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; no high temperature is needed.

Step 1: Identify the Role of KMnO4

In the titration of ferrous ammonium sulfate and potassium permanganate (KMnO_4), KMnO_4 acts as a strong oxidizing agent. It also functions as a self indicator, as the solution changes color to pink at the end point, indicating the completion of the reaction. Therefore, statement (a) is correct.

Step 2: Evaluate the Role of Temperature

In practice, no excessive temperatures are required for the titration of ferrous ammonium sulfate with potassium permanganate. Titrations are usually carried out at room temperature for accuracy. Thus, statement (b) is incorrect.

Step 3: Assess Ferrous to Ferric Conversion

During the titration, ferrous sulphate (FeSO_4) is oxidized to ferric sulphate (Fe_2(SO_4)_3) by the permanganate ion. This conversion from Fe(II) to Fe(III) confirms that statement (c) is correct.

Step 4: Determine the Incorrect Statement

Given the evaluation, statement (b) is incorrect regarding the conditions of the titration. Therefore, option (d), which states that both (b) and (c) are incorrect, is not entirely accurate since statement (c) is actually correct.

Key Concepts

Ferrous Ammonium Sulphate

Ferrous Ammonium Sulphate, chemically known as ext{FeSO}_4 ext{(NH}_4 ext{)}_2 ext{SO}_4 ext{·6H}_2 ext{O}, is a common reagent in chemistry, especially in redox titrations. It is composed of ferrous sulphate and ammonium sulphate bonded with water molecules, making it a double salt. The ferrous ion, ext{Fe}^{2+}, is the active component in reactions, which easily participates in electron transfer processes, notably oxidation-reduction reactions.
During redox reactions, ferrous ammonium sulphate acts as a reducing agent. It donates electrons to other substances, in this case, potassium permanganate ( ext{KMnO}_4). As it donates electrons, it undergoes oxidation itself. Specifically, it is oxidized from its ferrous state, ext{Fe}^{2+}, to its ferric state, ext{Fe}^{3+}. Ferrous ammonium sulphate is favored in many titrations because of its stability under normal laboratory conditions. It is less prone to oxidation by air compared to plain ferrous sulphate, offering more accurate results during titration processes. This means students can work at room temperature without worrying about complex environmental factors affecting the solution or reaction results.

Potassium Permanganate

Potassium Permanganate, with the chemical formula ext{KMnO}_4, is a well-known chemical in the field of redox chemistry. It is characterized by its striking purple color and high oxidative potential. In titration scenarios, ext{KMnO}_4 functions as a strong oxidizing agent.
The bright color of potassium permanganate makes it an excellent self-indicator in titrations. This means that unlike many titrants that require an additional indicator like phenolphthalein, ext{KMnO}_4 changes color on its own during the reaction process. As soon as the titration endpoint is reached, just one extra drop will cause the solution to turn pink, signifying that no ferrous ions are left to react.
In the process, the permanganate ion ( ext{MnO}_4^-) is reduced, changing from the manganese in its +7 oxidation state to a lower state, typically +2 (manganese ions, ext{Mn}^{2+}). This dramatic drop allows potassium permanganate to oxidize substances effectively, making it crucial in redox chemistry and many practical applications, from water treatment to laboratory analyses.

Oxidation-Reduction Reactions

Oxidation-Reduction Reactions, or redox reactions, involve the transfer of electrons between two substances. These reactions are foundational in chemistry, involving changes in the oxidation states of reactants. In titrations like the one between ferrous ammonium sulphate and potassium permanganate, redox reactions are essential to achieve accurate results.
In a redox titration, one reactant undergoes oxidation while the other undergoes reduction.

• **Oxidation** is the loss of electrons and an increase in oxidation state.
• **Reduction** is the gain of electrons and a decrease in oxidation state.

For the titration in focus:

• Ferrous ammonium sulphate ( ext{Fe}^{2+} ) is oxidized to ferric ( ext{Fe}^{3+} ).
• Potassium permanganate ( ext{MnO}_4^- ) is reduced from manganese +7 to manganese +2 state ( ext{Mn}^{2+} ).

Understanding redox reactions is crucial for students, as these reactions underpin many chemical and biological processes. Recognizing how electrons are transferred helps explain why certain outcomes, like color changes, occur during titrations. Each step is an opportunity to visualize how substances interact at the molecular level, reflecting broader principles in chemistry.