Problem 109 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of moles of \(\mathrm{KMnO}_{4}\) reduced by one mole of KI in alkaline medium is (a) 2 (b) 1 (c) 5 (d) 6

Step-by-Step Solution

Answer

2 moles of \(\mathrm{KMnO}_{4}\) are reduced by 1 mole of \(\mathrm{KI}\).

Step 1: Write the Balanced Reaction Equation

In an alkaline medium, the reaction between potassium permanganate \( (\mathrm{KMnO}_{4}) \) and potassium iodide \( (\mathrm{KI}) \) is represented as follows:\[ 2\mathrm{KMnO}_{4} + 1\mathrm{H}_{2}\mathrm{O} + 1\mathrm{I}_{2} \rightarrow 2\mathrm{MnO}_{2} + 2\mathrm{KOH} + 2\mathrm{KIO}_{3} \]

Step 2: Analyze the Reaction Stoichiometry

The stoichiometry of the balanced equation shows that 2 moles of \(\mathrm{KMnO}_{4}\) react with 1 mole of \(\mathrm{I}_{2}\). This indicates that the reduction of \(\mathrm{KMnO}_{4}\) happens on a 2:1 basis with respect to \(\mathrm{KI}\).

Step 3: Calculate the Moles of KMnO_4 Reduced

From the balanced equation, 2 moles of \(\mathrm{KMnO}_{4}\) are reduced by 1 mole of \(\mathrm{I}_{2}\), which means by 1 mole of image KI. Thus, for one mole of \(\mathrm{KI}\), 2 moles of \(\mathrm{KMnO}_{4}\) are reduced.

Key Concepts

Balanced Chemical Equations

A balanced chemical equation is essential in stoichiometry to ensure the conservation of mass. In a balanced equation, the number of atoms for each element is the same on both the reactant and product sides. This balance reflects the principle that matter cannot be created or destroyed in a chemical reaction.
To balance a chemical equation, one must adjust the coefficients (the numbers in front of molecules or atoms) to make sure both sides of the equation contain equal numbers of each type of atom. For example, in the reaction between potassium permanganate (\( \mathrm{KMnO}_{4} \)) and potassium iodide (\( \mathrm{KI} \)) in alkaline medium, the equation is balanced as follows:
\[ 2\mathrm{KMnO}_{4} + 1\mathrm{H}_{2}\mathrm{O} + 1\mathrm{I}_{2} \rightarrow 2\mathrm{MnO}_{2} + 2\mathrm{KOH} + 2\mathrm{KIO}_{3} \]
Notice that the coefficients ensure that the number of each type of atom is the same on both sides. This balance confirms the reaction adheres to the laws of chemical equations, providing a foundation for analyzing reaction details.

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, are a type of chemical reaction where the oxidation states of atoms are changed. It involves the transfer of electrons between two substances.
In redox reactions, one substance loses electrons (oxidation), while the other gains electrons (reduction). In the reaction between \( \mathrm{KMnO}_{4} \) and \( \mathrm{KI} \), \( \mathrm{KMnO}_{4} \) acts as an oxidizing agent and gets reduced, while \( \mathrm{KI} \) serves as the reducing agent and gets oxidized.

• \( \mathrm{KMnO}_{4} \) is reduced to \( \mathrm{MnO}_{2} \), which means it gains electrons.
• \( \mathrm{KI} \) is oxidized to \( \mathrm{KIO}_{3} \), losing electrons in the process.

Understanding which substances are oxidized and reduced helps students grasp the concepts of electron transfer and oxidation states, central to mastering the topic of redox reactions.

Chemical Reaction Analysis

Analyzing a chemical reaction involves understanding the changes that occur during the reaction. It is essential to identify the reactants and products, evaluate the stoichiometry, and understand the nature of the reaction: whether it is a combination, decomposition, single replacement, double replacement, or redox reaction.
For the reaction between \( \mathrm{KMnO}_{4} \) and \( \mathrm{KI} \) mentioned in the textbook problem, the analysis starts with recognizing it as a redox reaction. From there, attention turns to the stoichiometry.
The balanced equation reveals that 2 moles of \( \mathrm{KMnO}_{4} \) are reduced per mole of \( \mathrm{KI} \). Understanding the stoichiometry helps predict how much product is expected from given amounts of reactant, which is a crucial aspect of analyzing chemical reactions.

• Understand the ratio of reactants to products through coefficients in the balanced equation.
• Identify the role of each substance: oxidizing or reducing agent.

This type of analysis is vital for students to predict reaction yields, determine the limiting reactant, and apply these concepts in laboratory or practical applications.