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

Option (b) is incorrect; the titration is not conducted at high temperature.

Step 1: Understanding the Redox Titration

In the titration involving ferrous ammonium sulphate and potassium permanganate (\(\mathrm{KMnO}_{4}\)), potassium permanganate acts as an oxidant and also functions as a self-indicator due to its deep purple color, which disappears upon reduction.

Step 2: Evaluating Option (a)

Option (a) states that \(\mathrm{KMnO}_{4}\) acts as an oxidant and a self-indicator. This is accurate because \(\mathrm{KMnO}_{4}\) is known for its oxidizing property and its deep purple color is used as an indicator. Thus, option (a) is correct.

Step 3: Evaluating Option (b)

Option (b) mentions the maintenance of high temperature during the titration. This is incorrect as the titration is typically conducted at room temperature. Raising the temperature can affect the stability of potassium permanganate.

Step 4: Evaluating Option (c)

Option (c) describes the chemical reaction where ferrous sulphate is oxidized to ferric sulphate. This is correct because during the titration, the ferrous ion is oxidized by \(\mathrm{KMnO}_{4}\) to form ferric ion.

Step 5: Verifying Final Option (d)

Option (d) suggests that both (b) and (c) are incorrect. Since option (c) is correct, option (d) is incorrect. However, it does include the incorrectness of option (b), which indicates some truth to (d) but not in full context.

Key Concepts

Ferrous Ammonium Sulphate

Ferrous ammonium sulphate, often abbreviated as FAS, is a double salt frequently used in chemistry labs and various industrial applications. It is a light green crystalline solid that is quite stable and easy to use in different kinds of titrations.

FAS serves as an excellent reducing agent in redox reactions, which makes it ideal for titrations involving oxidizing agents. During a redox titration, FAS typically donates electrons to an oxidizing agent, such as potassium permanganate, which accepts these electrons.

Apart from being used in titration, FAS has several practical applications:

• It is used in analytical chemistry as a standard substance for volumetric analysis due to its stability.
• It serves as a catalyst in the process of removing impurities from wastewater.
• It can be used in photography as a reducing agent.

In the redox titration involving FAS and potassium permanganate, FAS is oxidized, turning ferrous ions into ferric ions, which highlights the essence of the redox process.

Potassium Permanganate

Potassium permanganate (\( \mathrm{KMnO}_{4} \)) is a powerful oxidizing agent, commonly used in redox titrations. It is well-known for its striking deep purple color, which makes it a unique choice as both a reactant and an indicator in redox reactions.

One of the remarkable features of \( \mathrm{KMnO}_{4} \) is its dual role in titration processes:

• Firstly, it acts as the oxidizing agent that accepts electrons during the titration process.
• Secondly, it functions as a self-indicator. As it reacts during the titration, the deep purple color fades to a colorless solution, signaling the endpoint of the titration.

In a typical redox titration involving \( \mathrm{KMnO}_{4} \) and ferrous ammonium sulphate, \( \mathrm{KMnO}_{4} \) accepts electrons from the ferrous ions, causing the purple color to disappear and indicating that the reduction process is complete.

Oxidizing Agents

In chemistry, oxidizing agents play a crucial role in redox reactions by accepting electrons from other substances. This electron acceptance leads to the oxidation of other reactants in the process.

Oxidizing agents are characterized by their ability to:

• Accept electrons readily from other substances, thereby gaining electrons themselves.
• Facilitate the oxidation process of substances that lose electrons.
• Act as key reactants in combustion, bleaching, and disinfection processes.

In the context of redox titrations, potassium permanganate often acts as the oxidizing agent. It's because of its strong affinity for electrons, which allows it to efficiently accept electrons from reducing agents like ferrous ammonium sulphate.

Chemical Indicators

Chemical indicators are essential tools in titrations to signal the completion of a reaction. They work by changing color at a particular phase of the titration process, providing a visual cue that helps chemists determine the endpoint of the reaction.

In redox titrations, self-indicating compounds like \( \mathrm{KMnO}_{4} \) are particularly beneficial. These compounds have a built-in color change, eliminating the need for external indicators:

• The intense purple color of \( \mathrm{KMnO}_{4} \) fades to colorless as it undergoes reduction, effectively marking the titration endpoint.
• Other chemical indicators may include substances like phenolphthalein or methyl orange used in different titration types to mark pH changes.
• Indicators are chosen based on the expected pH range or redox potential range where they change color.

The choice of using a self-indicator like \( \mathrm{KMnO}_{4} \) adds simplicity and accuracy to the titration process as it affords real-time visualization of the reaction's progression.