Problem 150 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements are correct? (a) \(\mathrm{KMnO}_{4}\) is a powerful oxidising agent. (b) \(\mathrm{KMnO}_{4}\) is a weaker oxidising agent than \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) in acid medium. (c) \(\mathrm{KMnO}_{4}\) is a stronger oxidising agent than \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) in acid medium. (d) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) oxidises a secondary alcohol to a ketone.

Step-by-Step Solution

Answer

Statements (a), (c), and (d) are correct.

Step 1: Determine Properties of KMnO4

Recognize that \( \mathrm{KMnO}_{4} \) is known for being a strong oxidizing agent. In chemical reactions, it often serves to oxidize substances by receiving electrons and getting reduced itself. This means that statement (a) is correct.

Step 2: Compare Oxidizing Strength in Acidic Medium

In an acidic medium, \( \mathrm{KMnO}_{4} \) is indeed a stronger oxidizing agent than \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \). Hence, it is incorrect to say that \( \mathrm{KMnO}_{4} \) is weaker. Therefore, statement (b) is incorrect, and statement (c) is correct.

Step 3: Examine Oxidation by K2Cr2O7

\( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) is known for oxidizing secondary alcohols to ketones. This means statement (d) is correct. The reaction typically follows:\[\mathrm{RCH(OH)R'} \xrightarrow{\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}} \mathrm{RCOR'}\]where a secondary alcohol \( \mathrm{RCH(OH)R'} \) is oxidized to a ketone \( \mathrm{RCOR'} \).

Key Concepts

KMnO4 properties

Potassium permanganate, often represented as \(\mathrm{KMnO}_4\), is renowned in the world of chemistry as a potent oxidizing agent. Its deep purple color not only makes it visually distinctive, but also contributes to its widespread use in various applications including titrations and disinfection.
One of the key properties of \(\mathrm{KMnO}_4\) is its ability to serve as an oxidizer. In redox reactions, \(\mathrm{KMnO}_4\) is reduced while it oxidizes another species, which means it gains electrons. This transformation results in the change of manganese's oxidation state from +7 to a lower state, typically +2 in acidic conditions.
Because of its strong oxidative properties, \(\mathrm{KMnO}_4\) finds applications in numerous reactions, including:

• Disinfecting and purifying water, where its oxidizing capacity is essential for removing organic matter.
• Conducting redox titrations, particularly in analyzing substances that can be oxidized.
• Serving as an antiseptic, thanks to its ability to kill germs and bacteria.

These attributes collectively underline the versatility and strength of \(\mathrm{KMnO}_4\) as an oxidizing compound.

K2Cr2O7 properties

Potassium dichromate, denoted as \(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\), is another strong oxidizing agent, often used in chemical analyses and organic chemistry. It is usually recognized by its bright orange crystalline appearance.
In redox reactions, \(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\) acts similarly to \(\mathrm{KMnO}_4\), where it is reduced as it oxidizes other substances. This involves the reduction of chromium from its +6 oxidation state to +3. The effectiveness of this compound is especially notable when used for oxidizing secondary alcohols to ketones, making it a valuable reagent in organic synthesis.
Key properties and uses include:

• Serves as a robust oxidizing agent, particularly useful in acidic medium.
• Widely employed in organic chemistry to oxidize alcohols to carbonyl compounds such as ketones and aldehydes.
• Utilized in cleaning glassware as a strong oxidant to remove organic residues.

Each of these roles highlights the effectiveness of \(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\) in both laboratory and industrial settings.

Redox reactions

Redox reactions, short for reduction-oxidation reactions, are fundamental chemical processes where the oxidation state of atoms changes through the transfer of electrons. These reactions are characterized by the movement of electrons from one substance to another.
Understanding redox involves grasping two interconnected processes:

• **Oxidation**: The process where a substance loses electrons and increases its oxidation state.
• **Reduction**: The process where a substance gains electrons, thus decreasing its oxidation state.

These two processes occur simultaneously, as the electrons lost by one molecule are gained by another.
In the context of oxidizing agents like \(\mathrm{KMnO}_4\) and \(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\), these compounds gain electrons during the reaction and are reduced, while the substances they react with lose electrons and are oxidized. This mutual electron exchange is crucial in numerous chemical processes, from energy production in biological systems to industrial manufacturing.
By breaking down these reactions into simpler parts, we get to see how electron flow drives change, making it an essential concept in understanding chemistry and its practical applications.