easily accepting electrons.

Key points about oxidizing agents include:

• They are reduced in a reaction
• Facilitate electron transfer from other substances
• Typically cause oxidation of other reactants